CHAPTER II
GENERAL ANATOMICAL CHARACTERS
I. TEGUMENTARY STRUCTURES
_Hair._—The external surface of the greater number of members of the class is thickly clothed with a peculiarly modified form of epidermis, commonly called hair. This consists of hard, elongated, slender, cylindrical or tapering, filiform, unbranched masses of epidermic material, growing from a short papilla sunk at the bottom of a follicle in the derm or true skin. Such hairs upon different parts of the same animal, or upon different animals, assume various forms, and are of various sizes and degrees of rigidity,—as seen in the delicate soft velvety fur of the Mole, the stiff bristles of the Pig, and the spines of the Hedgehog and Porcupine, all modifications of the same structures. Each hair is composed usually of a cellular pithy internal portion, containing much air, and a denser or more horny cortical part. In some animals, as Deer, the substance of the hair is almost entirely composed of the medullary or cellular substance, and it is consequently very easily broken; in others the horny part prevails almost exclusively, as in the bristles of the Wild Boar. In the Three-toed Sloth (_Bradypus_) the hairs have a central horny axis and a pithy exterior. Though generally nearly smooth, or but slightly scaly, the surface of some hairs is strongly imbricated, notably so in some Bats; while in the Two-toed Sloth (_Cholœpus_) the hairs are longitudinally grooved or fluted. Though usually more or less cylindrical or circular in section, hairs are often elliptical or flattened, as in the curly-haired races of men, the terminal portion of the hair of Moles and Shrews, and conspicuously in the spines of the Rodents _Xerus_ and _Platacanthomys_. Hair having a property of mutual cohesion or “felting,” which depends upon a roughened scaly surface and a tendency to curl, as in domestic Sheep (in which animal this property has been especially cultivated by selective breeding), is called “wool.”
In a large number of mammals hairs of one kind only are scattered pretty evenly over the surface; but in many there are two kinds, one longer, stiffer, and alone appearing on the surface, and the other shorter, finer, and softer, constituting the under fur, analogous to the down of birds. This under fur, or _pashm_ as it is called by the natives of Kashmir, is especially abundant in the mammals inhabiting the cold plateau of Tibet and the adjacent regions. In many cases hairs of a different character from those of the general surface grow in special regions, forming ridges or tufts on the median dorsal or ventral surface or elsewhere. The tail is very often completed in this way by variously disposed elongated hairs. The margins of the eyelids are almost always furnished with a special row of stiffish hairs, called _cilia_ or eyelashes; and in most mammals specially modified hairs, constituting the _vibrissæ_ or whiskers, and endowed, through the abundant nerve supply of their basal papillæ, with special tactile powers, grow from the lips and cheeks. In some mammals the hairy covering is partial and limited to particular regions; in others, as the Hippopotamus and the Sirenia, though scattered over the whole surface, it is extremely short and scanty; but in none is it reduced to so great an extent as in the Cetacea, in which it is limited to a few small bristles confined to the neighbourhood of the lips and nostrils, and often only present in the young or even fœtal condition.
Some kinds of hairs, as those of the mane and tail of the Horse, appear to persist throughout the lifetime of the animal; but more generally, as in the case of the body hair of the same animal, they are shed and renewed periodically, generally annually. Many mammals have a longer hairy coat in winter, which is shed as summer comes on; and some few, which inhabit countries covered in winter with snow, as the Arctic Fox, Variable Hare, and Ermine, undergo a complete change of colour in the two seasons, being white in winter, and gray or brown in summer. The several species of Cape Mole (_Chrysochloris_), the Desmans or Water Moles (_Myogale_), and _Potamogale velox_, are remarkable as being the only mammals whose hair reflects those iridescent tints so common in the feathers of tropical birds.
The principal and most obvious purpose of the hairy covering is to protect the skin against external influences, especially cold and damp. Its function in the hairless Cetacea is supplied by the specially modified and thickened layer of adipose tissue beneath the skin, called “blubber.”
_Colour._—From the consideration of hair we are easily led to that of colour. As a general rule, bright and primary colours are absent in the class; but among the Baboons we find brilliant patches of scarlet or blue on some of the bare portions of the body, and one of the South American Monkeys (_Brachyurus_) has its whole face of a bright crimson. The most general colours are various shades of gray, brown, and tawny, with a frequent tendency to whiteness of the ventral surface of the body; but among the Squirrels, and more especially those provided with a parachute for flying, we find brilliant russets, passing into orange and red. Dark brown or black is also not very uncommon, as in the Bears and the Sable Antelope of South Africa. Entirely white mammals are rare, and mostly characteristic of the polar regions, or of countries having a long and snowy winter. An entirely white Bat (_Diclidurus albus_) occurs, however, in South America. In the large majority of mammals that exhibit a varied coloration, the upper and most exposed parts of the surface present the richest and darkest colours, the under parts being pale or often quite white. The Ratels, Gluttons, _Ælurus_, Hamsters, and some others are exceptions to this rule. A large number of mammals having a ground colour of gray, tawny, or dun are marked by stripes or spots, which are generally of a darker hue than the ground colour, as in many Carnivora, but more rarely are lighter, as in the Fallow and Axis Deer and several species of Antelope. These stripes very generally run transversely to the axis of the body, as in the Tasmanian Thylacine, the Tiger, and the Zebra; but they may be longitudinal, as in several of the Civet family. There has been considerable discussion as to whether the striped or the spotted is the more primitive type of coloration; but no very conclusive arguments have been brought forward in favour of either view. It is, however, manifest that in several groups of mammals there is a tendency to lose the spots, and more rarely the stripes, and to assume a uniform colour. Thus the young of nearly all the species of Deer are spotted, whereas the adults of only the Fallow and Axis Deer are so marked. The same is true of most of the Pigs; and the young of the Malayan and American Tapirs are marked by light-coloured stripes and spots on a dark ground. In like manner the young of the Lion and the Puma exhibit distinct spots which disappear with advancing age. In most of our domestic horses of various shades of bay and brown we may detect “dappling” on the under hair when the outer coat has been removed, which is not apparent on the surface of the latter. Many varieties of the Ass and the Horse also exhibit a tendency to the presence of stripes on the legs, which would seem to indicate a descent from a striped Zebra-like type.
A peculiar feature, which is, however, common to many other groups of animals, is the tendency to what is known as melanism, or the production of black or dark individuals or races of particular species, due to an excess of pigment in the skin and hair. Thus we may have black Leopards and Jaguars, black Wolves, and black Rabbits.
The opposite to melanism, and of more frequent occurrence, is albinism—a condition in which the pigment or colouring matter usually present in the tissues constituting the external coverings of the body, and which gives them their characteristic hue, is absent. When it occurs the hair is of an opaque white, the claws, hoofs, etc., of a pale horn-colour, and the skin and eyes pink, in consequence of the colour of the blood which circulates through them being no longer concealed by the stronger hues of the pigments. An animal in this condition is called an _albino_. In complete albinism there is a total absence of pigment throughout the system. This condition occurs occasionally as an individual peculiarity among wild animals of many kinds; but it has never been perpetuated among them in distinct races or species. The disadvantage of absence of pigment in the eye, causing a certain amount of intolerance of light, is probably sufficient to account for this. Several races of true albinos, as White Ferrets, Rabbits, Rats, and Mice, have, however, been established under the protection of man, and in them this abnormal condition is propagated from generation to generation.
Partial albinism—a condition in which the absence of pigment is limited to portions of the surface, or, at all events, does not extend to the eyes—is much more common as an individual variation both in domestic and in wild animals. It is possible that the artificial conditions incident to domestication increase the tendency to its occurrence; but, whether this be so or not, it certainly becomes perpetuated more frequently among domesticated than among wild animals. This may be accounted for partly by its proving of no disadvantage to them, and partly by the frequent selection by man of animals of such colour in preference to others. The result is that there is no completely domestic animal of which white races do not exist. On the other hand, to most wild animals even partial albinism seems to be a disadvantage in the struggle for existence, since, except in the case of species inhabiting lands continually covered with snow, it renders them more conspicuous objects both to their enemies and their prey, and hence it is rarely perpetuated. In northern regions, however, a large proportion of species are regularly and normally of a white colour, either, as the Polar Bear, all the year through, or, as the Ermine or Stoat, Arctic Fox, and Alpine Hare, during the winter season. The coloration in these cases is obviously protective, as it is also to a great extent in many other instances throughout the class.
Among conspicuously coloured mammals, it has been observed that the vertical black and tawny stripes of the Tiger harmonise so well with the brown and green grasses of its native jungle as to render the animal almost invisible when lying among them; while the dappled hide of the Giraffe is said to agree equally well with the chequered splashes of light and shade in the clumps of tall mimosas among which it feeds. The uniformly tawny hue of the Lion accords well with the prevailing tint of its native desert; and any one who has seen an Elephant or Buffalo in the deep shades of an Indian forest will realise how perfectly adapted is their dull, slaty colour to concealment in such a spot. The dun colour of the Wild Ass of India is equally well suited to the sandy deserts of Kutch; it is also stated that the brilliant stripes of the Zebras of Africa are arranged in such proportion as exactly to match the pale tint which arid ground possesses when seen by moonlight.[1] The most remarkable instance of protective coloration is, however, to be found in the Sloths of South America, in which the coarse gray hairs so closely resemble a mass of lichenous growth that it is almost impossible to distinguish these animals when at rest from the gnarled and lichen-clad boughs from which they suspend themselves. This resemblance is increased by the fact that the hairs actually develop a growth of lichens upon themselves. That the sombre coloration of these animals has been produced to harmonise with their present surroundings seems to be evident by the circumstance that when the long hair is plucked off the under fur is seen to present a bold alternation of black and yellow stripes, which may probably be regarded as the original primitive coloration of this group.
_Scales, etc._—True scales, or flat imbricated plates of horny material, covering the greater part of the body, so frequently occurring in reptiles, are found only in one family of mammals, the _Manidæ_ or Pangolins; but these are also associated with hairs growing from the intervals between the scales, or on the parts of the skin not covered by them. Similarly, imbricated epidermic productions form the covering of the under surface of the tail of the flying Rodents of the genus _Anomalurus_; and flat scutes, with the edges in apposition, and not overlaid, clothe both surfaces of the tail of the Beaver, Rats, and others of the same order, and also of some Insectivores and Marsupials. The Armadillos alone have an ossified exoskeleton, composed of plates of true bony tissue, developed in the derm or corium, and covered with scutes of horny epidermis. Other epidermic appendages are the horns of Ruminants and Rhinoceroses,—the former being elongated, tapering, hollow caps of hardened epidermis of fibrillated structure, fitting on and growing from conical projections of the frontal bone, and always arranged in pairs, while the latter are of similar structure, but solid and without any internal bony support, and (in all existing species) situated in the median line. Callosities, or bare patches covered with hardened and thickened epidermis, are found covering the pads under the soles of the feet and undersurfaces of the toes of nearly all mammals, upon the ischial tuberosities of many Apes, the sternum of Camels, on the inner side of the limbs of the _Equidæ_, the grasping under surface of the tail of the prehensile-tailed Monkeys, etc. The greater part of the skin of both species of one-horned Asiatic Rhinoceros is immensely thickened and stiffened by increase of the tissue both of the derm and epiderm, constituting the well-known jointed “armour-plated” hide of those animals.
_Nails, Claws, and Hoofs._—With very few exceptions, the terminal extremities of the digits of both limbs are more or less protected or armed by epidermic plates or sheaths, constituting the various forms of nails, claws, or hoofs. These are wanting in the Cetacea alone. A perforated spur, with a special secreting gland in connection with it, is found attached to the hind leg of the males of the three genera of Monotremata, _Ornithorhynchus_, _Proechidna_, and _Echidna_.
_Odour-secreting Glands._—Besides the universally distributed sebaceous glands connected with the pilose system, most mammals have special glands situated in modified portions of the integument, often involuted to form a shallow recess or a deep sac with a narrow opening, situated in various parts of the surface of the body, and secreting odorous substances, by the aid of which individuals appear to recognise one another, and probably affording the principal means by which wild animals are able to become aware of the presence of other members of the species, even at great distances. Although the commencement of the modifications of portions of the external covering for the formation of special secretions may be at present difficult to understand, the principle of natural selection will readily explain how such organs become fixed and gradually increase in development in any species, especially as there would probably be a corresponding modification and increased sensibility of the olfactory organs. Such individuals as by the intensity and peculiarity of their scent had greater power of attracting the opposite sex would certainly be those most likely to leave descendants to inherit and in their turn propagate the modification.
To this group of structures belong the suborbital gland or “crumen” of Antelopes and Deer, the frontal gland of the Muntjac and of Bats of the genus _Hipposiderus_, the submental gland of the Chevrotains and of _Taphozous_ and some other Bats, the post-auditory follicle of the Chamois, the temporal gland of the Elephant, the lateral glands of the Musk-Shrew, the dorsal gland of the Peccary, the inguinal glands of Antelopes, the preputial glands of the Musk-Deer and Beaver (already alluded to in connection with the use made of their powerfully odorous secretion in medicine and perfumery) and also of the Swine and Hare, the anal glands of Carnivora, the perineal gland of the Civet (also of commercial value), the caudal glands of the Fox and Goat, the gland on the humeral membrane of Bats of the genus _Saccopteryx_, the post-digital gland of the Rhinoceros, the interdigital glands of the Sheep and many Ruminants, and numerous others. In some of these cases the glands are peculiar to, or more largely developed in, the male; in others they are found equally developed in both sexes.
II. DENTAL SYSTEM
The dental system of mammals may be considered rather more in detail than space permits for some other portions of their structure, not only on account of the important part it plays in the economy of the animals of this class, but also for its interest to zoologists as an aid in the classification and identification of species. Owing to the imperishable nature of their tissues, teeth are preserved for an indefinite time, and in the case of extinct species frequently offer the only indications available from which to derive an idea of the characters, affinities, and habits of the animals to which they once belonged. Hence even their smallest modifications have received great attention from comparative anatomists, and they have formed the subject of many special monographs.[2]
Teeth are present in nearly all mammals, and are applied to various purposes. They are, however, mainly subservient to the function of alimentation, being used either in procuring food, by seizing and killing living prey or gathering and biting off portions of vegetable material, and more indirectly in tearing or cutting through the hard protective coverings of food substances, as the husks and shells of nuts, or in pounding, crushing, or otherwise mechanically dividing the solid materials before swallowing, so as to prepare them for digestion in the stomach. Certain teeth are also in many animals most efficient weapons of offence and defence, and for this purpose alone, quite irrespective of subserviency to the digestive process, are they developed in the male sex of many herbivorous animals, in the females of which they are absent or rudimentary.
Teeth belong essentially to the tegumentary or dermal system of organs, and, as is well seen in the lower vertebrates, pass by almost insensible gradations into the hardened spines and scutes formed upon the integument covering the outer surface of the body; but in mammals they are more specialised in structure and limited in locality. In this class they are developed only in the gums or fibro-mucous membrane covering the alveolar borders of the upper and lower jaws, or, in other words, the premaxillary and maxillary bones and the mandible. In the process of development, for the purpose of giving them that support which is needful for the performance of their functions, they almost always become implanted in the bone,—the osseous tissue growing up and moulding itself around the lengthening root of the tooth, so that ultimately they become apparently parts of the skeleton. In no mammal, however, does ankylosis or bony union between the tooth and jaw normally take place, as in many fishes and reptiles,—a vascular layer of connective tissue, the alveolo-dental membrane, always intervening.[3] The presence of two or more roots, frequently met with in the cheek-teeth of mammals, implanted in corresponding distinct sockets of the jaw, is now peculiar to animals of this class.[4]
_Structure._—The greater number of mammalian teeth when fully formed are not simple and homogeneous in structure, but are composed of several distinct tissues, which are enumerated below.
The _pulp_, a soft substance, consisting of a very delicate gelatinous connective tissue, in which numerous cells are imbedded, and abundantly supplied with blood-vessels and nerves, constitutes the central axis of all the basal part of the tooth, and affords the means by which the vitality of the whole is preserved. The nerves which pass into the pulp and endow the tooth with sensibility are branches of the fifth pair of cranial nerves. The pulp occupies a larger relative space, and performs a more important purpose, in the young growing tooth than afterwards, as by the calcification and conversion of its outer layers the principal hard constituent of the tooth, the dentine, is formed. In teeth which have ceased to grow the pulp occupies a comparatively small space, which in the dried tooth is called the pulp-cavity. This communicates with the external surface of the tooth by a small aperture at the apex of the root, through which the branches of the blood-vessels and nerves, by which the tooth receives its nutrition and sensitiveness, pass in to be distributed in the pulp. In growing teeth the pulp-cavity is widely open, while in advanced age it often becomes obliterated, and the pulp itself entirely converted into bone-like material.
The _dentine_ or _ivory_ forms the principal constituent of the greater number of teeth. When developed in its most characteristic form, it is a very hard but elastic substance, white, with a yellowish tinge, and slightly translucent. It consists of an organic matrix, something like, but not identical with, that of bone, richly impregnated with calcareous salts (chiefly calcium phosphate), these constituting in a fresh human tooth 72 per cent of its weight. When subjected to microscopical examination it is seen to be everywhere permeated by nearly parallel branching tubes which run, in a slightly curving or wavy manner, in a general direction from the centre towards the free surface of the tooth. These tubes communicate by open mouths with the pulp-cavity, and usually terminate near the periphery of the dentine by closed ends or loops, though in Marsupials and certain other mammals they penetrate into the enamel. They are occupied in the living tooth by soft gelatinous fibrils connected with the cells of the pulp. A variety of dentine, permeated by canals containing blood-vessels, met with commonly in fishes and in some few mammals, as the _Megatherium_, is called vaso-dentine. Other modifications of this tissue occasionally met with are called osteodentine and secondary dentine,—the latter being a dentine of irregular structure which often fills up the pulp-cavity of old animals.
The _enamel_ constitutes a thin investing layer, complete or partial, of the outer or exposed and working surface of the dentine of the crown of the teeth of most mammals. This is the hardest tissue met with in the animal body, containing from 95 to 97 per cent of mineral substances (chiefly calcium phosphate and some carbonate, with traces of fluoride). Its ultimate structure consists of prismatic fibres, placed generally with their long axes at right angles to the free surface of the tooth. Enamel is easily distinguished from dentine with the naked eye by its clear, bluish-white, translucent appearance.
The _cement_ or _crusta petrosa_ is always the most externally placed of the hard tissues of which teeth are composed, as will be understood when the mode of development of these organs is considered. It is often only found as a thin layer upon the surface of the root; but sometimes, as in the complex-crowned molar teeth of the Horse and Elephant, it is a structure which plays a very important part, covering and filling in the interstices between the folds of the enamel. In appearance, histological structure, and chemical composition it is closely allied to osseous tissue, containing lacunæ and canaliculi, though only when it is of considerable thickness are Haversian canals present in it.
_Development._—The two principal constituents of the teeth, the dentine and the enamel, are developed from the two layers of the mucous membrane of the jaw—the dentine from the deeper or vascular, the enamel from the superficial or epithelial layer. The latter dips down into the substance of the gum, and forms the enamel-organ or germ, the first rudiment of the future tooth, which is constantly present even in those animals in which the enamel is not found as a constituent of the perfectly-formed tooth. Below the mass of epithelial cells thus embedded in the substance of the gum, and remaining connected by a narrow neck of similar structure with the epithelium of the surface, a portion of the vascular areolar tissue becomes gradually separated and defined from that which surrounds it, and assumes a distinct form, which is that of the crown of the future tooth,—a single cone in the case of simple teeth, or with two or more eminences in the complex forms. This is called the dental papilla or dentine germ, and by the gradual conversion of its tissue into dentine the bulk of the future tooth is formed, the uncalcified central portion remaining as the pulp. The conversion of the papilla into hard tissue commences at the outer surface of the apex, and gradually proceeds downwards and inwards, so that the form of the papilla exactly determines the form of the future dentine, and no alteration either in shape or size of this portion of the tooth, when once calcified, can take place by addition to its outer surface. In the meanwhile, calcification of a portion of the cells of the enamel-organ, which adapts itself like a cap round the top of the dentinal papilla, and has assumed a somewhat complex structure, results in the formation of the enamel-coating of the crown of the tooth. While these changes are taking place the tissues immediately surrounding the tooth-germ become condensed and differentiated into a capsule, which appears to grow up from the base of the dental papilla, and encloses both this and the enamel-germ, constituting the follicle or tooth-sac. By the ossification of the inner layer of this follicle the cement is formed. This substance, therefore, unlike the dentine, increases from within outwards, and its growth may accordingly be the cause of considerable modification of form and enlargement, especially of the roots, of certain teeth, as those of Seals and some Cetacea. The delicate homogeneous layer coating the enamel surface of newly-formed teeth, in which cement is not found in the adult state, and known as Nasmyth’s membrane, is considered by Tomes as probably a film of this substance, too thin to exhibit its characteristic structure, though by others it is believed to be derived from the external layer of the enamel-organ. The homology of the teeth with the dermal appendages, hairs, scales, and claws, has already been alluded to, and it will now be seen that in both cases two of the primary embryonic layers are concerned in their development—the mesoblast and epiblast—although in very different proportions respectively. Thus in the hair or nail the part derived from the epiblast forms the principal bulk of the organ, the mesoblast only constituting the papilla or matrix. But in the tooth the epiblastic portion is limited to the enamel, and is always of relatively small bulk and often absent, while the dentine (the principal constituent of the tooth) and the cement are formed from the mesoblast.
When more than one set of teeth occur in mammals, those of the second set are developed in a precisely similar manner to the first, but the enamel-germ, instead of being derived directly from an independent part of the oral epithelium, is formed from a budding out of the neck of the germ of the tooth succeeded. In the case of the true molars, which have no predecessors, the germ of the first has an independent origin, but that of the others is derived from the neck of the germ of the tooth preceding it in the series. The foundations of the permanent teeth are thus laid as it were almost simultaneously with those of their predecessors, although they remain in many cases for years before they are developed into functional activity.
Although the commencement of their formation takes place at an early period of embryonic life, teeth are in nearly all mammals still concealed beneath the gum at the time of birth. The period of eruption, or “cutting” of the teeth as it is called, that is, their piercing through and rising above the surface of the mucous membrane, varies much in different species. In some, as Seals, the whole series of teeth appears almost simultaneously; but more often there are considerable intervals between the appearance of the individual teeth, the front ones usually coming into place first, and those at the back of the mouth at a later period.
_Forms of Teeth._—The simplest form of tooth may be exemplified on a large scale by the tusk of the Elephant (Fig. 1, I.) It is a hard mass almost entirely composed of dentine, of a conical shape at first, but during growth becoming more and more cylindrical or uniform in width. The enamel-covering, present on the apex in its earliest condition, soon disappears, but a thin layer of cement covers the circumference of the tooth throughout life. In section it will be seen that the basal portion is hollow, and contains a large conical pulp, as broad at the base as the tooth itself, and deeply imbedded in the bottom of a recess, or socket, in the maxillary bone. This pulp continues to grow during the lifetime of the animal, and at the same time is converted at its surface into dentine. The tooth therefore continually elongates, but the use to which the animal subjects it in its natural state causes the apex to wear away, at a rate generally proportionate to the growth at the base, otherwise it would become of inconvenient length and weight. Such teeth of indefinite growth are said to be “rootless,” or to have “persistent pulps.”
[Illustration: FIG. 1.—Diagrammatic Sections of various forms of Teeth. I. Incisor or tusk of Elephant, with pulp-cavity persistently open at base. II. Human incisor during development, with root imperfectly formed, and pulp-cavity widely open at base. III. Completely formed human incisor, with pulp-cavity contracted to a small aperture at the end of the root. IV. Human molar, with broad crown and two roots. V. Molar of the Ox, with the enamel covering the crown deeply folded, and the depressions filled up with cement. The surface is worn by use; otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cement by dots.]
One of the corresponding front teeth of man (Fig. 2, II. and III.) may be taken as an example of a very different condition. After its crown is fully formed by calcification of the germ, the pulp, though continuing to elongate, begins to contract in diameter; a neck or slight constriction is formed; and the remainder of the pulp is converted into the root (often, but incorrectly, called “fang”), a tapering conical process imbedded in the alveolar cavity of the bone, and having at its extremity a minute perforation, through which the vessels and nerves required to maintain the vitality of the tooth enter the pulp-cavity, which is very different from the widely open cavity at the base of the growing tooth. When the crown of the tooth is broad and complex in character, instead of having a single root, it may be supported by two or more roots, each of which is implanted in a distinct alveolar recess or socket, and to the apex of which a branch of the common pulp-cavity is continued (Fig. 1, IV.) Such teeth are called “rooted teeth.” When they have once attained their position in the jaw, with the neck a little way above the level of the free margin of the alveolus, and embraced by the gum or tough fibrovascular membrane covering the alveolar border, and having the root fully formed, they can never increase in length or alter their position; if they appear to do so in old age, it being only in consequence of absorption and retrocession of the surrounding alveolar margins. If, as often happens, their surface wears away in mastication, it is never renewed. The open cavity at the base of the imperfectly developed tooth (Fig. 1, II.) causes it to resemble the persistent condition of the rootless tooth. The latter is therefore a more primitive condition, the formation of the root being a completion of the process of tooth development. Functionally it is, however, difficult to say that the one is a higher form than the other, since they both serve important and different purposes in the animal economy.
As is almost always the case in nature, intermediate conditions between these two forms of teeth are met with. Thus some teeth, as the molars of the Horse, and of many Rodents, are for a time rootless, and have growing pulps producing very long crowns with parallel sides, the summits of which may be in use and beginning to wear away while the bases are still growing; but ultimately the pulp contracts, forms a neck and distinct roots, and ceases to grow. The canine tusks of the Musk Deer and of the Walrus have persistent pulps, and are open at their base until the animal is of advanced age, when they close, and the pulp ceases to be renewed. The same sometimes happens in the tusks of very old Boars.
The simplest form of the crown of a tooth is that of a cone; but this may be variously modified. Thus it may be flattened, with its edges sharp and cutting, and pointed at the apex, as in the laterally compressed premolars of most Carnivora; or it may be chisel- or awl-shaped, with a straight truncated edge, as in the human incisors; or it may be broad, with a flat or rounded upper surface. Very often there is a more or less prominent ridge encircling the whole or part of the base of the crown just above the neck, called the cingulum, which serves as a protection to the edge of the gum in masticating, and is most developed in flesh-eating and insectivorous animals, in which the gums are liable to be injured by splinters of bone or other hard fragments of their food. The form of the crown is frequently rendered complex by the development upon its surface of elevations or tubercules called cusps or cones, or by ridges usually transverse, but sometimes variously curved or folded. When the crown is broad and the ridges are greatly developed, as in the molars of the Elephant, Horse, and Ox (Fig. 1, V.), the interspaces between them are filled with cement, which supports them and makes a solid compact mass of the whole tooth. When such a tooth wears away at the surface by friction against the opposed tooth of the other jaw, the different density of the layers of the substances of which it is composed—enamel, dentine, and cement—arranged in characteristic patterns, causes them to wear unequally, the hard enamel ridges projecting beyond the others, and thus giving rise to a grinding surface of great mechanical advantage.
_Succession._—The dentition of all mammals consists of a definite set of teeth, almost always of constant and determinate number, form, and situation, and, with few exceptions, persisting in a functional condition throughout the natural term of the animal’s life. In many species these are the only teeth which the animal ever possesses,—the set which is first formed being permanent, or, if accidentally lost, or decaying in extreme old age, not being replaced by others. These animals are called Monophyodont. But in the larger number of mammals, certain of the teeth are preceded by others, which may be only of a very transient, rudimentary, and functionless character (being in the Seals, for example, shed either before or within a few days after birth), or may be considerably developed, and functionally occupy the place of the permanent teeth for a somewhat lengthened period, during the growth and development of the latter and of the jaws. In all cases these teeth disappear (by the absorption of their roots and shedding of the crowns) before the frame of the animal has acquired complete maturity, as evidenced by the coalescence of the epiphyses of the osseous system. As these teeth are, as a general rule, present during the period in which the animal is nourished by the milk of the mother, the name of “milk-teeth” (French _dents de lait_, German _milchzähne_) has been commonly accorded to them, although it must be understood that the epoch of their presence is by no means necessarily synchronous with that of lactation. Animals possessing such teeth are called Diphyodont. No mammal is known to have more than two sets of teeth; and the definite and orderly replacement of certain members of the series is a process of quite a different nature from the indefinite succession which takes place in all the teeth continuously throughout the lifetime of the lower vertebrates.
When the milk-teeth are well developed, and continue in place during the greater part of the animal’s growth, as is especially the case with the Ungulata, and, though to a less degree, with the Primates and Carnivora, their use is obvious, since taken all together they form structurally a complete epitome on a small scale of the more numerous and larger permanent set (see Fig. 3), and, consequently, are able to perform the same functions, while time is allowed for the gradual maturation of the latter, and especially while the jaws of the growing animal are acquiring the size and strength sufficient to support the permanent teeth. Those animals, therefore, that have a well-developed and tolerably persistent set of milk-teeth may be considered to be in a higher state of development, as regards their dentition, than those that have the milk-teeth absent or rudimentary.
It is a very general rule that individual teeth of the milk and permanent set have a close relationship to one another, being originally formed, as mentioned above, in exceedingly near proximity, and with, at all events so far as the enamel-germ is concerned, a direct connection. Moreover, since the latter ultimately come to occupy the position in the alveolar border temporarily held by the former, they are spoken of respectively as the predecessors or successors of each other. But it must be understood that milk-teeth may be present which have no successors in the permanent series, and, what is far more general, permanent teeth may have no predecessors in the milk series.
The complete series of permanent teeth of most mammals forms a complex machine, with its several parts adapted for different functions,—the most obvious structural modification for this purpose being an increased complexity of the individual components of the series from the anterior towards the posterior extremity of such series. Since, as has just been said, the complete series of the milk teeth often presents structurally and functionally a similar machine, but composed of fewer individual members, and the anterior of which are as simple, and the posterior as complex as those occupying corresponding positions in the permanent series,—and since the milk-teeth are only developed in relation to the anterior or lateral, never to the most posterior of the permanent series,—it follows that the hinder milk-teeth are usually more complex than the teeth of which they are the predecessors in the permanent series, and represent functionally, not their immediate successors, but those more posterior permanent teeth which have no direct predecessors. This character is clearly seen in those animals in which the various members of the molar series are well differentiated from each other in form, as the Carnivora, and also in Man.
In animals which have two sets of teeth the number of those of the permanent series which are preceded by milk-teeth varies greatly, being sometimes, as in Marsupials and some Rodents, as few as one on each side of each jaw, and sometimes including the larger portion of the series.
Although there are difficulties in some cases in arriving at a satisfactory solution of the question, it is, on the whole, safest to assume that when only one set of teeth is present, this corresponds to the permanent teeth of the Diphyodonts. When this one set is completely developed, and remains in use throughout the animal’s life, there can be no question on this subject. When, on the other hand, the teeth are rudimentary and transient, as in the Whalebone Whales, it is possible to consider them as representing the milk series; but there are weighty reasons in favour of the opposite conclusion.[5]
_Arrangement, Homologies, and Notation of Teeth._—The teeth of the two sides of the jaws are always alike in number and character, except in cases of accidental or abnormal variation, and in the one remarkable instance of constant deviation from bilateral symmetry among mammals, the tusks of the Narwhal (_Monodon_), in which the left is of immense size, and the right rudimentary. In certain mammals, such as the Dolphins and some Armadillos, which have a very large series of similar teeth, not always constant in number in different individuals, there may be differences in the two sides; but, apart from these, in describing the dentition of any mammal, it is quite sufficient to give the number and characters of the teeth of one side only. Since the teeth of the upper and the lower jaws work against each other in masticating, there is a general correspondence or harmony between them, the projections of one series, when the mouth is closed, fitting into corresponding depressions of the other. There is also a general resemblance in the number, characters, and mode of succession of both series, so that, although individual teeth of the upper and lower jaws may not be in any strict sense of the term homologous parts, there is a great convenience in applying the same descriptive terms to the one as are used for the other.
[Illustration: FIG. 2.—Upper and Lower Teeth of one side of the Mouth of a Dolphin (_Lagenorhynchus_) as an example of the homodont type of dentition. The bone covering the outer side of the roots of the teeth has been removed to show their simple character.]
The simplest dentition as a whole is that of many species of Dolphin (Fig. 2), in which the crowns are single-pointed, slightly curved cones, and the roots also single and tapering, and all alike in form from the anterior to the posterior end of the series, though it may be with some slight difference in size, those at the two extremities of the series being rather smaller than the others. Such a dentition is called Homodont, and in the case cited, as the teeth are never changed, it is also Monophyodont. Such teeth are adapted only for catching slippery living prey, as fish.
In a very large number of mammals the teeth of different parts of the series are more or less differentiated in character, and have different functions to perform. The front teeth are simple and one-rooted, and are adapted for cutting and seizing. They are called “incisors.” The back- or cheek-teeth have broader and more complex crowns, tuberculated or ridged, and are supported on two or more roots. They crush or grind the food, and are hence called “molars.” Many animals have, between these two sets, a tooth at each corner of the mouth, longer and more pointed than the others, adapted for tearing or stabbing, or for fixing struggling prey. From the conspicuous development of such teeth in the Carnivora, especially the Dogs, they have received the name of “canines.” A dentition with its component parts so differently formed that these distinctive terms are applicable to them is called Heterodont. In most cases, though by no means invariably, animals with Heterodont dentition are also Diphyodont.
This general arrangement is extremely obvious in a considerable number of mammals; and closer examination shows that, under very great modification in detail, there is a remarkable uniformity of essential characters in the dentition of a large number of members of the class belonging to different orders and not otherwise closely allied; so much so indeed that it has been possible (chiefly through the researches of Sir Richard Owen) to formulate a common plan of dentition from which the others have been derived by the alteration of some and suppression of other members of the series, and occasionally, but very rarely, by addition. The records of palæontology fully confirm this view, as by tracing back many groups now widely separated in dental characters we find a gradual approximation to a common type. In this generalised form of mammalian dentition (which is best exemplified in the genera _Anoplotherium_ and _Homalodontotherium_) the entire number of teeth present is 44, or 11 above and 11 below on each side. Those of each jaw are placed in continuous series without intervals between them; and, although the anterior teeth are simple and single-rooted, and the posterior teeth complex and with several roots, the transition between the two kinds is gradual.
In dividing and grouping such teeth for the purpose of description and comparison, more definite characters are required than those derived merely from form or function. The first step towards a classification has been made by the observation that the upper jaw is composed of two bones, the premaxilla and the maxilla, and that the suture between these bones separates the three anterior teeth from the others. These three teeth, then, which are implanted by their roots in the premaxilla, form a distinct group, to which the name of “incisor” is applied. This distinction is, however, not so important as it appears at first sight, for, as mentioned when speaking of the development of the teeth, their connection with the bone is only of a secondary nature, and, although it happens conveniently for our purpose that in the great majority of cases the segmentation of the bone coincides with the interspace between the third and fourth tooth of the series, still, when it does not happen to do so, as in the case of the Mole, we must not give too much weight to this fact, if it contravenes other reasons for determining the homologies of the teeth. The eight remaining teeth of the upper jaw offer a natural division, inasmuch as the posterior three never have milk-predecessors; and, although some of the anterior teeth may be in the same case, the particular one preceding these three always has such a predecessor. These three then are grouped apart as the “molars,” or, since some of the teeth in front of them often have a molariform character, “true molars.” Of the five teeth between the incisors and molars the most anterior, or that which is usually situated close behind the premaxillary suture, almost always, as soon as any departure takes place from the simplest and most homogeneous type, assumes a lengthened and pointed form, and is the tooth so developed as to constitute the “canine” or “laniary” tooth of the Carnivora, the tusk of the Boar, etc. It is customary therefore to call this tooth, whatever its size or form, the “canine.” The remaining four are the “premolars” or “false molars.” This system of nomenclature has been objected to as being artificial, and in many cases not descriptive, the distinction between premolars and canine especially being sometimes not obvious; but the terms are now in such general use, and are so practically convenient—especially if, as it is best to do in all such cases, we forget their original signification and treat them as arbitrary signs—that it is not likely they will be superseded by any that have been proposed as substitutes for them.
With regard to the lower teeth the difficulties are greater, owing to the absence of any suture corresponding to that which defines the incisors above; but since the number of the teeth is the same, the corresponding teeth are preceded by milk-teeth, and in the large majority of cases it is the fourth tooth of the series which is modified in the same way as the canine (or fourth tooth) of the upper jaw, it is quite reasonable to adopt the same divisions as with the upper series, and to call the first three, which are implanted in the part of the mandible opposite to the premaxilla, the incisors, the next the canine, the next four the premolars, and the last three the molars. It may be observed that when the mouth is closed, especially when the opposed surfaces of the teeth present an irregular outline, the corresponding upper and lower teeth are not exactly opposite, otherwise the two series could not fit into one another; but as a rule the points of the lower teeth shut into the interspaces in front of the corresponding teeth of the upper jaw. This is seen very distinctly in the canine teeth of the Carnivora, and is a useful guide in determining the homologies of the teeth of the two jaws. Objections have certainly been made to this view, because, in certain rare cases, the tooth which, according to it, would be called the lower canine has the form and function of an incisor (as in Ruminants and Lemurs), and on the other hand (as in _Cotylops_, an extinct Ungulate from North America) the tooth that would thus be determined as the first premolar has the form of a canine; but it should not be forgotten that, as in all such cases, definitions derived from form and function alone are quite as open to objection as those derived from position and relation to surrounding parts, or still more so.
_Dental formulæ._—For the sake of brevity the complete dentition, arranged according to these principles, is often described by the following formula, the numbers above the line representing the teeth of the upper, those below the line those of the lower jaw:—incisors ³⁻³⁄₃₋₃, canines ¹⁻¹⁄₁₋₁, premolars ⁴⁻⁴⁄₄₋₄, molars ³⁻³⁄₃₋₃ = ¹¹⁻¹¹⁄₁₁₋₁₁; total 44. Since, however, initial letters may be substituted for the names of each group, and it is quite unnecessary to give more than the numbers of the teeth on one side of the mouth, the formula may be conveniently abbreviated into—
_i_ ³⁄₃, _c_ ¹⁄₁, _p_ ⁴⁄₄, _m_ ³⁄₃ = ¹¹⁄₁₁; total 44.
The individual teeth of each group are always enumerated from before backwards, and by such a formula as the following—
_i_ 1, _i_ 2, _i_ 3, _c_, _p_ 1, _p_ 2, _p_ 3, _p_ 4, _m_ 1, _m_ 2, _m_ 3 ------------------------------------------------------------------------- _i_ 1, _i_ 2, _i_ 3, _c_, _p_ 1, _p_ 2, _p_ 3, _p_ 4, _m_ 1, _m_ 2, _m_ 3
or more briefly—
1,2,3 1 1,2,3,4 1,2,3 _i_ ------ _c_ -- _p_ -------- _m_ -----. 1,2,3, 1, 1,2,3,4, 1,2,3
A special numerical designation is thus given by which each one can be indicated. In mentioning any single tooth, such a sign as _m¹_⁄ will mean the first upper molar, ⁄_m₁_ the first lower molar, and so on. The use of such signs saves much time and space in description.[6]
It was part of the view of the founder of this system of dental notation that, at least throughout the group of mammals whose dentition is derived from this general type, each tooth has its strict homologue in all species, and that in those cases in which fewer than the typical number are present (as in all existing mammals except the genera _Sus_, _Gymnura_, _Talpa_, and _Myogale_), the teeth that are missing can be accurately defined. According to this view, when the number of incisors falls short of three it is assumed that the absent ones are missing from the outer and posterior end of the series. Thus, when there is but one incisor present, it is _i_ 1; when two, they are _i_ 1 and _i_ 2. Furthermore, when the premolars and the molars are below their typical number, the absent teeth are missing from the fore part of the premolar series, and from the back part of the molar series. If this were invariably so, the labours of those who describe teeth would be greatly simplified; but there are so many exceptions that a close scrutiny into the situation, relations, and development of a tooth is required before its nature can be determined, and in some cases the evidence at our disposal is scarcely sufficient for the purpose. In other instances, however, as among the Polyprotodont Marsupials, we have decisive evidence to show that the missing premolar teeth are not those at the extremity of the series.
[Illustration: FIG. 3.—Milk and Permanent Dentition of Upper (I.) and Lower (II.) Jaw of the Dog (_Canis familiaris_), with the symbols by which the different teeth are commonly designated. The third upper molar (_m._3) is the only tooth wanting in this animal to complete the typical heterodont mammalian dentition.]
The milk-dentition is expressed by a similar formula, _d_ for deciduous or _m_ for milk being commonly prefixed to the letter expressive of the nature of the tooth. Since the three molars, and almost invariably the first premolar of the permanent series, have no predecessors, the typical milk-dentition would be expressed as follows—_di_ ³⁄₃, _dc_ ¹⁄₁, _dm_ ³⁄₃, = ⁷⁄₇, total 28. In a few Ungulates, however, such as the Hyrax and Tapir, and in some instances the Rhinoceros and the extinct _Palæotherium_, the whole of the four premolars are preceded by milk-teeth; when we have the fullest development of cheek-teeth in the whole of the Eutheria. The teeth which precede the premolars of the permanent series are all called molars in the milk-dentition, although as a general rule, in form and function they represent in a condensed form the whole premolar and molar series of the adult. When there is a marked difference between the premolars and molars of the permanent dentition, the first milk-molar resembles a premolar, while the last has the characters of the posterior true molar.
The dentition of all the members of the orders Primates, Carnivora, Insectivora, Chiroptera, and Ungulata can clearly be derived from the above-described generalised type. The same may be said of the Rodents, and even the Proboscidea, though at least in the existing members of the order with greater modification. It is also apparent in certain extinct Cetacea, as _Zeuglodon_ and _Squalodon_, but it is difficult to find any traces of it in existing Cetacea, Sirenia, or any of the so-called Edentata. All the Marsupials, different as they are in their general structure and mode of life, and variously modified as is their dentition, present in this system of organs some deep-lying common characters which show their unity of origin. The generalised type to which their dentition can be reduced presents considerable resemblance to that of the placental mammals, yet differing in details. It is markedly heterodont, and susceptible of division into incisors, canines, premolars, and molars upon the same principles. The whole number is, however, not limited to forty-four. The incisors may be as numerous as five on each side above, and they are almost always different in number in the upper and the lower jaw. The premolars and molars are commonly seven, as in the placental mammals, but their arrangement is reversed, as there are four true molars and three premolars.
The larger number of incisive and molar teeth among the Marsupials suggests that their additional teeth have disappeared in the Eutheria,[7] and Mr. O. Thomas has endeavoured to construct a generalised dental formula from which both the Marsupial and Eutherian modifications may have been derived by the suppression of particular teeth. Thus the hypothetical formula _i_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, _c_ ¹⁄₁, _p_ ¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, by the loss of the fifth lower incisor, and of the second premolars (which we know to be those which disappear in the Marsupials) and the fifth molars, will give _i_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₀, _c_ ¹⁄₁, _p_ ¹,⁰,³,⁴⁄₁,₀,₃,₄, _m_ ¹,²,³,⁴⁄₁,₂,₃,₄; or the formula of the Opossum (_Didelphys_), usually written _i_ ⁵⁄₄, _c_ ¹⁄₁, _p_ ³⁄₃, _m_ ⁴⁄₄. Again, in the same formula the loss of the fourth and fifth incisors in both jaws, and also of the fourth molars, gives us _i_ ¹,²,³,⁰,⁰⁄₁,₂,₃,₀,₀, _c_ ¹⁄₁, _p_ ¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³⁄₁,₂,₃, or the formula of a typical Eutherian, like the Pig, which we generally write as _i_ ³⁄₃, _c_ ¹⁄₁, _p_ ⁴⁄₄, _m_ ³⁄₃. Such a generalised formula will admit of modification into that of all existing, and a large number of fossil Marsupials, but it is possible that some of the Mesozoic types may have had more than four premolars, although there is no absolutely decisive evidence that such was the case. The presence of seven or eight true molars in some Mesozoic forms merely entails the addition of two or three additional figures to the ideal generalised formula.
The milk-dentition of all known Marsupials, existing or extinct, is (if not entirely absent) limited to a single tooth on either side of each jaw, this being the predecessor of the last permanent premolar. And if the view that the milk-dentition is an additional series grafted upon the original permanent series be correct, it is evident that we have in this single replacement the first stage of this additional development.
In very few mammals are teeth entirely absent. Even in the Whalebone Whales their germs are formed in the same manner and at the same period of life as in other mammals, and even become partially calcified, but they never rise above the gums, and completely disappear before the birth of the animal. In some species of the order Edentata, the true Anteaters and the Pangolins, no traces of teeth have been found at any age. The adult Monotremata are likewise devoid of teeth of the same structure as those of ordinary mammals; but well-developed molars occur in the young _Ornithorhynchus_, although no traces of teeth have hitherto been detected in _Echidna_.
_Modifications of the Teeth in Relation to their Functions._—The principal functional modifications noticed in the dentition of mammalia may be roughly grouped as piscivorous, carnivorous, insectivorous, omnivorous, and herbivorous, each having, of course, numerous variations and transitional conditions.
The essential characters of a piscivorous dentition are best exemplified in the Dolphins, and also (as modifications of the carnivorous type) in the Seals. This type consists of an elongated, rather narrow mouth, wide gape, with numerous subequal, conical, sharp-pointed, recurved teeth, adapted simply to rapidly seize, but not to divide or masticate, active, slippery, but not powerful prey. All animals which feed on fish as a rule swallow and digest them entire, a process which the structure of prey of this nature, especially the intimate interblending of delicate, sharp-pointed bones with the muscles, renders very advantageous, and for which the above-described type of dentition is best adapted.
The carnivorous type of dentition is shown in its most specialised development among existing mammals in the _Felidæ_. The function being here to seize and kill struggling animals, often of large size and great muscular power, the canines are immensely developed, trenchant, and piercing, and are situated wide apart, so as to give the firmest hold when fixed in the victim’s body. The jaws are as short as is consistent with the free action of the canines, so that no power may be lost. The incisors are very small, so as not to interfere with the penetrating action of the canines, and the crowns of the molar series are reduced to scissor-like blades, with which to pare off the soft tissues from the large bones, or to divide into small pieces the less dense portions of the bones for the sake of nutriment afforded by the blood and marrow they contain. The gradual modification between this and the two following types will be noticed in their appropriate places.
In the most typical insectivorous animals, as the Hedgehogs and Shrews, the central incisors are elongated, pointed, and project forwards, those of the upper and lower jaw meeting like the blades of a pair of forceps, so as readily to secure small active prey, quick to elude capture, but powerless to resist when once seized. The crowns of the molars are covered with numerous sharp edges and points, which, working against each other, rapidly cut up the hard-cased insects into little pieces fit for swallowing and digestion.
The omnivorous type, especially that adapted for the consumption of soft vegetable substances, such as fruits of various kinds, may be exemplified in the dentition of Man, of most Monkeys, and of the less modified Pigs. The incisors are moderate, subequal, and cutting. If the canines are enlarged, it is usually for other purposes than those connected with food, and only in the male sex. The molars have their crowns broad, flattened, and elevated into rounded tubercles. The name _Bunodont_, or hillock-toothed, has been proposed for molars of this type, and will frequently be found convenient.
In the most typically herbivorous forms of dentition, as seen in the Horse and Kangaroo, the incisors are well developed, trenchant, and adapted for cutting off the herbage on which the animals feed; the canines are rudimentary or suppressed; the molars are large, with broad crowns, which in the simplest forms have strong transverse ridges, but may become variously complicated in the higher degrees of modification which this type of tooth assumes.
Various forms of teeth of this type will be noticed among the Ungulates and Rodents.
The natural groups of mammals, or those which in our present state of knowledge we have reason to believe are truly related to each other, may each contain examples of more than one of these modifications. Thus the Primates have both omnivorous and insectivorous forms. The Carnivora show piscivorous, carnivorous, insectivorous, and omnivorous modifications of their common type of dentition. The Ungulata and the Rodentia have among them the omnivorous and various modifications, both simple and complex, of the herbivorous type. The Marsupialia exhibit examples of all forms, except the purely piscivorous. Other orders, more restricted in number or in habits, as the Proboscidea and Cetacea, naturally do not show so great a variety in the dental structure of their members.
_Taxonomy._—In considering the taxonomic value to be assigned to the modifications of teeth of mammals, two principles, often opposed to each other, which have been at work in producing these modifications, must be held in view:—(1) the type, or ancestral form, as we generally now call it, characteristic of each group, which in most mammals is itself derived from the still more generalised type described above; and (2) variations which have taken place from this type, generally in accordance with special functions which the teeth are called upon to fulfil in particular cases. These variations are sometimes so great as completely to mask the primitive type, and in this way the dentition of many animals of widely different origin has come to present a remarkable superficial resemblance, as in the case of the Wombat (a Marsupial), the Aye-Aye (a Lemur), and the Rodents, or as in the case of the Thylacine and the Dog. In all these examples indications may generally be found of the true nature of the case by examining the earlier conditions of dentition; for the characters of the milk-teeth or the presence of rudimentary or deciduous members of the permanent set will generally indicate the route by which the specialised dentition of the adult has been derived. It is perhaps owing to the importance of the dental armature to the well-being of the animal in procuring its sustenance, and preserving its life from the attacks of enemies, that great changes appear to have taken place so readily, and with such comparative rapidity, in the forms of these organs—changes often accompanied with but little modification in the general structure of the animal. Of this proposition the Aye-Aye (_Chiromys_) among Lemurs, the Walrus among Seals, and the Narwhal among Dolphins form striking examples; since in all these forms the superficial characters of their dentition would entirely separate them from the animals with which all other evidence (even including the mode of development of their teeth) proves their close affinity.
[Illustration: FIG. 4.—Molar teeth of Mesozoic Mammals (enlarged). Triconodont type—1, _Dromatherium_; 2, _Microconodon_; 3, _Amphilestes_; 4, _Phascolotherium_; 5, _Triconodon_. Tritubercular type—6, 7, _Spalacotherium_; 10, _Asthenodon_. Tubercular sectorial type—8, _Amphitherium_; 9, _Peramus_; 11-13, _Amblotherium_; 14 (?) _Amblotherium_. _pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_, metacone, in the upper teeth; and protoconid, hypoconid, paraconid, and metaconid in the lower. 6 and 15 are upper molars, and the rest lower molars. (After Osborn.)]
_Trituberculism._—Recent researches, and more especially those of Professors Cope and Osborn, tend to show that almost all of the extremely different forms of tooth-structure found among Mammals may be traced to one common type, in which the crown of each tooth carried three cusps, and hence termed the _tritubercular_ type; these three cusps being arranged in a triangle, with the apex directed inwardly in the upper teeth (Fig. 4, ₆), and outwardly in the lower ones (Fig. 4, ₇). It is further probable that this tritubercular type was itself derived from a type of dentition in which the teeth were in the form of almost a quite simple cone; such a presumably primitive type of dentition—being apparently retained among some existing Edentates, like the Armadillos, while it is possible that we should regard the dentition of the existing Cetacea (Fig. 2) as a reversion to the same primitive type. None of the Mesozoic mammals at present known exhibit this simple conical type of teeth, although we have an approximation to it in the extremely generalised genus _Dromatherium_. Starting then from this presumed simple cone it appears that the teeth of _Dromatherium_ (Fig. 4, ₁) present the first stage towards trituberculism, the crown of each tooth having one main cone, with minute lateral cusps, and the root being grooved. In the next or true Triconodont stage (Fig. 4, ₃₋₅) the crown has become elongated antero-posteriorly, and consists of one central and two lateral cones or cusps, while the root is divided. From this the transition is easy to the tritubercular type, in which the three cusps, instead of being placed in a line, are arranged in a triangle; the upper teeth (Fig. 4, ₆) having one inner and two outer cusps, while the reverse condition obtains in those of the lower jaw (Fig. 4, ₇). These three cusps of the simple tritubercular tooth are collectively designated as the primitive triangle; in the upper tooth the inner cusp is termed the protocone, the antero-external one the paracone, and the postero-external the metacone; the corresponding cusps of the lower tooth being named protoconid, paraconid, and metaconid—the protoconid being here on the outer side of the crown.
It is thus apparent that in the first, or haplodont type, as well as in the triconodont type, the upper and lower molars are alike; while in the simple tritubercular type they have a similar pattern, but with the arrangement of the cusps reversed. This simple tritubercular type occurs in the Mesozoic genus _Spalacotherium_ (Fig. 4, ₆ and ₇), and apparently in the existing _Chrysochloris_; but in the majority of tritubercular forms, while this primitive triangle forms the main portion of the crown, other secondary cusps are added, the homologies of which in the upper and lower teeth are somewhat doubtful. At the same time that we have the addition of these secondary cusps we also find trituberculism differentiating into a secodont and a bunodont series, according as to whether the dentition becomes of a cutting or a crushing type.
Thus in the lower molars (Fig. 4, ₈ and ₉) we very frequently find the three cusps of the primitive triangle elevated and connected by cross crests, while there is an additional low posterior heel or talon, which may be termed the hypoconid. This tubercular-sectorial sub-type, as it is termed, is found in the lower molars of many Polyprotodont Marsupials and Insectivores, and it also occurs in the lower carnassial teeth of the true Carnivora. The presence of two cusps (inner and outer) to the talon converts this modification into a quinquetubercular form; while, by the suppression of one of the three primitive cusps, it develops into the quadritubercular type of the bunodont series.
[Illustration: FIG. 5.—Diagram of two upper and two lower left quadritubercular molars in mutual apposition. The cusps and ridges of the upper molars in double lines, and those of the lower in black lines. The lower molars are looked at from below, as if transparent. _pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_, metacone; _ml_, protoconule; _pl_, metaconule; _prd_, protoconid; _hyd_, hypoconid; _pad_, paraconid; _med_, metaconid; _end_, entoconid. (After Osborn.)]
In the upper molars the primitive triangle in the secodont series may remain purely tricuspid; but the addition of intermediate cusps, both in the secodont and bunodont series, may give rise to a quinquetubercular type; these intermediate cusps being respectively designated as the protoconule and metaconule (Fig. 5, _ml_, _pl_). Finally, in the bunodont series, the addition of a postero-internal cusp (Fig. 5, _hy_), termed the hypocone, forms the sextubercular molar.
The following table exhibits, in a collective form, the names and relations of all the above-mentioned cusps, and the letters by which they are indicated in the figures:—
UPPER MOLARS.
Antero-internal cusp = protocone = _pr_. Postero ” or 6th cusp = hypocone = _hy_. Antero-external cusp = paracone = _pa_. Postero ” ” = metacone = _me_. Anterior intermediate cusp = protoconule = _ml_. Posterior ” ” = metaconule = _pl_.
LOWER MOLARS.
Antero-external cusp = protoconid = _prd_. Postero ” ” = hypoconid = _hyd_. Antero-internal or 5th cusp = paraconid = _pad_. Intermediate (or in quadritubercular molars antero-internal) cusp = metaconid = _med_. Postero-internal cusp = entaconid = _end_.
The common occurrence of trituberculism in the mammals of the earlier geological epochs is, as remarked by Osborn, very significant of the uniformity of molar origin. Thus, among the Mesozoic mammals (with the exception of the group known as Multituberculata, in which the molars are constructed on a different type), trituberculism occurs in the great majority of the genera; while out of 82 species, belonging to five different suborders from the Lowest or Puerco Eocene of the United States, all but four exhibit this feature; and the same holds good for the mammals of the corresponding European horizon. At the present day trituberculism persists in the Lemuroidea, Insectivora, Carnivora, and Marsupialia. In the Carnivora there is a tendency to lose the metaconid, while in the bunodont molars of the Ungulata it is the paraconid that disappears.
III. THE SKELETON.
_Definition._—The skeleton is a system of hard parts, forming a framework which supports and protects the softer organs and tissues of the body. It consists of dense fibrous and cartilaginous tissues, portions of which remain through life in this state, but the greater part is transformed during the growth of the animal into bone or osseous tissue. This is characterised by a peculiar histological structure and chemical composition, being formed mainly of a gelatinous basis, strongly impregnated with salts of calcium, chiefly phosphate, and disposed in a definite manner, containing numerous minute nucleated spaces or cavities called lacunæ, connected together by delicate channels or canaliculi, which radiate in all directions from the sides of the lacunæ. Parts composed of bone are, next to the teeth, the most imperishable of all the organs of the body, often retaining their exact form and internal structure for ages after every trace of all other portions of the organisation has completely disappeared, and thus, in the case of extinct animals, affording the only means of attaining a knowledge of their characters and affinities.[8]
In the Armadillos and their extinct allies alone is there an ossified exoskeleton, or bony covering developed in the skin. In all other mammals the skeleton is completely internal. It may be described as consisting of an axial portion belonging to the head and trunk, and an appendicular portion belonging to the limbs. There are also certain bones called splanchnic, being developed within the substance of some of the viscera. Such are the _os cordis_ and _os penis_ found in some mammals.
It is characteristic of all the larger bones of the mammalia that their ossification takes its origin from several distinct centres. One near the middle of the bone, and spreading throughout its greater portion, constitutes the _diaphysis_, or “shaft,” in the case of the long bones. Others near the extremities, or in projecting parts, form the _epiphyses_, which remain distinct during growth, but ultimately coalesce with the rest of the bone.
_Axial skeleton._—The axial skeleton consists of the skull, the vertebral column (prolonged at the posterior extremity into the tail), the sternum, and the ribs.
_Skull._—In the _skull_ of adult mammals, all the bones, except the lower jaw, the auditory ossicles, and the bones of the hyoid arch, are immovably articulated together, their edges being in close contact, and often interlocking by means of fine denticulations projecting from one bone and fitting into corresponding depressions of the other; they are also held together by the investing periosteum, or fibrous membrane, which passes directly from one to the other, and permits no motion, beyond perhaps a slight yielding to external pressure. In old animals there is a great tendency for the different bones to become actually united by the extension of ossification from one to the other, with consequent obliteration of the sutures. The cranium, thus formed of numerous originally independent ossifications, which may retain throughout life more or less of their individuality, or be all fused together, according to the species, the age, or even individual peculiarity, consists of a brain-case, or bony capsule for enclosing and protecting the brain, and a face for the support of the organs of sight, smell, and taste, and of those concerned in seizing and masticating the food. The brain-case articulates directly with the anterior cervical vertebra, by means of a pair of oval eminences, called condyles, placed on each side of the large median foramen which transmits the spinal cord. It consists of a basal axis, continuous serially with the axes or centra of the vertebræ, and of an arch above, roofing over and enclosing the cavity which contains the cephalic portion of the central nervous system (see Fig. 6). The base with its arch is composed of three segments placed one before the other, each of which is comparable to a vertebra with a greatly expanded neural arch. The hinder or occipital segment consists of the basioccipital, exoccipital, and supraoccipital bones; the middle segment of the basisphenoid, alisphenoid, and parietal bones; and the anterior segment of the presphenoid, orbitosphenoid, and frontal bones. The axis is continued forwards into the mesethmoid, or septum of the nose, around which the bones of the face are arranged in a manner so extremely modified for their special purposes that anatomists who have attempted to trace their serial homologies with the more simple portions of the axial skeleton have arrived at very diverse interpretations. The characteristic form and structure of the face of mammals is mainly dependent upon the size and shape of (1) the orbits, a pair of cup-shaped cavities for containing the eyeball and its muscles, which may be directed forwards or laterally, placed near together or wide apart, and may be completely or only partially encircled by bone; (2) the nasal fossæ, or cavities on each side of the median nasal septum, forming the passage for the air to pass between the external and the internal nares, and containing in their upper part the organ of smell; (3) the zygomatic arch, a bridge of bone for the purpose of muscular attachment, which extends from the side of the face to the skull, overarching the temporal fossa; (4) the roof of the mouth, with its alveolar margin for the implantation of the upper teeth. The face is completed by the mandible, or lower jaw, consisting of two lateral rami, articulated by a hinge joint with the squamosal (a cranial bone interposed between the posterior and penultimate segment of the brain-case, where also the bony capsule of the organ of hearing is placed), each being composed of a single solid piece of bone, and the two united together in the middle line in front, at the symphysis,—which union may be permanently ligamentous or become completely ossified. Into the upper border of the mandibular rami the lower teeth are implanted.
[Illustration: FIG. 6.—Longitudinal and vertical section of the skull of a Dog (_Canis familiaris_), with mandible and hyoid arch. _an_, Anterior narial aperture; _MT_, maxillo-turbinal bone; _ET_, ethmo-turbinal; _Na_, nasal; _ME_, ossified portion of the mesethmoid; _CE_, cribriform plate of the ethmo-turbinal; _Fr_, frontal; _Pa_, parietal; _IP_, interparietal; _SO_, supraoccipital; _ExO_, exoccipital; _BO_, basioccipital; _Per_, periotic; _BS_, basisphenoid; _Pt_, pterygoid; _AS_, alisphenoid; _OS_, orbitosphenoid; _PS_, presphenoid; _PI_, palatine; _VO_, vomer; _Mx_, maxilla; _PMx_, premaxilla; _sh_, stylohyal; _eh_, epihyal; _ch_, ceratohyal; _bh_, basihyal; _th_, thyrohyal; _s_, symphysis of mandible; _cp_, coronoid process; _cd_, condyle; _a_, angle; _id_, inferior dental canal. The mandible is displaced downwards, to show its entire form; the * indicates the part of the cranium to which the condyle is articulated.[9]]
In addition to the bones already mentioned as entering into the formation of the cranium, there are many others, the most important of which may be briefly noticed. The anterior extremity of the skull is formed by the premaxillæ (Figs. 6, 7, _PMx_), which carry the incisors; behind them are the maxillæ, in which all the remaining upper teeth are implanted. Both the premaxillæ and maxillæ meet in a median suture on the palate, where they form a floor to the nasal passage; this floor being continued backwards by the plate-like palatines, at the hinder extremity of which the posterior nares are usually situated. In a few instances, however, as in certain Edentates and Cetaceans, the small pair of bones forming the posterior continuation of the lateral borders of the palatines, and known as the pterygoids (Fig. 6, _Pt_), likewise meet in the middle line below the nasal passage, and thus cause the aperture of the posterior nares to be situated near the occiput. On the upper, or frontal aspect of the cranium the paired nasals roof over the nasal passage and fill the interval left between the premaxilla and maxilla of either side. Behind the nasals and maxillæ, the anterior part of the brain-case is formed by the large paired frontals (Figs. 6, 7, _Fr_), behind which are the parietals, which may be of still larger size, and form the greater part of the brain-case. A median interparietal ossification (Fig. 6, _IP_) may divide the parietals posteriorly, and is itself articulated with the supraoccipital, to the lateral borders of which the parietals are also joined. The squamosal (Fig. 7, _Sq_) forms the lateral wall of the hinder part of the brain-case, and articulates superiorly with the parietal, and posteriorly with the exoccipital. The glenoid cavity (Fig. 8), for the reception of the articular condyle of the mandible, is formed by the inferior portion of the squamosal, at the point where it gives off the zygomatic process to form the hinder portion of the zygomatic arch. The middle portion of that arch is formed by the jugal, or malar bone (Fig. 7, _Ma_), which articulates posteriorly with the zygomatic process of the squamosal, and anteriorly with the maxilla. The jugal (as in Fig. 7) may also articulate with a small bone situated on the anterior border of the orbit known as the lachrymal. It is important to observe that the zygomatic or temporal arch is a squamoso-maxillary one, and that an arcade thus composed is found elsewhere only among the extinct Anomodont reptiles, which have already been mentioned as showing signs of mammalian affinity. The relative position occupied by the orbito- and alisphenoid is sufficiently indicated in Fig. 7.
[Illustration: FIG. 7.—Side view of skull of Cape Jumping Hare (_Pedetes caffer_). × ⅗ _PMx_, Premaxilla; _Mx_, maxilla, _Ma_, jugal or malar; _Fr_, frontal; _L_, lachrymal; _Pa_, parietal; _Na_, nasal; _Sq_, squamosal; _Ty_, tympanic; _ExO_, exoccipital; _AS_, alisphenoid; _OS_, orbitosphenoid; _Per_, mastoid bulla.]
Wedged in between the squamosal and the bones of the occipital and basisphenoidal region are the bones connected with the organ of hearing, known as the periotic and tympanic. The position of the periotic, which encloses the labyrinth or essential organ of hearing, is shown in Fig. 6. The periotic is divided into a very dense antero-internal moiety known as the petrosal, and a postero-external or mastoid portion (Fig. 8), which appears on the outer wall of the brain-case. The tympanic is produced horizontally outwards to form the external auditory meatus or tube of the ear, while the inner and under surface is frequently dilated into a shell-like auditory bulla (Fig. 8). The small bones of the internal ear known as the malleus, incus, and stapes are contained in the membranous _tympanic cavity_, which is situated in a space left among this group of bones. Further mention of these bones is made below under the head of the sense organs.
In the Carnivora and some other groups the foramina on the base of the skull for the passage of blood-vessels and nerves are of considerable taxonomic importance. The position of the more important of these foramina is indicated in Fig. 8; but for details the reader may refer to the work on the _Osteology of the Mammalia_ already mentioned. Attention may, however, be particularly directed to the so-called alisphenoid canal, the position of which is shown in Fig. 8, since this is a feature of some importance in the classification of the Carnivora. This canal is a short channel running horizontally forward from near the foramen ovale through the alisphenoid, and opening anteriorly with the foramen rotundum; it is traversed by the external carotid artery.
[Illustration: FIG. 8.—The right half of the hinder part of the base of the cranium of the Wolf (_Canis lupus_). _c_, Condyloid foramen; _l_, foramen lacerum posticum; _car_, carotid canal; _e_, eustachian canal; _o_, foramen ovale; _a_, posterior, and _a′_, anterior aperture of alisphenoid canal; _P_, paroccipital process of exoccipital; _m_, mastoid process of periotic; _am_, external auditory meatus; _g_, glenoid foramen, below which is the glenoid cavity for the condyle of the mandible. (Flower, _Proc. Zool. Soc._, 1869, p. 25.)]
Only in those species, as Man and the smaller kinds of the Primates and some other orders, in which the brain holds a large relative proportion to the rest of the body, does the external form of the skull receive much impress from the real shape of the cavity containing the brain. The size and form of the mouth, and the modifications of the jaws for the support of teeth of various shape and number, the ridges and crests on the cranium for the attachment of the muscles necessary to put this apparatus in motion, and outgrowths of bone for the enlargement of the external surface required for the support of sense organs or of weapons, such as horns or antlers (which outgrowths, to prevent undue increase of weight, are filled with cells containing air), cause the principal variations in the general configuration of the skull. These variations are, however, only characteristically developed in perfectly adult animals, and are in many cases more strongly marked in the male than the female sex. Throughout all the later stages of growth up to maturity the size and form of the brain-case remain comparatively stationary, while the accessory parts of the skull rapidly increase and assume their distinctive development characteristic of the species.
The hyoidean apparatus in mammals (Fig. 6) supports the tongue and larynx, and consists of an inferior median portion termed the basihyal, from which two pairs of half arches, or cornua, extend upwards and outwards. The anterior is the more important, being connected with the periotic bone of the cranium. It may be almost entirely ligamentous, but more often has several ossifications, the largest of which is usually the stylohyal. The posterior cornu (thyrohyal) is united at its extremity with the thyroid cartilage of the larynx, which it suspends in position. The median portion, or basihyal, is sometimes, as in the Howling Monkeys, enormously enlarged and hollowed, admitting into its cavity an air-sac connected with the organ of voice.
[Illustration: FIG. 9.—Anterior surface of Human thoracic vertebra (fourth). _c_, Body or centrum; _nc_, neural canal; _p_, pedicle, and _l_, lamina of the arch; _t_, transverse process; _az_, anterior zygapophysis.]
_Vertebral Column._—The _vertebral column_ consists of a series of distinct bones called vertebræ, arranged in close connection with each other along the dorsal side of the neck and trunk, and in the median line.[10] It is generally prolonged posteriorly beyond the trunk, to form the axial support of the appendage called the tail. Anteriorly it is articulated with the occipital region of the skull. The number of distinct bones composing the vertebral column varies greatly among the Mammalia, the main variation being due to the degree of elongation of the tail. Apart from this, in most mammals the number is not far from thirty, though it may fall as low as twenty-six (as in some Bats), or rise as high as forty (_Hyrax_ and _Cholœpus_). The different vertebræ, with some exceptions, remain through life quite distinct from each other, though closely connected by means of fibrous structures which allow of a certain, but limited, amount of motion between them. The exceptions are the following:—(1) near the posterior part of the trunk, in nearly all mammals which possess completely developed hinder limbs, two or more vertebræ become ankylosed together to form the “sacrum,” or portion of the vertebral column to which the pelvic girdle is attached; (2) in some species of Whales and Armadillos there are constant ossific unions of certain vertebræ of the cervical region.
[Illustration: FIG. 10.—Side view of the first lumbar vertebra of a Dog (_Canis familiaris_). _s_, Spinous process; _az_, anterior zygapophysis; _pz_, posterior zygapophysis; _m_, metapophysis; _a_, anapophysis; _t_, transverse process.]
Although the vertebræ of different regions of the column of the same animal or of different animals present great diversities of form, yet there is a certain general resemblance among them, or a common plan on which they are constructed, which is more or less modified by alteration of form or proportions, or by the addition or suppression of parts to fit them to fulfil their special purpose in the economy. An ordinary or typical vertebra consists, in the first place, of a solid piece of bone, termed the body or centrum (Fig. 9, _c_), of the form of a disk or short cylinder. The bodies of contiguous vertebræ are connected together by a very dense, tough, and elastic material called the “intervertebral substance,” of peculiar and complex arrangement. This substance forms the main, and in some cases the only, union between the vertebræ. Its elasticity provides for the vertebræ always returning to their normal relation to each other and to the column generally, when they have been disturbed therefrom by muscular action. A process (_p_) arises on each side from the dorsal surface of the body. These processes, meeting in the middle line above, form an arch, surmounting a space or short canal (_nc_). Since it contains the posterior prolongation of the great cerebro-spinal nervous axis, or spinal cord, this space is called the neural canal, and the arch the neural arch, in contradistinction to another arch on the ventral surface of the body of the vertebræ, called the hæmal arch. The latter is, however, never formed in mammals by any part of the vertebra itself, but by certain distinct bones placed more or less in apposition to it, namely the ribs in the thoracic, and the “chevron bones” in the caudal region. In most cases the arch of one vertebra is articulated with that of the next by distinct surfaces with synovial joints, placed one on each side, called “zygapophyses” (_az_, _pz_), but these are often entirely wanting when flexibility is more needed than strength, as in the greater part of the caudal region of long-tailed animals. In addition to the body and the arch, there are certain projecting parts called processes, chiefly serving for the attachment of the numerous muscles which move the vertebral column. Of these two are single and median, viz. the spinous process, neural spine, or neurapophysis (_s_), arising from the middle of the upper part of the arch, and the hypapophysis from the under surface of the body. The latter, however, is as frequently absent as the former is constant. The other processes are paired and lateral. They are the transverse processes (_t_), of which there may be two, an upper and a lower, in which case the former is called, in the language of Owen (to whom we are indebted for the terminology of the parts of vertebræ in common use), “diapophysis,” and the latter “parapophysis.” Other processes less constantly present are called respectively “metapophyses” (_m_) and “anapophyses” (_a_).
The vertebral column is divided for convenience of description into five regions—the cervical, thoracic or dorsal, lumbar, sacral, and caudal. This division is useful, especially as it is not entirely arbitrary, and in most cases is capable of ready definition; but at the contiguous extremities of the regions the characters of the vertebræ of one are apt to blend into those of the next region, either normally or as peculiarities of individual skeletons.
[Illustration: FIG. 11.—Anterior surface of sixth cervical vertebra of Dog. _s_, Spinous process; _az_, anterior zygapophysis; _v_, vertebrarterial canal; _t_, transverse process; _t′_, its inferior lamella.]
_Cervical Vertebræ._—The _cervical_ region constitutes the most anterior portion of the column, or that which joins the cranium. The vertebræ which belong to it are either entirely destitute of movable ribs, or if they have any these are small, and do not join the sternum. As a general rule they have a considerable perforation through the base of the transverse process (the vertebrarterial canal, Fig. 11, _v_); or, as it is sometimes described, they have two transverse processes, superior and inferior, which meet at their extremities to enclose a canal. This, however, rarely applies to the last vertebra of the region, in which only the upper transverse process is usually developed. The transverse process, moreover, very often sends down near its extremity a more or less compressed plate (inferior lamella), which, being considered serially homologous with the ribs of the thoracic vertebræ (though not developed autogenously), is often called the “costal” or “pleurapophysial” plate. This is usually largest on the sixth, and altogether wanting on the seventh vertebra. The first and second cervical vertebræ, called respectively “atlas” and “axis,” are specially modified for the function of supporting and permitting the free movements of the head. They are not united together by the intervertebral substance, but connected only by ordinary ligaments and synovial joints.
The cervical region in mammals presents the remarkable peculiarity that, whatever the length or flexibility of the neck, the number of vertebræ is the same, viz. seven, with the exception of the Manatee and Hoffman’s Two-toed Sloth (_Cholœpus hoffmanni_), which both have but six, and the Three-toed Sloth (_Bradypus tridactylus_), which has nine, though in this case the last two usually support movable ribs, which are not sufficiently developed to reach the sternum.
According to Parker there may occasionally be eight cervicals in the Pangolins (_Manis_).
_Dorsal Vertebræ._—The _dorsal_ (or, as it would be more correctly termed, _thoracic_) region consists of the vertebræ succeeding those of the neck, which have ribs movably articulated to them. These ribs arch round the thorax—the anterior one, and usually the greater number of those that follow, being attached below to the sternum.
_Lumbar Vertebræ._—The _lumbar_ region consists of those vertebræ of the trunk in front of the sacrum which bear no movable ribs. It may happen that, as the ribs decrease in size posteriorly (the last being sometimes more or less rudimentary), the step from the thoracic to the lumbar region may be gradual and rather undetermined in a given species; but most commonly this is not the case, and the distinction is as well defined here as in any other region. As a general rule there is a certain relation between the number of the thoracic and lumbar vertebræ, the whole number being tolerably constant in a given group of animals, and any increase of the one being at the expense of the other. Thus in all known Artiodactyle Ungulata there are 19 dorso-lumbar vertebræ; but these may consist of 12 dorsal and 7 lumbar vertebræ, or 13 dorsal and 6 lumbar, or 14 dorsal and 5 lumbar. The smallest number of dorso-lumbar vertebræ in mammals occurs in some Armadillos, which have but 14. The number found in Man, the higher Apes, and most Bats, viz. 17, is exceptionally low; 19 prevails in the Artiodactyla, nearly all Marsupials, and very many Rodents; 20 or 21 in Carnivora and most Insectivora; and 23 in Perissodactyla. The highest and quite exceptional numbers are in the Two-toed Sloth (_Cholœpus_) 27, and the Hyrax 30. The prevailing number of rib-bearing vertebræ is 12 or 13, any variation being generally in excess of these numbers.
_Sacral Vertebræ._—The _sacral_ region offers more difficulties of definition. Taking the human “os sacrum” as a guide for comparison, it is generally defined as consisting of those vertebræ between the lumbar and caudal regions which are ankylosed together to form a single bone. It happens, however, that the number of such vertebræ varies in different individuals of the same or nearly allied species, especially as age advances, when a certain number of the tail vertebræ generally become incorporated with the true sacrum. Other suggested tests—as those vertebræ which have a distinct additional (pleurapophysial) centre of ossification between the body and the ilium, those to which the ilium is directly articulated, or those in front of the insertion of the ischiosacral ligaments—being equally unsatisfactory or unpractical, the old one of ankylosis, as it is found to prevail in the average condition of adults in each species, is used in the enumeration of the vertebræ in the following pages. The Cetacea, having no iliac bones, have no part of the vertebral column modified into a sacrum.
[Illustration: FIG. 12.—Anterior surface of fourth caudal vertebræ of Porpoise (_Phocæna communis_). _s_, Spinous process; _m_, metapophysis; _t_, transverse process; _h_, chevron bone.]
_Caudal Vertebræ._—The _caudal_ vertebræ are those placed behind the sacrum, and terminating the vertebral column. They vary in number greatly—being reduced to 5, 4, or even 3, in a most rudimentary condition, in Man and in some Apes and Bats, and being numerous and powerfully developed, with strong and complex processes, in many mammals, especially among the Edentata, Cetacea, and Marsupialia. The highest known number, 46, is possessed by the African Long-tailed Pangolin. Connected with the under surface of the caudal vertebræ of many mammals which have the tail well developed are certain bones formed more or less like an inverted arch, called chevron bones, or by the French _os en V_. These are always situated nearly opposite to an intervertebral space, and are generally articulated both to the vertebra in front and the vertebra behind, but sometimes chiefly or entirely either to one or the other.
In some of the Anomodont Reptiles and Labyrinthodont Amphibians these chevrons are attached to the intercentra—or imperfect disks alternating with the true centra—which suggests that they are primarily intercentral elements which have been transferred to the edges of the centra by the disappearance of the intercentra.
_Sternum._—The _sternum_ of mammals is a bone, or generally a series of bones, placed longitudinally in the mesial line, on the inferior or ventral aspect of the thorax, and connected on each side with the vertebral column by a series of more or less ossified bars called “ribs.” It is present in all mammals, but varies much in character in the different groups. It usually consists of a series of distinct segments placed one before the other, the anterior being called the presternum or “manubrium sterni” of human anatomy, and the posterior the xiphisternum, or xiphoid or ensiform process, while the intermediate segments, whatever their number, constitute the mesosternum or “body.” In the Whalebone Whales the presternum alone is developed, and but a single pair of ribs is attached to it.
[Illustration: FIG. 13.—Human sternum and sternal ribs. _ps_, Presternum; _ms_, mesosternum; _xs_, xiphisternum; _c_, point of attachment of clavicle; 1 to 10, the cartilaginous sternal ribs.]
_Ribs._—The _ribs_ form a series of long, narrow, and more or less flattened bones, extending laterally from the sides of the vertebral column, curving downwards towards the median line of the body below, and mostly joining the sides of the sternum. The posterior ribs, however, do not directly articulate with that bone, but are either attached by their extremities to the edges of each rib in front of them, and thus only indirectly join the sternum, or else they are quite free below, meeting no part of the skeleton. These differences have given rise to the division into “true” and “false” ribs (by no means good expressions), signifying those that join the sternum directly and those that do not; and of the latter, those that are free below, are called “floating” ribs. The portion of each rib nearest the vertebral column and that nearest the sternum differ in their characters, the latter being usually but imperfectly ossified, or remaining permanently cartilaginous. These are called “costal cartilages,” or when ossified “sternal ribs.”
[Illustration: FIG. 14.—Sternum and strongly ossified sternal ribs of Great Armadillo (_Priodon gigas_). _ps_, Presternum; _xs_, xiphisternum.]
In the anterior part of the thorax the vertebral extremity of each rib is divided into two parts, “head” or “capitulum,” and “tubercle”; the former is attached to the side of the body of the vertebra, the latter to its transverse process; the former attachment corresponds to the interspace between the vertebræ, the head of the rib commonly articulating partly with the hinder edge of the body of the vertebra antecedent to that which bears its tubercle. Hence the body of the last cervical vertebra usually supports part of the head of the first rib. In the posterior part of the series the capitular and tubercular attachments commonly coalesce, and the rib is attached solely to its corresponding vertebra. The number of pairs of ribs is of course the same as that of the thoracic vertebræ.
The circumstance that in some of the Anomodont reptiles and Labyrinthodonts the capitula of the ribs articulate with the intercentral elements of the vertebral column has suggested, as in the instance of the chevron bones, that the intercentral capitular articulation of the ribs of mammals is a feature directly inherited from those extinct types by the gradual disappearance of the intercentra.
_Appendicular Skeleton._—The appendicular portion of the framework consists, when completely developed, of two pairs of limbs, anterior and posterior (Fig. 15).
[Illustration: FIG. 15.—Skeleton of Lion (_Felis leo_). _cd_, Caudal vertebræ; _cp_, carpus; _cr_, coracoid process of scapula, _cv_, cervical vertebræ; _d_, dorsal vertebræ; _fb_, fibula; _fm_, femur; _h_, humerus; _il_, ilium; _isch_, ischium; _l_, lumbar vertebræ; _m_, metatarsus; _mc_, metacarpus; _p_, patella; _pb_, pubis; _ph_, phalanges; _pv_, pelvis; _r_, radius; _s_, sacral vertebræ; _sc_, scapula; _sk_, skull; _tb_, tibia; _ts_, tarsus; _u_, ulna; _zy_, zygomatic arch.]
_Anterior Limb._—The anterior limb is present and fully developed in all mammals, being composed of a shoulder girdle and three segments belonging to the limb proper; viz. the upper arm or brachium, the forearm or antebrachium, and the hand or manus.
_Shoulder-girdle._—The _shoulder_ or _pectoral girdle_ in the large majority of mammals is in a rudimentary or rather modified condition, compared with that in which it exists in other vertebrates. In the Monotremata (_Ornithorhynchus_ and _Echidna_) alone is the ventral portion, or coracoid, complete and articulated with the sternum below, as in the Sauropsida; and in this group alone do we find an anterior ventral element, apparently corresponding with the pre-coracoid of the Anomodont reptiles, although generally known as the epi-coracoid. In all other mammals the coracoid, though ossified from a distinct centre, forms only a process, sometimes a scarcely distinct tubercle, projecting from the anterior border of the glenoid cavity of the scapula. The last-named cavity, which in the Monotremes is formed jointly by the scapula and coracoid, receives the head of the humerus, or arm-bone. The scapula is always well developed, and generally broad and flat (whence its vernacular name “blade bone”), with a ridge called the “spine” on its outer surface, which usually ends in a free curved process, the “acromion.” As the scapula affords attachment to many of the muscles which act upon the anterior limb, its form and the development of its processes are greatly modified according to the uses to which the member is put. Thus it is most reduced and simple in character in those animals whose limbs are mere organs of support, as the Ungulates; and most complex when the limbs are also used for grasping, climbing, or digging. The development or absence of the clavicle or “collar-bone,” an accessory bar which connects the sternum with the scapula and steadies the shoulder-joint, has a somewhat similar relation, though its complete absence in the Bears shows that this is not an invariable rule. A complete clavicle is found in Man and all the Primates, in Chiroptera, all Insectivora (except _Potamogale_), in many Rodents, in most Edentates, and in all Marsupials, except _Perameles_. More or less rudimentary clavicles (generally suspended freely in the muscles) are found in the Cat, Dog, and most Carnivora, _Myrmecophaga_, and some Rodents. Clavicles are altogether absent in most of the _Ursidæ_, all the Pinnipedia, _Manis_ among Edentates, the Cetacea, Sirenia, Ungulates, and some Rodents.
The Monotremes are peculiar in possessing a T-shaped interclavicle like that of many reptiles, lying upon the sternum, and articulating superiorly with the clavicles.
_Brachium and Antebrachium._—The proximal segment of the anterior or pectoral limb proper contains a single bone, the humerus, and the second segment two bones, the radius and the ulna, placed side by side, and articulating with the humerus at their proximal, and with the carpus at their distal extremity (Fig. 15). In their primitive and unmodified condition these bones may be considered as placed one on each border of the limb, the radius being preaxial or anterior, and the ulna postaxial or posterior, when the distal or free end of the limb is directed outwards, or away from the trunk. This is their position in the earliest embryonic condition, and is best illustrated among adult mammals in the Cetacea, where the two bones are fixed side by side and parallel to each other. In the greater number of mammals the bones assume a very modified and adaptive position, usually crossing each other in the forearm, the radius in front of the ulna, so that the preaxial bone (radius), though external (in the ordinary position of the limb) at the upper end, is internal at the lower end; and the hand, being mainly fixed to the radius, also has its preaxial border internal. In the large majority of mammals the bones are fixed in this position, but in some few, as in Man, a free movement of crossing and uncrossing—or pronation and supination, as it is termed—is allowed between them, so that they can be placed in their primitive parallel condition, when the hand (which moves with the radius) is said to be supine, or they may be crossed, when the hand is said to be prone.
The humerus frequently has a foramen piercing the inner border of the distal extremity, known as the entepicondylar foramen, which corresponds with a similar one found in the Anomodont reptiles. The hollow in the head of the ulna for the reception of the head of the humerus is known as the greater sigmoid cavity, and that for the head of the radius as the lesser sigmoid cavity (Fig. 16). The term olecranon is applied to that process of the ulna which forms the prominence of the elbow.
[Illustration: FIG. 16.—Outer aspect of the proximal extremity of the right ulna of a Bear (_Ursus_). _a_, Anterior tubercle; _ol_, olecranon; _b_, greater sigmoid cavity; _c_, lesser do.]
In most mammals walking on four limbs, in which the hand is permanently prone, the ulna is much reduced in size, and the radius increased, especially at the upper end; so that the articular surface of the latter, instead of being confined to the external side of the trochlea of the humerus, extends all across its anterior surface, and the two bones, instead of being external and internal, are anterior and posterior. In many hoofed or Ungulate mammals, and in Bats, the ulna is reduced to little more than its upper articular extremity, and firmly ankylosed to the radius—stability of these parts being more essential than mobility.
_Manus._—The terminal segment of the anterior limb is the hand or manus. Its skeleton consists of three divisions: (1) the “carpus,” a group of small, more or less rounded or angular bones with flattened surfaces applied to one another, and, though articulating by synovial joints, having scarcely any motion between them; (2) the “metacarpus,” a series of elongated bones placed side by side, with their proximal ends articulating by almost immovable joints with the carpus; (3) the “phalanges” or bones of the digits, usually three in number to each, articulating to one another by freely movable hinge-joints, the first being connected in like manner to the distal end of the metacarpal bone to which it corresponds.
[Illustration: FIG. 17.—Dorsal surface of the right manus of a Water Tortoise (_Chelydra serpentina_). After Gegenbaur. U, Ulna; R, radius; _u_, ulnare; _i_, intermedium; _r_, radiale; _c_, centrale; 1-5, the five bones of the distal row of the carpus; _m¹_-_m⁵_, the five metacarpals.]
_Carpus._—To understand thoroughly the arrangement of the bones of the carpus in mammals, it is necessary to study their condition in some of the lower vertebrates. Fig. 17 represents the manus in one of its fullest and at the same time most generalised forms, as seen in one of the Water Tortoises (_Chelydra serpentina_). The carpus consists of two principal rows of bones. The upper or proximal row contains three bones, to which Gegenbaur has applied the terms _radiale_ (_r_), _intermedium_ (_i_), and _ulnare_ (_u_), the first being on the radial or preaxial side of the limb.[11] The lower or distal row contains five bones, called _carpale_ 1, 2, 3, 4, and 5 respectively, commencing on the radial side. Between these two rows, in the middle of the carpus, is a single bone, the _centrale_ (_c_). In this very symmetrical carpus it will be observed that the _radiale_ supports on its distal side two bones, _carpale_ 1 and 2; the _intermedium_ is in a line with the _centrale_ and _carpale_ 3, which together form a median axis of the hand, while the _ulnare_ has also two bones articulating with its distal end, viz. _carpale_ 4 and 5. Each of the carpals of the distal row supports a metacarpal.
In the carpus of the Mammalia there are usually two additional bones developed in the tendons of the flexor muscles, one on each side of the carpus, which may be called the radial and ulnar sesamoid bones; the latter, which is the more constant and generally larger, is commonly known as the pisiform bone. The fourth and fifth carpals of the distal row are always united into a single bone, and the centrale is very often absent. As a general rule all the other bones are present and distinct, though it not unfrequently happens that two may have coalesced to form a single bone, or one or more may be altogether suppressed.
The following table shows the principal names in use for the various carpal bones,—those in the second column being the terms generally employed by English anatomists:—
_Radiale_ = Scaphoid = _Naviculare_. _Intermedium_ = Lunar = _Semilunare_, _Lunatum_. _Ulnare_ = Cuneiform = _Triquetrum_, _Pyramidale_. _Centrale_ = Central = _Intermedium_ (Cuvier). _Carpale_ 1 = Trapezium = _Multangulum majus_. _Carpale_ 2 = Trapezoid = _Multangulum minus_. _Carpale_ 3 = Magnum = _Capitatum_. _Carpale_ 4 } = Uneiform = _Hamatum_, _Uncinatum_. _Carpale_ 5 }
The radial and ulnar sesamoids are regarded by Bardeleben[12] as the rudiments of a prepollex and a postminimus digit; the primitive number of digits being thus supposed to have been seven. These bones have been observed in all orders of mammals having five complete digits. Occasionally, as in _Pedetes caffer_, the so-called prepollex consists of two bones, of which the distal one bears a distinct nail-like horny covering. In _Bathyergus maritimus_ the pisiform, or postminimus, is likewise double; the two elements being regarded by their describer as representing the carpal and metacarpal of the presumed seventh digit.
Similarly in the posterior limb the tibial sesamoid, and a fibular ossification corresponding to the pisiform, are regarded as representing a prehallux and a postminimus.
_Metacarpus and Phalanges._—The metacarpal bones, with the digits which they support, are never more than five in number, and are described numerically—first, second, etc., counting from the radial towards the ulnar side. The digits are also sometimes named (1) the pollex, (2) index, (3) medius, (4) annularis, (5) minimus. One or more may be in a rudimentary condition, or altogether suppressed. If one is absent, it is most commonly the first. Excepting the Cetacea, no mammals have more than three phalanges to each digit, but they may occasionally have fewer by suppression or ankylosis. The first or radial digit is an exception to the usual rule, one of its parts being constantly absent, since, while each of the other digits has commonly a metacarpal and three phalanges, it has only three bones altogether; whether the missing one is a metacarpal or one of the phalanges is a subject which has occasioned much discussion, and has not yet been satisfactorily decided. The terminal phalanges of the digits are usually specially modified to support the nail, claw, or hoof, and are called “ungual phalanges.” In walking, some mammals (as the Bears) apply the whole of the lower surface of the carpus, metacarpus, and phalanges to the ground; to these the term “plantigrade” is applied. Many others (as nearly all the existing Ungulata) only rest on the last one or two phalanges of the toes, the first phalanx and the metacarpals being vertical and in a line with the forearm. These are called “digitigrade.” Intermediate conditions exist between these two forms, to which the terms “phalangigrade” (as the Camel) and “subplantigrade” (as in most Carnivora), are applied. When the weight is borne entirely on the distal surface of the ungual phalanx, and the horny structures growing around it, as in the Horse, the mode of progression is called “unguligrade.”
In the Chiroptera the digits are enormously elongated, and support a cutaneous expansion constituting the organ of flight. In the Cetacea the manus is formed into a paddle, being covered by continuous integument, which conceals all trace of division into separate digits, and shows no sign of nails or claws. In the Sloths the manus is long and very narrow, habitually curved, and terminating in two or three pointed curved claws in close apposition with each other, and incapable, in fact, of being divaricated; so that it is reduced to the condition of a hook, by which the animal suspends itself to the boughs of the trees among which it lives. These are only examples of the endless modifications to which the distal extremity of the limb is subjected in adaptation to the various purposes to which it is applied.
_Posterior Limb._—The posterior limb is constructed upon a plan very similar to that of the anterior extremity. It consists of a pelvic girdle and three segments belonging to the limb proper, viz. the thigh, the leg, and the foot or pes (Fig. 15).
_Pelvic Girdle._—The pelvic girdle is present in some form in all mammals, though in the Cetacea and the Sirenia it is in an exceedingly rudimentary condition. In all mammals except those belonging to the two orders just named, each lateral half of the pelvic girdle consists essentially, like the corresponding part of the anterior limb, of a flattened rod of bone crossing the long axis of the trunk, having an upper or dorsal and a lower or ventral end. The upper end diverges from that of the opposite side, but the lower end approaches, and, in most cases, meets it, forming a symphysis, without the intervention of any bone corresponding to the sternum. The pelvic girdle differs from the shoulder girdle in being firmly articulated to the vertebral column, thus giving greater power to the hinder limb in its function of supporting and propelling the body. Like the shoulder girdle, it bears on its outer side, near the middle, a cup-shaped articular cavity (“acetabulum”), into which the proximal end of the first bone of the limb proper is received. Each lateral half of the girdle is called the “os innominatum,” or innominate bone, and consists originally of three bones which unite at the acetabulum. The “ilium” or upper bone is that which articulates with the sacral vertebræ. Of the two lower bones the anterior or “pubis” unites with its fellow of the other side at the symphysis; the posterior is the “ischium.” These lower elements form two bars of bone, united above and below, but leaving a space between them in the middle, filled only by membrane, and called the “thyroid” or “obturator” foramen. The whole circle of bone formed by the two innominate bones and the sacrum is called the pelvis. In the Monotremata and Marsupialia, a pair of thin, flat, elongated ossifications called epipubic or marsupial bones are attached to the fore part of the pubis, and project forward into the muscular wall of the abdomen.
_Thigh and Leg._—The first segment of the limb proper has one bone, the femur, corresponding with the humerus of the anterior limb. The second segment has two bones, the tibia and fibula, corresponding with the radius and ulna. These bones always lie in their primitive unmodified position, parallel to each other, the tibia on the preaxial and the fibula on the postaxial side, and are never either permanently crossed or capable of any considerable amount of rotation, as in the corresponding bones of the fore limb. In the ordinary walking position the tibia is internal, and the fibula external. In many mammals the fibula is in a more or less rudimentary condition, and it often ankyloses with the tibia at one or both extremities. The patella or “knee-cap,” which is found in an ossified condition in all mammals, with the exception of some of the Marsupialia, is a large sesamoid bone developed in the tendon of the extensor muscles of the thigh, where the tendon passes over the front of the knee-joint, to which it serves as a protection. There are frequently smaller ossicles, one or two in number, situated behind the femoral condyles, called “fabellæ.” The processes for the attachment of muscles near the upper end of the femur are termed trochanters; and the third trochanter, found on the hinder aspect of the shaft of this bone in many forms is of considerable taxonomic importance.
_Pes._—The terminal segment of the hind limb is the foot or pes. Its skeleton presents in many particulars a close resemblance to that of the manus, being divisible into three parts: (1) a group of short, more or less rounded or square bones, constituting the tarsus; (2) a series of long bones placed side by side, forming the metatarsus; and (3) the phalanges of the digits or toes.
The bones of the tarsus of many of the lower Vertebrata closely resemble both in number and arrangement those of the carpus, as shown in Fig. 17. They have been described in their most generalised condition by Gegenbaur under the names expressed in the first column of the following table. The names in the second column are those by which they are generally known to English anatomists, while in the third column some synonyms occasionally employed are added.
_Tibiale (?)_ } = Astragalus[13] = _Talus_. _Intermedium_ } _Fibulare_ = Calcaneum = _Os calcis_. _Centrale_ = Navicular = _Scaphoideum_. _Tarsale_ 1 = Internal cuneiform = _Entocuneiforme_. _Tarsale_ 2 = Middle cuneiform = _Mesacuneiforme_. _Tarsale_ 3 = External cuneiform = _Ectocuneiforme_. _Tarsale_ 4 } = Cuboid. _Tarsale_ 5 }
The bones of the tarsus of mammals present fewer diversities of number and arrangement than those of the carpus. The proximal row (see Fig. 18) always consists of two bones, namely the astragalus (_a_), which probably represents the coalesced scaphoid and lunar of the hand, and the calcaneum (_c_). The former is placed more to the dorsal side of the foot than the latter, and almost exclusively furnishes the tarsal part of the tibio-tarsal or ankle-joint. The calcaneum, placed more to the ventral or “plantar” side of the foot, is elongated backwards to form a more or less prominent tuberosity, the “tuber calcis,” to which the tendon of the great extensor muscles of the foot is attached. The navicular bone (_n_) is interposed between the proximal and distal row on the inner or tibial side of the foot, but on the outer side the bones of the two rows come into contact. The distal row, when complete, consists of four bones, which, beginning on the inner side, are the three cuneiform bones, internal (_c¹_), middle (_c²_), and external (_c³_), articulated to the distal surface of the navicular, and the cuboid (_cb_), articulated with the calcaneum. Of these the middle cuneiform is usually the smallest in animals in which all five digits are developed; but when the hallux is wanting the internal cuneiform may be rudimentary or altogether absent. The three cuneiform bones support respectively the first, second, and third metatarsals, and the cuboid supports the fourth and fifth; they thus exactly correspond with the four bones of the distal row of the carpus.
[Illustration: FIG. 18.—Bones of the right Human foot. _T_, Tarsus; _M_, metatarsus; _Ph_, phalanges, _c_, calcaneum; _a_, astragalus; _cb_, cuboid; _n_, navicular; _c¹_, internal cuneiform; _c²_, middle cuneiform; _c³_, external cuneiform. The digits are indicated by Roman numerals, counting from the tibial to the fibular side.]
In addition to these constant tarsal bones, there may be supplemental or sesamoid bones: one situated near the middle of the tibial side of the tarsus, largely developed in many Carnivora and Rodentia; another, less frequent, on the fibular side; and a third, often developed in the tendons of the plantar surface of the tarsus, is especially large in Armadillos. There is also usually a pair of sesamoid bones on the plantar aspect of each metatarso-phalangeal articulation. In the young of the carnivorous genus _Crytoprocta_ there may be a second centrale, which usually coalesces with the ectocuneiform.
The metatarsal bones never exceed five in number, and the phalanges follow the same numerical rule as in the manus, never exceeding three in each digit. Moreover, the first digit, counting from the tibial side, or hallux, resembles the pollex of the hand in always having one segment less than the other digits. As the function of the hind foot is more restricted than that of the hand the modifications of its structure are less striking. In the Cetacea and the Sirenia it is entirely wanting, though in some existing members of the first-named order rudiments of the bones of both the first and second segments of the limb have been detected, and a femur is present in the Miocene Sirenian _Halitherium_.
IV. THE DIGESTIVE SYSTEM.
_General Considerations._—The search after the purpose which every modification of structure subserves in the economy is always full of interest, and, if conducted with due caution and sufficient knowledge of all the attendant circumstances, may lead to important generalisations. It must always be borne in mind, however, that adaptation to its special function is not the only cause of the particular form or structure of an organ, but that this form, having in all probability been arrived at by the successive and gradual modification of some other different form from which it is now to a greater or less degree removed, has other factors besides use to be taken into account. In no case is this principle so well seen as in that of the organs of digestion. These may be considered as machines which have to operate upon alimentary substances in very different conditions of mechanical and chemical combination, and to reduce them in every case to the same or precisely similar materials; and we might well imagine that the apparatus required to produce flesh and blood out of coarse fibrous vegetable substances would be different from that which had to produce exactly the same results out of ready-made flesh or blood; and in a very broad sense we find that this is so. Thus, if we take a large number of carnivorous animals, belonging to different fundamental types, and a large number of herbivorous animals, and strike a kind of average of each, we shall find that there is, pervading the first group, a general style, if we may use the expression, of the alimentary organs, different from that of the others. That is to say, there is a specially carnivorous and a specially herbivorous modification of these parts. But, if function were the only element which has guided such modification, it might be inferred that, as one form must be supposed to be best adapted in its relation to a particular kind of diet, that form would be found in all the animals consuming such diet. But this is far from being the case. Thus the Horse and the Ox, for instance—two animals whose food in the natural state is precisely similar—are most different as regards the structure of their alimentary canal, and the processes involved in the preparation of that food. Again, the Seal and the Porpoise, both purely fish-eaters, which seize, swallow, and digest precisely the same kind of prey, in precisely the same manner, have a totally different arrangement of the alimentary canal. If the Seal’s stomach is adapted in the best conceivable manner for the purpose it has to fulfil, why is not the Porpoise’s stomach an exact facsimile of it, and _vice versâ_? We can only answer that the Seal and Porpoise belong to different natural groups of animals, formed either on different primitive types, or descended from differently constructed ancestors. On this principle only can we account for the fact that, whereas, owing to the comparatively small variety of the different alimentary substances met with in nature, few modifications would appear necessary in the organs of digestion, there is really endless variety in the parts devoted to this purpose.
_Mouth._—The digestive apparatus of mammals, as in other vertebrates, consists mainly of a tube with an aperture placed at or near either extremity of the body,—the oral and the anal orifice,—and furnished with muscular walls, the fibres of which are so arranged as by their regular alternate contraction and relaxation to drive onwards the contents of the tube from the first to the second of these apertures. The anterior or commencing portion of this tube and the parts around it are greatly and variously modified in relation to the functions assigned to them of selecting and seizing the food, and preparing it by various mechanical and chemical processes for the true digestion which it has afterwards to undergo before it can be assimilated into the system. For this end the tube is dilated into a chamber or cavity called the mouth, bordered externally by the lips, which are usually muscular and prehensile, and supported by a movable framework carrying the teeth; the structure and modifications of which have been already described. The roof of the mouth is formed by the palate, terminating behind by a muscular, contractile arch, having in Man and some few other species a median projection called the uvula, beneath which the mouth communicates with the pharynx. The anterior part of the palate is composed of mucous membrane tightly stretched over the flat or slightly concave bony lamina separating the mouth from the nasal passages, and is generally raised into a series of transverse ridges, which sometimes, as in Ruminants, attain a considerable development. In the floor of the mouth, between the rami of the mandible, and supported behind by the hyoidean apparatus, lies the tongue; an organ the free surface of which, especially in its posterior part, is devoted to the sense of taste, but which also, by its great mobility (being composed almost entirely of muscular fibres), performs important mechanical functions connected with masticating and procuring food. Its modifications of form in different mammals are very numerous. Between the long, extensile, vermiform tongue of the Anteaters, which is essential to the peculiar mode of feeding of those animals, and the short, sessile, and almost functionless tongue of the Porpoise, every intermediate condition is found. Whatever the form, the upper surface is always covered with numerous fine papillæ, in which the terminal filaments of the gustatory nerve are distributed.
_Salivary Glands._—The fluid known as the saliva is secreted by an extensive and complex system of glands discharging into the cavity of the mouth (buccal cavity), the position and relation of some of which are exhibited in the woodcut on the next page (Fig. 19).
[Illustration: FIG. 19.—Salivary Glands of the Genet. _A_, Right side of the head dissected; _p_, parotid gland; _d_, Steno’s duct; _sm_, submaxillary gland, traversed by the jugular veins (_jv_); _o_, aperture of Steno’s duct. _B_, Part of the head with the lip drawn up to show (_st.d_) aperture of Steno’s duct; _z.gl_, zygomatic gland; _o_, aperture of do.; _z_, zygomatic arch (Mivart, _Proc. Zool. Soc._ 1882, p. 504.)]
This apparatus consists of small glands embedded in the mucous membrane or submucous tissue lining the cavity of the mouth, which are of two kinds (the follicular and the racemose), and of others in which the secreting structure is aggregated in distinct masses removed some distance from the cavity; other tissues besides the lining membrane being usually interposed, and pouring their secretion into the cavity by a distinct tube or duct, which traverses the mucous membrane. To the latter alone the name of “salivary glands” is ordinarily appropriated, although the distinction between them and the smaller racemose glands is only one of convenience for descriptive purposes, their structure being more or less nearly identical; and, since the fluids secreted by all become mixed in the month, their functions are, at all events in great part, common. Under the name of salivary glands are commonly included—(1) the “parotid” (_p_), situated very superficially on the side of the head, below or around the cartilaginous external auditory meatus, and the secretion of which enters the mouth by a duct (often called Steno’s or Stenson’s) which crosses the masseter muscle and opens into the upper and back part of the cheek (Fig. 19); and (2) the “submaxillary” (_sm_), situated in the neck, near or below the angle of the mandible, and sending a long duct (Wharton’s) forwards to open on the forepart of the floor of the cavity of the mouth, below the apex of the tongue. These are the most largely developed and constant of the salivary glands, being met with in various degrees of development in almost all animals of the class. Next in constancy are (3) “the sublingual,” closely associated with the last-named, at all events in the locality in which the secretion is poured out; and (4) the “zygomatic” (_z.gl_), found only in some animals in the cheek, just under cover of the anterior part of the zygomatic arch, its duct entering the buccal cavity near that of the parotid.
The most obvious function common to the secretion of these various glands, and to that of the smaller ones placed in the mucous membrane of the lips, the cheeks, the tongue, the palate, and fauces, is the mechanical one of moistening and softening the food, to enable it the more readily to be tasted, masticated, and swallowed, though each kind of gland may contribute in different manner and different degree to perform this function. The saliva is, moreover, of the greatest importance in the first stage or introduction to the digestive process, as it dissolves or makes a watery extract of all soluble substances in the food, and so prepares them to be further acted on by the more potent digestive fluids met with subsequently in their progress through the alimentary canal. In addition to these functions it seems now well established by experiment that saliva serves in Man and many animals to aid directly in the digestive process, particularly by its power of inducing the saccharine transformation of amylaceous substances. As a general rule, in mammals the parotid saliva is more watery in its composition, while that of the submaxillaries, and still more the sublingual, contains more solid elements and is more viscid;—so much so that some anatomists consider the latter, together with the small racemose glands of the cheeks, lips, and tongue, as mucous glands, retaining the name of salivary only for the parotid. These peculiar properties are sometimes illustrated in a remarkable degree, as, for example, the great secretion of excessively viscid saliva which lubricates the tongue of the Anteaters and Armadillos, associated with enormously developed submaxillary glands; while, on the other hand, the parotids are of great size in those animals which habitually masticate dry and fibrous food.
_Stomach._—After the preparation which the aliment has undergone in the mouth,—the extent of which varies immensely in different forms, being reduced almost to nothing in such animals as the Seals and Cetaceans, which, to use the familiar expression, “bolt” their food entire, and most fully carried out in the Ruminants, which “chew the cud,”—it is swallowed, and carried along the œsophagus by the action of its muscular coats into the stomach. In the greater number of mammals this organ is a simple saccular dilatation of the alimentary canal, as in Figs. 20, 21, but in others it undergoes remarkable modifications and complexities. The lining of the stomach is thickly beset with tubular glands, which are generally considered to belong to two different forms, recognisable by their structure, and different in their function—the most numerous and important secreting the gastric juice (the active agent in stomachic digestion), and hence called “peptic” glands, while the others are concerned only in the elaboration of mucus. The relative distribution of these glands in different regions of the walls of the stomach varies greatly in different animals, and in many species there are large tracts of the mucous membrane which do not secrete a fluid having the properties of gastric juice, but often constitute more or less distinct cavities devoted to storing and perhaps softening or otherwise preparing the food for digestion. Sometimes there is a great aggregation of glands forming distinct thickened patches of the stomach wall, as in the Beaver and Koala, or even collected in pyriform pouches with a common narrow opening into the cavity, as in the Manatee and the curious African Rodent _Lophiomys_. The action of the gastric fluid is mainly exerted upon the nitrogenous elements of the food, which it dissolves and modifies so as to render them capable of undergoing absorption, effected partly by the blood-vessels of the stomach, although the greater part, passes through the pylorus, an aperture surrounded by a circular muscular valve, into the intestinal canal. Here it comes in contact with the secretion of a vast number of small glands called the crypts of Lieberkuhn, somewhat similar to those of the stomach; and also of several special glands of a different character, namely, the small racemose, duodenal, or Brunner’s glands, the pancreas, and the liver; the position of the ducts of the two latter organs being indicated in Fig. 20.
[Illustration: FIG. 20.—Stomach and pancreas of the Genet. Posterior or dorsal surface, _œ_, Œsophagus; _s_, pancreas; _pd_, pancreatic duct; _bd_, biliary duct from the liver. (From Mivart, _Proc. Zool. Soc._ 1882, p. 305.)]
_Intestinal Canal._—The intestinal canal varies greatly in relative length and capacity in different animals, and it also offers manifold peculiarities of form, being sometimes a simple cylindrical tube of nearly uniform calibre throughout, but more often subject to alterations of form and capacity in different portions of its course,—the most characteristic and constant being the division into an upper and narrower, and lower and wider portion, called respectively the small and the large intestine, the former being divided quite arbitrarily and artificially into duodenum, jejunum, and ileum, and the latter into colon and rectum. One of the most striking peculiarities of this part of the alimentary canal is the frequent presence of a diverticulum or blind pouch, the _caput cæcum coli_, as it was first called, a name generally abbreviated into “cæcum,” situated at the junction of the large and the small intestine, a structure presenting an immense variety of development, from the smallest bulging of a portion of the side wall of the tube to a huge and complex sac, greatly exceeding in capacity the whole of the remainder of the alimentary canal. It is only in herbivorous animals that the cæcum is developed to this great extent, and among these there is a curious complementary relationship between the size and complexity of this organ and that of the stomach. Where the latter is simple the cæcum is generally the largest, and _vice versâ_. Both the cæcum and colon are often sacculated, a disposition caused by the arrangement of the longitudinal bands of muscular tissue in their walls; but the small intestine is always smooth and simple-walled externally, though its lining membrane often exhibits various contrivances for increasing the absorbing surface without adding to the general bulk of the organ, such as the numerous small villi by which it is everywhere beset, and the more obvious transverse, longitudinal, or reticulating folds projecting into the interior, met with in many animals, of which the “valvulæ conniventes” of Man form well-known examples.
[Illustration: FIG. 21.—Diagrammatic plan of the general arrangement of the alimentary canal in a typical Mammal. _o_, Œsophagus; _st_, stomach; _p_, pylorus; _s_, _s_, small intestine; (abbreviated); _c_, cæcum; _l_, _l_, large intestine or colon, ending in _r_, the rectum.]
Besides the crypts of Lieberkuhn found throughout the intestinal canal, and the glands of Brunner confined to the duodenum, there are other structures in the mucous membrane, about the nature of which there is still much uncertainty, called “solitary” and “agminated” glands; the latter being more commonly known by the name of “Peyer’s patches.” These were formerly supposed to be secretory organs, which discharged some kind of fluid into the intestine, but are now more generally considered to belong to that group of structures of somewhat mysterious function of which the lymphatic and lacteal glands are members. The solitary glands are found scattered irregularly throughout the whole intestinal tract; the agminated, on the other hand, are always confined to the small intestine, and are most abundant in its lower part. They are subject to great variation in number and in size, and even in different individuals of the same species, and also differ in character at different periods of life, becoming atrophied in old age.
_Liver._—The distinct glands situated outside the walls of the intestinal canal, but which pour their secretion into it, are the pancreas and the liver. The latter is the more important on account of its size, if not on account of the direct action of its secretion in the digestive process. This large gland, so complex in structure and function, is well developed in all mammals, and its secreting tube, the bile-duct, always opens into the duodenum, or that portion of the canal which immediately succeeds the stomach. It is situated on the right side of the abdomen in contact with the diaphragm and the stomach, but varies greatly in relative size, and also in form, in different groups of mammals. In most mammals a gall-bladder, consisting of a pyriform diverticulum from the bile-duct, is present, but in many this appendage is wanting, and it is difficult to find the rationale of its presence or absence in relation to use or any other circumstance in the animal economy.
The descriptions of the livers of various animals to be met with in treatises or memoirs on comparative anatomy are very difficult to understand for want of a uniform system of nomenclature. The difficulty usually met with arises from the circumstance that this organ is divided sometimes, as in Man, Ruminants, and the Cetacea, into two main lobes, which have been always called respectively right and left, and in other cases, as in the lower Monkeys, Carnivora, Insectivora, and several other orders, into a larger number of lobes. Among the latter the primary division usually appears at first sight tripartite, the whole organ consisting of a middle, called “cystic” or “suspensory” lobe, and two lateral lobes, called respectively right and left lobes. This introduces confusion in describing livers by the same terms throughout the whole series of mammals, since the right and left lobes of the Monkey or Dog, for instance, do not correspond with parts designated by the same names in Man and the Sheep. There are, moreover, conditions where neither the bipartite nor the tripartite system of nomenclature will answer, so that we should have considerable difficulty in describing them without some more general system. In order to arrive at such a system it appears desirable to consider the liver in all cases as primarily divided by the umbilical vein (see Fig. 22, _u_) into two segments, right and left. This corresponds with its development and with the condition characteristic of the organ in the inferior classes of vertebrates. The situation of this division can almost always be recognised in adult animals by the persistence of some traces of the umbilical vein in the form of the round ligament, and by the position of the suspensory ligament.
[Illustration: FIG. 22.—Diagrammatic plan of the inferior surface of a multilobed liver of a Mammal. The posterior or attached border is uppermost. _u_, Umbilical vein of the fœtus, represented by the round ligament in the adult, lying in the umbilical fissure; _dv_, the ductus venosus; _vc_, the inferior vena cava; _p_, the vena portæ entering the transverse fissure; _llf_, the left lateral fissure; _rlf_, the right lateral fissure; _cf_, the cystic fissure; _ll_, the left lateral lobe; _lc_, the left central lobe; _rc_, the right central lobe; _rl_, the right lateral lobe; _s_, the Spigelian lobe; _c_, the caudate lobe; _g_, the gall-bladder.]
When the two main parts into which the liver is thus divided are entire, as in Man, the Ruminants, and Cetacea, they may be spoken of as the right and left lobes; when fissured, as the right and left segments of the liver, reserving the term lobe for the subdivisions. This will involve no ambiguity, for the terms right and left lobe will no longer be used for divisions of the more complex form of liver. In the large majority of mammals each segment is further divided by a fissure, more or less deep, extending from the free towards the attached border, which are called right and left lateral fissures (Fig. 22, _rlf_ and _llf_). When these are more deeply cut than the umbilical fissure (_u_), the organ has that tripartite or trefoil-like form just spoken of, but it is easily seen that it is really divided into four regions or lobes, those included between the lateral fissures being the right and left central (_rc_ and _lc_) separated by the umbilical fissure, and those beyond the lateral fissures on each side being the right and left lateral lobes (_rl_ and _ll_). The essentially bipartite character of the organ and its uniformity of construction throughout the class are thus not lost sight of, even in the most complex forms. The left segment of the liver is rarely complicated to any further extent, except in some cases by minor or secondary fissures marking off small lobules, generally inconstant and irregular, and never worthy of any special designation. On the other hand, the right segment is usually more complex. The gall-bladder, when present, is always attached to the under surface of the right central lobe, sometimes merely applied to it, in other cases deeply embedded in its substance. In many instances the fossa in which it is sunk is continued to the free margin of the liver as an indent, or even a tolerably deep fissure (_cf_). The portal fissure (_p_), through which the portal vein and hepatic artery enter and the bile-duct emerges from the liver, crosses the right central lobe transversely, near the attached border of the liver. The right lateral lobe always has the great vena cava (_vc_) either grooving its surface or tunnelling through its substance near the inner or left end of its attached border; and a prolongation of this lobe to the left, between the vein and the portal fissure, sometimes forming a mere flat track of hepatic substance, but more often a prominent tongue-shaped process, is the so-called “Spigelian lobe” (_s_). From the under surface, of the right lateral lobe a portion is generally partially detached by a fissure, and called the “caudate lobe” (_c_). In Man this lobe is almost obsolete, but in most mammals it is of considerable magnitude, and has very constant and characteristic relations. It is connected by an isthmus at the left (narrowest or attached) end to the Spigelian lobe, behind which isthmus the vena cava is always in relation to it, channelling through or grooving its surface. It generally has a pointed apex, and is deeply hollowed to receive the right kidney, to the upper and inner side of which it is applied.
Considerations derived from the comparatively small and simple condition of the liver of the Ungulata, compared with its large size and complex form in the Carnivora, have led to the perhaps too hasty generalisation that the first type is related to a herbivorous and the latter to a carnivorous diet. The exceptions to such a proposition are very numerous. The fact of the great difference between the liver of the Cetacea and that of the Seals cannot be accounted for by difference of habits of life, though it perhaps may be by difference of origin.[14]
V. CIRCULATORY, ABSORBENT, RESPIRATORY, AND URINARY SYSTEMS.
_Blood._—The blood of mammals is always red, and during the life of the animal hot, having a nearly uniform temperature, varying within a few degrees on each side of 100° Fahr. The corpuscles are, as usual in the vertebrates, of two kinds: (1) colourless, spheroidal, nucleated, and exhibiting amœboid movements; while (2) the more numerous, on which depends the characteristic hue of the fluid in which they are suspended, are coloured, non-nucleated, flattened, slightly biconcave discs, with circular outline in all known species except the Camels and Llamas, where they have the elliptical form characteristic of the red corpuscles of nearly all the other vertebrates, though adhering to the mammalian type in the absence of nucleus and relatively small size. As a rule they are smaller as well as more numerous than in other classes, but vary considerably in size in different species, and not always in relation to the magnitude of the animal; a Mouse, for instance, having as large corpuscles as a Horse. Within the limits of any natural group there is, however, very often some such relation, the largest corpuscles being found among the large species and the smallest corpuscles among the small species of the group, but even to this generalisation there are many exceptions. The transverse diameter of the red corpuscles in Man averages ¹⁄₃₂₀₀ of an inch, which is exceptionally large, and only exceeded by the Elephant (¹⁄₂₇₄₅), and by some Cetacea and Edentata. They are also generally large in Apes, Rodents, and the Monotremata, and small in the Artiodactyles, least of all in the Chevrotains (_Tragulus_), being in _T. javanicus_ and _meminna_ not more than ¹⁄₁₂₃₂₅.[15]
_Heart._—The heart of mammals consists of four distinct cavities, two auricles and two ventricles. Usually the ventricular portion is externally of conical form, with a simple apex, but in the Sirenia it is broad and flattened, and a deep notch separates the apical portion of each ventricle. A tendency to this form is seen in the Cetacea and the Seals. It is characteristic of mammals alone among vertebrates that the right auriculo-ventricular valve is tendinous like the left, consisting of flaps held in their place by fibrous ends (_chordæ tendiniæ_) and arising from projections of the muscular walls of the ventricular cavity (_musculi papillares_). In the Monotremata a transition between this condition and the simple muscular flap of the Sauropsida is observed. In most of the larger Ungulates a distinct but rather irregular ossification (_os cordis_) is developed in the central tendinous portion of the base of the heart.
_Blood-vessels._—The orifices of the aorta and pulmonary artery are each guarded by three semilunar valves. The aorta is single, and arches over the left bronchial tube. After supplying the tissues of the heart itself with blood by means of the coronary arteries, it gives off large vessels (“carotid”) to the head and (“brachial”) to the anterior extremities. The mode in which these vessels arise from the aorta varies much in different mammals, and the study of their disposition affords some guide to classification. In nearly all cases the right brachial and carotid have a common origin (called the “innominate artery” in anthropotomy). The other two vessels may come off from this, as is the rule in Ungulates, the common trunk constituting the “anterior aorta” of veterinary anatomy; or they may be detached in various degrees, both arising separately from the aorta, as in Man, or the left carotid from the innominate and the left brachial from the aorta, a very common arrangement; or the last two from a common second or left innominate, as in some Bats and Insectivores. The aorta, after giving off the intercostal arteries, passes through the diaphragm into the abdomen, and, after supplying the viscera of that cavity by means of the gastric, hepatic, splenic, mesenteric, renal, and spermatic vessels, gives off in the lumbar region a large branch (iliac) to each of the hinder extremities, which also supplies the pelvic viscera, and is continued onwards in the middle line, greatly diminished in size, along the under surface of the tail as the caudal artery. In certain mammals, arterial plexuses, called _retia mirabilia_, formed by the breaking up of the vessel into an immense number of small trunks, which may run in a straight course parallel to one another (as in the limbs of Sloths and Slow Lemurs), or form a closely packed network, as in the intracranial plexuses of Ruminants, or a sponge-like mass of convoluted vessels, as in the intercostals of Cetaceans, are peculiarities of the vascular system the meaning of which is not in all cases clearly understood. In the Cetacea they are obviously receptacles for containing a large quantity of oxygenated blood available during the prolonged immersion, with consequent absence of respiration, to which these animals are subject.
The vessels returning the blood to the heart from the head and upper extremities usually unite, as in Man, to form the single _vena cava superior_ or precaval vein, but in some Insectivores, Chiroptera, and Rodents, in the Elephant, and all Marsupials and Monotremes, the two superior caval veins enter the right auricle without uniting, as in birds. In Seals and some other diving mammals there is a large venous sinus or dilatation of the inferior vena cava immediately below the diaphragm. In the Cetacea the purpose of this is supplied by the immense abdominal venous plexuses. As a rule the veins of mammals are furnished with valves, but these are said to be altogether wanting in the Cetacea, and in the superior and inferior cava, subclavian and iliac veins, the veins of the liver (both portal and hepatic), heart, lungs, kidneys, brain, and spinal cord of other mammals. Many of the veins within the cranium are included in spaces formed by the separation of the laminæ of the dura mater, and do not admit of being dilated beyond a certain size; these are termed sinuses. The portal circulation in mammals is limited to the liver, the portal vein being formed by the superior and inferior mesenteric, the splenic, the gastro-epiploic, and the pancreatic veins. The kidney is supplied solely by arterial blood, and its veins empty their contents only into the inferior cava.
_Lymphatic Vessels._—The _absorbent_ or _lymphatic_ system of vessels is very fully developed in the Mammalia. Its ramifications extend through all the soft tissues of the body, and convey a colourless fluid called lymph, containing nucleated corpuscles, and also, during the process of digestion, the chyle, a milky fluid taken up by the lymphatics (here called lacteals) of the small intestine, and pour them into the general vascular system, where they mix with the venous blood. The lymphatic vessels of the hinder extremities, as well as those from the intestinal canal, unite in the abdomen to form the “thoracic duct,” the hinder end or commencement of which has a dilatation called the _receptaculum chyli_. This duct, which is of irregular size and sometimes double, often dividing and uniting again in its course, or even becoming plexiform, passes forwards close to the bodies of the thoracic vertebræ, and empties itself, by an orifice guarded by a valve, into the great left brachio-cephalic vein, having previously received the lymphatics from the thorax and the left side of the head and left anterior extremity. The lymphatics from the right side of the head and right anterior limb usually enter by a small distinct trunk into the corresponding part of the right brachio-cephalic vein. The duct, and also the principal lymphatic vessels, are provided with valves.
Lymphatic glands, rarely met with in the Sauropsida, are usually present in mammals, both in the general and in the lacteal system; the latter being called “mesenteric glands.” They are round or oval masses, situated upon the course of the vessels, which break up in them and assume a plexiform arrangement, and then reunite as they emerge. No structures corresponding to the pulsating “lymphatic hearts” of the lower vertebrates have been met with in mammals.
_Ductless Glands._—Associated with the vascular and lymphatic systems are certain bodies (the functions of which are not properly understood), usually, on account of their general appearance, grouped together under the name of “ductless glands.” The largest of these is the “spleen,” which is single, and always placed in mammals in relation to the fundus or left end of the stomach, to which it is attached by a fold of peritoneum. It is dark-coloured and spongy in substance, and has a depression or “hilus” on one side, into which the splenic artery, a branch of the cœliac axis of the abdominal aorta, enters, and from which the vein joining the portal system emerges. The spleen varies much in size and form in different mammals, being relatively very small in the Cetacea. It is sometimes almost spherical, but more often flattened, oval, triangular, or elongated, and occasionally, as in Monotremes and most Marsupials, triradiate. The “suprarenal bodies” or “adrenals” are two in number, each situated either in contact with, or at a short distance in front of the anterior extremity of the kidney. They are abundantly supplied with nerves, and are much larger relatively in early than in adult life. The “thyroid bodies,” of which there are generally two, though in Man and some other species they are connected by an isthmus passing across the middle line, are constant in mammals, though only met with in a rudimentary condition, if at all, in other vertebrates. They are situated in the neck, in contact with the sides of the anterior extremity of the trachea. The “thymus” lies in the anterior part of the thorax, between the sternum and the great vessels at the base of the heart, and differs from the thyroid in being median and single, and having a central cavity. It attains its greatest development during the period in which the animal is nourished by its mother’s milk, and then it diminishes, and generally disappears before full growth is attained.
_Nostrils._—Mammals breathe occasionally through the mouth, but usually, and in many cases exclusively, through the nostrils or _nares_. Those are apertures, always paired (except in the toothed Cetacea, where they unite to form a single external opening), and situated at the fore part of the face, generally at or beneath the end of the muzzle, a median prominence above the mouth. This is sometimes elongated to form a proboscis, to the extremity of which the nostrils are carried, and which attains its maximum of development in the Elephant. In the Cetacea the nostrils are situated at a considerable distance behind the anterior end of the face, upon the highest part of the head, and are called “blowholes,” from the peculiar mode of respiration of those animals. The nostrils are kept open by means of cartilages surrounding their aperture, which many animals have the power of moving so as to cause partial dilatation or contraction. In diving animals, as Seals and Cetacea, they can be completely closed at will so as to prevent the entrance of water when beneath the surface. The passage to which the nostrils lead is in most mammals filled by a more or less complex sieve-like apparatus, formed of the convoluted turbinal bones and cartilages, over which a moist, vascular, ciliated mucous membrane is spread, which intercepts particles of dust, and also aids in warming the inspired air before it reaches the lungs. In the Proboscidea, in which these functions are performed by the walls of the long tubular proboscis, this apparatus is entirely wanting.
_Trachea._—The narial passages have the organ of smell situated in their upper part, and communicate posteriorly with the pharynx, and through the glottis with the “trachea” or windpipe, a tube by which the air is conveyed to and from the lungs. The permanent patency of the trachea during the varied movements of the neck is provided for by its walls being stiffened by a series of cartilaginous rings or hoops, which in most mammals are incomplete behind. Having entered the thorax, the trachea bifurcates into the two bronchi, one of which enters, and, dividing dichotomously, ramifies through each lung. In some of the Cetacea and Artiodactyla a third bronchus is given off from the lower part of the trachea, above its bifurcation and enters the right lung.
_Larynx._—The upper end of the trachea is modified into the organ of voice or “larynx,” the air passing through which to and from the lungs is made use of to set the edges of the “vocal cords,” or fibrous bands stretched one on each side of the tube, into vibration. The larynx is composed of several cartilages, such as the “thyroid,” the “cricoid,” and the “arytenoid” which are moved upon one another by muscles, and suspended from the hyoidean arch. By alteration of the relative position of these cartilages the cords can be tightened or relaxed, approximated or divaricated, as required to modulate the tone and volume of the voice. A median tongue-shaped fibro-cartilage at the top of the larynx, the “epiglottis,” protects the “glottis,” or aperture by which the larynx communicates with the pharynx, from the entry of particles of food during deglutition. The form of the larynx and development of the vocal cords present many variations in different members of the class, the greatest modification from the ordinary type being met with in the Cetacea, where the arytenoid cartilages and epiglottis are united in a tubular manner, so as to project into the nasal passage, and, being grasped by the muscular posterior margin of the palate, provide a direct channel of communication from the lungs to the external surface. An approach to this condition is met with in the Hippopotamus and some other Ungulates; it is indeed so general as an abnormality, that Howes suggests that an internarial epiglottis may have been a primitive feature common throughout the class. Nearly all mammals have a voice, although sometimes it is only exercised at seasons of sexual excitement. Some Marsupials and Edentates appear to be quite mute. In no mammal is there an inferior larynx, or “syrinx,” as in birds.
_Diaphragm._—The thoracic cavity of mammals differs from that of the Sauropsida in being completely separated from the abdomen by a muscular partition, the “diaphragm,” attached to the vertebral column, the ribs, and the sternum. This is much arched, with the convexity towards the thorax, so that when its fibres contract and it is flattened the cavity of the thorax is increased, and when they are relaxed the cavity is diminished.
_Lungs._—The lungs are suspended freely in the thorax, one on each side of the heart, being attached only by the root, which consists of the bronchus or air-tube and pulmonary arteries and veins by which the blood is passed backwards and forwards between the heart and the lungs. The remaining part of the surface of each lung is covered by serous membrane, the “pleura”; and whatever the state of distension or contraction of the chest-wall, is accurately in contact with it. Inspiration is effected by the contraction of the diaphragm and by the intercostal and other muscles elevating or bringing forward the ribs, and thus throwing the sternum farther away from the vertebral column. As the surface of the lung must follow the chest-wall, the organ itself is expanded, and air rushes in through the trachea to fill all the minute cells in which the ultimate ramifications of the bronchi terminate. In ordinary expiration very little muscular power is expended, the elasticity of the lungs and surrounding parts being sufficient to cause a state of contraction and thus drive out at least a portion of the air contained in the cells, when the muscular stimulus is withdrawn. The lungs are sometimes simple externally, as in the Sirenia (where they are greatly elongated) and the Cetacea, but are more often divided by deep fissures into one or more lobes. The right lung is usually larger and more subdivided than the left. It often has a small distinct lobe behind, wanting on the left side, and hence called _lobulus azygos_.
_Air-sacs._—Most mammals have in connection with the air passages certain diverticuli or pouches containing air, the use of which is not always easy to divine. The numerous air sinuses situated between the outer and inner tables of the bones of the head, represented in Man by the antrum of Highmore and the frontal and sphenoidal sinuses, and attaining their maximum of development in the Indian Elephant, are obviously for the mechanical purpose of allowing expansion of the osseous surface without increase of weight. They are connected with the nasal passages. The Eustachian tubes pass from the back of the pharynx to the cavity of the tympanum, into which and the mastoid cells they allow air to pass. In the _Equidæ_ there are large post-pharyngeal air-sacs in connection with them. The Dolphins have an exceedingly complicated system of air-sacs in connection with the nasal passages just within the nostrils, and the Tapirs, Rhinoceroses, and Horses have blind sacs in the same situation. In the males of some Seals (_Cystophora_ and _Macrorhinus_) large pouches, which the animal can inflate with air, and which are not developed in the young animal or the female, arise from the upper part of the nasal passages, and lie immediately under the skin of the face. These appear analogous, although not in the same situation, to the gular pouch of the male Bustard. The larynx frequently has membranous pouches in connection with it, into which air passes. These may be lateral and opening just above the vocal cords, when they constitute the _sacculi laryngis_, found in a rudimentary state in Man, and attaining an enormous development, so as to reach to the shoulders and axillæ, in some of the Anthropoid Apes; or they may be median, opening in front either above or below the thyroid and cricoid cartilages, as in the Howling and other Monkeys, and also in the Whalebone Whales and Great Anteater.
_Urinary Organs._—The kidneys of mammals are more compact and definite in form than in other vertebrates, being usually more or less oval, with an indent on the side turned towards the middle line, from and into which the vessels and ducts pass. They are distinctly divided into a cortical secretory portion, composed mainly of convoluted tubes, and containing the so-called Malpighian bodies; and a medullary excreting portion, formed of straight tubes converging towards a papilla, embraced by the commencement of the ureter or duct of the organ. The kidneys of some mammals, as most Monkeys, Carnivores, Rodents, etc., are simple, with a single papilla into which all the renal tubuli enter. In others, as Man, there are many pyramids of the medullary portion, each with its papilla, opening into a division (calyx) of the upper end of the ureter. Such kidneys, either in the embryonic condition only, or throughout life, are lobulated on the surface. In some cases, as in Bears, Seals, and especially the Cetacea, the lobulation is carried further, the whole organ being composed of a mass of renules, loosely united by connective tissue, and with separate ducts, which soon join to form the common ureter.
_Bladder._—In all mammals except the Monotremes the ureters terminate by slit-like valvular openings in the urinary bladder. This receptacle when filled discharges its contents through the single median urethra, which in the male is almost invariably included in the penis, and in the females of some species of Rodents, Insectivores, and Lemurs has a similar relation to the clitoris. In the Monotremes, though the bladder is present, the ureters do not enter into it, but join the urino-genital canal some distance below it, with the orifice of the genital duct intervening.
VI. NERVOUS SYSTEM AND ORGANS OF SENSE.
_Brain._—The brain of mammals shows a higher condition of organisation than that of other vertebrates. The cerebral hemispheres have a greater preponderance compared with other parts, especially to the so-called optic lobes, or corpora quadrigemina, which are completely concealed by them. The commissural system of the hemispheres is much more complex, both fornix and corpus callosum being present in some form; and when the latter is rudimentary, as in Marsupials and Monotremes, its deficiency is made up for by the great size of the anterior commissure. The lateral lobes of the cerebellum, wanting in lower vertebrates, are well developed and connected by a transverse commissure, the pons Varolii. The whole brain, owing especially to the size of the cerebral hemispheres, is considerably larger relatively to the bulk of the animal than in other classes, but it must be recollected that the size of its brain depends upon many circumstances besides the degree of intelligence which an animal possesses, although this is certainly one. Man’s brain is many times larger than that of all other known mammals of equal bulk, and even three times as large as that of the most nearly allied Ape. Equal bulk of body is here mentioned, because, in drawing any conclusions from the size of the brain compared with that of the entire animal, it is always necessary to take into consideration the fact that in every natural group of closely allied animals the larger species have much smaller brains relatively to their general size than the smaller species, so that, in making any effective comparison among animals belonging to different groups, species of the same size must be selected. It may be true that the brain of a Mouse is, as compared with the size of its body, larger than that of a Man, but, if it were possible to reduce an animal having the general organisation of a Man to the size of a Mouse, its brain would doubtless be very many times larger; and conversely, as shown by the rapid diminution of the relative size of the brain in all the large members of the Rodent order, a Mouse magnified to the size of a Man would, if the general rule were observed, have a brain exceedingly inferior in volume. Although the brain of the large species of Whales is, as commonly stated, the smallest in proportion to the bulk of the animal of any mammal, this does not invalidate the general proposition that the Cetacea have very large brains compared with terrestrial mammals, like the Ungulata, or even the aquatic Sirenia, as may be proved by placing the brain of a Dolphin by the side of that of a Sheep, a Pig, or a Manatee of equal general weight. It is only because the universally observed difference between the slower ratio of increase of the brain compared with that of the body becomes so enormous in these immense creatures that they are accredited with small brains.
The presence or absence of “sulci” or fissures on the surface of the hemisphere, dividing it into “convolutions” or “gyri,” and thus increasing the superficies of the cortical gray matter, as well as allowing the pia mater with its nutrient blood-vessels to penetrate into the cerebral substance, follow somewhat similar rules. The sulci are related partly to the high or low condition of organisation of the species, but also in a great degree to the size of the cerebral hemispheres. In very small species of all groups, even the Primates, they are absent, and in the largest species of groups so low in the scale as the Marsupials and Edentates they are found. They reach their maximum of development in the Cetacea.
The accompanying woodcut (Fig. 23) shows the principal parts of a mammalian brain, as seen from the superior, lateral, and inner surfaces. The sylvian fissure (_sf_) is one of the most constant of the sulci found in the hemispheres.
[Illustration: FIG. 23.—Brain of the Genet (_Genetta tigrina_). A, From above; B, from the right side; C, inner surface of right hemisphere; _cc_, corpus callosum; _c.m.s_, calloso-marginal sulcus; _c_, notch representing central sulcus of other forms; _d_, depression on superior lateral gyrus of hemisphere; _hg_, hippocampal gyrus; _i_, inferior lateral gyrus of hemisphere; _m_, middle lateral gyrus of do.; _s_, superior lateral gyrus of do.; _os_, supraorbital sulcus of do.; _sf_, sylvian fissure of do.; _ol_, olfactory lobes. The deeply convoluted part behind the cerebral hemisphere is the cerebellum, below which lies the medulla oblongata, or commencement of the spinal cord. (Mivart, _Proc. Zool. Soc._ 1882, p. 516.)]
The researches of Palæontologists, founded upon studies of casts of the interior of the cranial cavity of extinct forms, have shown that, in many natural groups of mammals, if not in all, the brain has increased in size, and also in complexity of surface foldings, with the advance of time,—indicating in this, as in so many other respects, a gradual progress from a lower to a higher type of development.
_Nerves._—The twelve pairs of cranial nerves generally recognised in vertebrates are usually all found in mammals, though the olfactory nerves are excessively rudimentary, if not altogether absent, in the Toothed Whales. The spinal cord, or continuation of the central nervous axis, lies in the canal formed by the neural arches of the vertebræ, and gives off the compound double-rooted nerves of the trunk and the extremities, corresponding in number to the vertebræ, through the interspaces between which they pass out to their destination. The cord is somewhat enlarged at the two points where it gives off the great nerves to the anterior and the posterior extremities, which, from their interlacements soon after their origin, are called respectively the brachial and lumbar plexuses. The ganglionic or sympathetic portion of the nervous system is well developed, and presents few modifications.
_Sense of Touch._—The sense of touch is situated in the skin generally, but is most acute in certain regions more or less specialised for the purpose by the presence of tactile papillæ, such as portions of the face, especially the lips and end of the snout, and the extremities of the limbs when these are used for other purposes than mere progression, and the under surface of the end of the tail in some Monkeys. The “vibrissæ” or long stiff bristles situated on the face of many mammals are rendered extremely sensitive to touch by the abundant supply of branches from the fifth nerve to their basal papillæ. In Bats the extended wing membranes, and probably also the large ears and the folds and prominences of skin about the face of some species, are so sensitive as to receive impressions even from the different degrees of resistance of the air, and so enable the animals to avoid coming in contact with obstacles to their nocturnal flight.
_Taste and Smell._—The organs of the other special senses are confined to the head. Taste is situated in the papillæ scattered on the dorsal surface of the tongue. The organ of smell is present in all mammals except the Toothed Whales. It consists of a ramification of the olfactory nerves over a plicated, moist, mucous membrane, supported by folded plates of bone, placed on each side of the septum nasi in the roof, or often in a partially distinct upper chamber, of the nasal passage, so arranged that, of the air passing into the lungs in inspiration, some comes in contact with it, causing the perception of any odorous particles with which it may be charged. Many mammals possess intense powers of smelling certain odours which others are quite unable to appreciate, and the influence which this sense exercises over the well-being of many species is very great, especially in indicating the proximity of others of the same kind, and giving warning of the approach of enemies. The development and modification of the sense of smell is probably associated with that of the odorous secretion of the cutaneous glands.
_Sight._—The organ of sight is quite rudimentary, and even concealed beneath the integument, in some burrowing Rodents and Insectivores, and is most imperfectly developed in the _Platanista_, or Freshwater Dolphin of the rivers of India. In all other mammals the eyeball has the structure characteristic of the organ in the higher Vertebrata, consisting of parts through which the rays of light are admitted, regulated, and concentrated upon the sensitive expansion of the optic nerve lining the posterior part of the ball. A portion of the fibrovascular and highly pigmented layer, the choroid, which is interposed between the retina and the outer sclerotic coat, is in many mammals modified into a brilliantly-coloured light-reflecting surface, the _tapetum lucidum_. There is never a pecten or marsupium like that of the Sauropsida, nor is the sclerotic ever supported by a ring of flattened ossicles, as is so frequently the case in the lower vertebrated classes. The eyeball is moved in various directions by a series of muscles—the four straight, two oblique, and, except in the higher Primates, a posterior retractor muscle called choanoid. The superior oblique muscle passes through a tendinous pulley fastened to the roof of the orbit, which is a feature not found beyond the limits of the mammalian class. The eye is protected by the lids, generally distinctly separated into an upper and a lower movable flap, which, when closed, meet over the front of the eye in a more or less nearly horizontal line: but sometimes, as in the Sirenia, the lids are not distinct, and the aperture is circular, closing to a point. In almost all mammals below the Primates, except the Cetacea, a “nictitating membrane” or third eyelid is placed at the inner corner of the eyeball, and works horizontally across the front of the ball within the true lids. Its action is instantaneous, being apparently for the purpose of cleaning the front of the transparent cornea;—a function unnecessary in animals whose eyes are habitually bathed in water, and which in Man and his nearest allies is performed by winking the true eyelids. Except in Cetacea the surface of the eye is kept moist by the secretion of the lachrymal gland, placed under the upper lid at its outer side, and the lids are lubricated by the Harderian and Meibornian glands, the former being situated at the inner side of the orbit, and especially related to the nictitating membrane, the latter in the lining membrane of the lids.
_Hearing._—The organ of hearing is inclosed in a bony capsule (periotic) situated in the side of the head, intercalated between the posterior (occipital) and the penultimate (parietal) segment of the skull. It has, in common with other vertebrates, three semicircular canals and a vestibule, but the cochlea is more fully developed than in the Sauropsida, and, except in the Monotremes, spirally convoluted. The tympanic cavity is often dilated below, forming a smooth rounded prominence on the base of the skull, the auditory bulla (Fig. 8). The three principal ossicles, the “malleus,” “incus,” and “stapes,” are always present, but variable in characters. In the Sirenia, Cetacea, and Seals they are massive in form, being in the first-named order of larger size than in any other mammals. In the Cetacea the malleus is ankylosed to the tympanic; but in other mammals it is connected only with the membrana tympani. The stapes in the lower orders—Edentates, Marsupials, and Monotremes—has a great tendency to assume the columnar form of the corresponding bone in Sauropsida, its two rami entirely or partially coalescing.[16] The tympanic membrane (drum of the ear) forms the outer wall of the cavity. In the fœtal state it is level with the external surface of the skull, and remains so permanently in a few mammals as the American Monkeys; but commonly, by the growth of the squamosal bone, it becomes deeply buried at the bottom of a bony tube (_meatus auditorus externus_), which is continued to the surface of the skin in a fibrous or fibro-cartilaginous form. In Whales, owing to the thickness of the subcutaneous adipose tissue, this meatus is of great length, and is also extremely narrow. In most aquatic and burrowing animals it opens upon the surface by a simple aperture, but in the large majority of the class there is a projecting fold of skin, strengthened by fibro-cartilages, called the pinna, auricle, or “external ear,” of very variable size and shape, generally movably articulated on the skull, and provided with muscles to vary its position; this pinna helping to collect and direct the vibrations of sound into the meatus.
VII. REPRODUCTIVE ORGANS.
_Testes._—In the male the testes retain nearly their primitive or internal position throughout life in the Monotremata, Sirenia, Cetacea, most Edentata, Hyracoidea, Proboscidea, and Seals, but, in other groups they either periodically (as in Rodentia, Insectivora, and Chiroptera) or permanently pass out of the abdominal cavity through the inguinal canal, forming a projection beneath the skin of the perineum, or becoming suspended in a distinct pouch of integument called the scrotum. All the Marsupials have a pedunculated scrotum, the position of which differs from that of other mammals, being in front of, instead of behind, the preputial orifice. As regards the presence, absence, or comparative size and number of the accessory generative glands—prostate, vesicular, and Cowper’s glands, as they are called—there is much variation in different groups of mammals.
_Penis._—The penis is almost always completely developed, consisting of two corpora cavernosa attached to the ischial bones, and of a median corpus spongiosum enclosing the urethra, and forming the glans at the distal portion of the organ. In Marsupials, Monotremes, and the Sloths and Anteaters, the corpora cavernosa are not attached directly to the ischia, and in the last-named the penis is otherwise of a very rudimentary character, the corpus spongiosum not being present. In many Marsupials the glans penis is bifurcated. In most Primates, Carnivora, Rodentia, Insectivora, and Chiroptera, but in no other orders, an _os penis_ is present.
_Ovaries and Oviduct._—In the female, the ovaries permanently retain their original abdominal position, or only descend a short distance into the pelvis. They are of comparatively smaller size than in other vertebrates, have a definite flattened oval form, and are enclosed in a more or less firm “tunica albiginea.” The oviduct has a trumpet-like, and usually fimbriated abdominal aperture, and is more or less differentiated into three portions:—(1) a contracted upper part, called in Man and the higher mammals the “Fallopian tube”; (2) an expanded part with muscular walls, in which the ovum undergoes the changes by which it is developed into the fœtus, called the “uterus”; (3) a canal, the “vagina,” separated from the last by a valvular aperture, and terminating in the urino-genital canal, or common urinal and genital passage, which in higher mammals is so short as scarcely to be distinct from the vagina. The complete distinction of the oviducts of the two sides throughout their whole length, found in all lower vertebrates, only occurs in this class in Monotremes; a prevailing mammalian characteristic being their more or less perfect coalescence in the middle line to form a single median canal. In the Marsupials this union only includes the lower part of the vagina; but in most Placentals it extends to the whole vagina and a certain portion of the uterus, which cavity is then described as “bicornuate.” In the higher mammals, as in Man, and also in some of the Edentates, the whole of the uterus is single, the contracted upper portion of the oviducts or Fallopian tubes, as they are then called, entering its upper lateral angles by small apertures. In certain lower forms the urino-genital canal opens with the termination of the rectum into a common cloaca, as in other vertebrates; but it is characteristic of the majority of the class that the two orifices are more or less distinct externally.
_Mammary Glands._—Mammary glands secreting the milk by which the young are nourished during the first portion of their existence after birth, are present in both sexes in all mammals, though usually only functional in the female. In the Monotremes alone their orifices are mere scattered pores in the skin, but in all other forms they are situated upon the end of conical elevations, called mammillæ, or teats, which, taken into the mouth of the young animal, facilitate the process of sucking. These are always placed in pairs upon some part of the ventral surface of the body, but vary greatly in number and position in different groups. In the Cetacea, where the prolonged action of sucking would be incompatible with their subaqueous life, the ducts of the glands are dilated into large reservoirs from which the contents are injected into the mouth of the young animal by the action of a compressor muscle.
_Secondary Sexual Characters._—Secondary sexual characters, or modifications of structure peculiar to one sex, but not directly related to the reproductive function, are very general in mammals. They almost always consist of the acquisition or perfection of some character by the male as it attains maturity, which is not found in the female or the young in either sex. In a large number of cases these clearly relate to the combats in which the males of many species engage for the possession of the females during the breeding season; others are apparently ornamental, and of many it is still difficult to apprehend the meaning. Many suggestions on this subject will, however, be found in the chapters devoted to it in Darwin’s work on _The Descent of Man and Selection in Relation to Sex_, where most of the best-known instances are collected. Superiority of size and strength in the male of many species is a well-marked secondary sexual character related to the purpose indicated above, being probably perpetuated by the survivors or victors in combats transmitting to their descendants those qualities which gave them advantages over others of their kind. To the same category belong the great development of the canine teeth of the males of many species which do not use these organs in procuring their food, as the Apes, Swine, Musk and some other Deer, the tusk of the male Narwhal, the antlers of Deer, which are present in most cases only in the males, and the usual superiority in size and strength of the horns of the _Bovidæ_. Other secondary sexual characters, the use of which is not so obvious, or which may only relate to ornament, are the presence of masses or tufts of long hair on different parts of the body, as the mane of the male Lion and Bison, the beards of some Ruminants and Bats (as _Taphozous melanopogon_), Monkeys, and of Man, and all the variations of coloration in the sexes, in which, as a general rule, the adult male is darker and more vividly coloured than the female. Here may also be mentioned the presence or the greater development of odoriferous glands in the male, as in the Musk Deer, and the remarkable perforated spur with its glands and duct, so like the poison-tooth of the venomous serpents, found in the males of both _Ornithorhynchus_ and _Echidna_, the use of which is at present unknown.
_Placenta._—The development of the mammalian ovum, and the changes which the various tissues and organs of the body undergo in the process of growth, are too intricate subjects to be explained without entering into details incompatible with the limits of this work, especially as they scarcely differ, excepting in their later stages, from those of other vertebrates, upon which, owing to the greater facilities these present for examination and study, the subject has been more fully worked out. There are, however, some points which require notice, as peculiar to the mammalian class, and as affording at least some hints upon the difficult subject of the affinities and classification of the members of the group.
The nourishment of the fœtus during intra-uterine life takes place through the medium of certain structures, partly belonging to the fœtus itself and partly belonging to the inner parietes of the uterus of the parent. These in their complete form constitute the complex organ called the “placenta,” serving as the medium of communication between the mother and fœtus, and in which the physiological processes that are concerned in the nutrition of the latter take place; but as we shall see, though a placenta, in the usual acceptation of the term, is peculiar to the mammalian class, it is not in all of its members that one is developed. The structures to which we shall have especially to refer are the outer tunic of the ovum, to which, however formed, the term “chorion” is commonly applied, and two sac-like organs connected with the body-cavity of the embryo, both formed from the splanchnic mesoblast, lined by a layer of the hypoblast. These are the “umbilical vesicle” or “yolk-sac” and the “allantois.”
The umbilical vesicle is a thin membrane enclosing the yolk, which by the doubling in of the ventral walls of the embryo becomes gradually formed into a distinct sac external to the body, with a pedicle (the omphalo-enteric duct) by which for a time a communication is maintained between its cavity and the intestinal canal. In the walls of this sac blood-vessels (omphalo-meseraic or vitelline) are developed in connection with the vascular system of the embryo, through which, either by their contact with the outer surface of the walls of the ovum, or by the absorption through them of the contents of the yolk-sac, the nutrition of the embryo in the lower vertebrates chiefly takes place. In mammals the umbilical vesicle plays a comparatively subordinate part in the nourishment of the fœtus, its function being generally superseded by the allantois.
The last-named sac commences at a very early period as a diverticulum from the hinder end of the alimentary tract of the embryo. Its proximal portion afterwards becomes the urinary bladder, the contracted part between this and the cavity of the allantois proper constituting the urachus, which passes out of the body of the fœtus at the umbilicus together with the vitelline duct. The mesoblastic tissue of the walls of the allantois soon becomes vascular; its arteries are supplied with fœtal blood by the two hypogastric branches of the iliacs, or main divisions of the abdominal aorta, and the blood is returned by venous trunks uniting to form the single umbilical vein which runs to the under surface of the liver, where, part of it joining the portal vein and part entering the vena cava directly, it is brought to the heart. These are the vessels which, with their surrounding membranes, constitute the umbilical cord—the medium of communication between the fœtus and the placenta, when that organ is fully developed.
The egg membranes of the Monotremes present many points of agreement with those of the ovum of the Marsupials,[17] and differ from those of the Placental types. Thus Monotremes and Marsupials agree in having a vitelline membrane, which appears between the young ovum and the follicular epithelium, persisting in the one case until the time of hatching, and in the other till a late uterine stage. There are also several other common features fully described in Mr. Caldwell’s memoir, but which cannot be detailed in this work.
In the Marsupialia the observations made many years ago by Sir R. Owen upon the development of the Kangaroo have been confirmed by those of Dr. H. C. Chapman,[18] while Dr. Selenka,[19] and Professor H. F. Osborn[20] have contributed important evidence as to the structure and relations of the fœtal membranes of the Opossums and others. It thus appears that up to the period of the very premature birth of these animals the outer covering of the ovum, or false chorion, is free from persistent villi, and not adherent to the epithelium of the uterine walls; for, although fitting into the folds of the latter, it is perfectly and readily separable in its entire extent from them. The umbilical vesicle or yolk-sac is large, vascular, and adherent to a considerable portion of the false chorion or subzonal membrane, while the allantois is relatively small, and although the usual blood-vessels can be traced into it, it does not appear to contract any connection with the false chorion, and, therefore, much less with the walls of the uterus, of such a nature as to constitute a placenta. In some forms, however, such as the Opossums, the umbilical vesicle or yolk-sac develops temporary villi, which unite with the subzonal membrane, or false chorion, to form a disc-like area closely attached to the cells covering the utricular glands of the uterine epithelium, and thus forming a so-called _yolk-sac placenta_. The function of this organ is considered to be the transmission of the secretions of the utricular glands to the embryo by means of the umbilical vesicle; the function of the allantois being either respiratory or the absorption of the fluid secreted in the uterine cavity by the utricular glands.
While in the uterus the nourishment of the fœtus seems, therefore, to be derived from the umbilical vesicle, as in reptiles and birds, rather than from the uterine walls by means of the allantoic vessels, as in the higher mammals. The latter vessels, in fact, play even a much less important part in the development of these animals, not only than in the placental mammals, but even than in the Sauropsida, for they can scarcely have the respiratory function assigned to them in that group: pulmonary respiration and the lacteal secretion of the mother very early superseding all other methods of providing the due supply, both of oxygen and of food required for the development and growth of the young animal. In this sense the Marsupials may be looked upon as the most typically “mammalian” of the whole class. In no other group do the milk-secreting glands play such an important part in providing for the continuity of the race.
In the third primary division of the Mammalia, the so-called Placentalia, the umbilical vesicle generally does not quite unite with the chorion, and disappears as development proceeds, so that no trace of it can be seen in the membranes of an advanced embryo; but it may persist until the end of the intra-uterine life as a distinct sac in the umbilical cord, or lying between the allantois and amnion. The disappearance or persistence of the umbilical vesicle does not, according to our present knowledge, appear to be correlated with a higher or lower general grade of development, as might be presupposed. It is stated to have been found in Man even up to the end of intra-uterine life, and also in the Carnivora, while in the Ungulata and Cetacea it disappears at an earlier age. In many, if not all, of the Rodentia, Insectivora, and Chiroptera, it plays a more important part, becoming adherent to a considerable part of the inner surface of the chorion, to which it conveys blood-vessels, although villi do not appear to be developed from the surface of this part, as they are on the portion of the chorion supplied by the allantoic vessels. These orders thus present to a certain extent a transitional condition from the Marsupials, although essentially different, in possessing the structures next to be described.
The special characteristic of the whole of the placental mammals constituting the majority of the class, is that the allantois and its vessels become intimately blended with a smaller or greater part of the parietes of the ovum, forming a structure on the outer surface of which villi are developed, and which, penetrating into corresponding cavities of the “decidua,” or soft, vascular, hypertrophied lining membrane of the uterus, constitutes the placenta. This organ may be regarded, as Sir William Turner says, both in its function and in the relative arrangement of its constituent textures, as a specially modified secreting gland, the ducts of which are represented by the extremities of the blood-vessels of the fœtal system. The passage of material from the maternal to the fœtal-system of vessels is not a simple percolation or diffusion through their walls, but is occasioned by the action of a layer of cells derived from the maternal or uterine structures, and interposed between the blood-vessels of the maternal part of the placenta and those of the villi covering the chorion, in which the embryonic vessels ramify.
The numerous modifications in the details of the structure of this organ relate to augmenting the absorbing capacity of the vessels of the chorion, and are brought about either by increasing the complexity of the fœtal villi and maternal crypts over a limited area, or by increasing the area of the part of the chorion covered by the placental villi, or by various combinations of the two methods.
The first class of variations has given rise to a distinction into two principal kinds of placenta: (1) simple or non-deciduate, and (2) deciduate. In the former the fœtal villi are received into corresponding depressions of the maternal surface, from which at the period of parturition they are simply withdrawn. In the second, or more complex form, the relation is more intimate, a layer of greater or less thickness of the lining membrane of the uterus, called “decidua,” becoming so intimately blended with the chorion as to form part of the placenta proper, or that structure which is cast off as a solid body at parturition. In other words, in the one case the line of separation between the placenta and uterus at birth takes place at the junction of the fœtal and maternal structures, in the other through the latter, so that a portion of them, often of considerable thickness, and containing highly organised structures, is cast off with the former. It was once thought that the distinction between these two forms of placentation is so important as to constitute a sufficiently valid basis for a primary division of the placental mammals into two groups. It has, however, been shown that the distinction is one rather of degree than of kind, as intermediate conditions may exist, and it is probable that in different primary groups the simpler, non-deciduate form may have become developed independently into one or other of the more complex kinds.
Apart from its intimate structure, the placenta may be met with of very varied general form. It may consist of villi scattered more or less regularly over the greater part of the surface of the chorion, the two extremities or poles being usually more or less bare. This form is called the “diffused placenta.” It is probably a primitive condition, from which most of the others are derived, although its existence must presuppose the absence of the umbilical vesicle as a constituent of the chorionic wall. It is found at present in the Manis among Edentates, the Cetacea, the Perissodactyle Ungulates, and the Camels, Pigs, and Chevrotains among the Artiodactyles. Such placentæ are always non-deciduate. Recent observations by Sir W. Turner on the placentation of the Dugong show that the Sirenia present the peculiarity of having a zonary placenta, which is either entirely or in great part non-deciduate, and is, therefore, transitional between the diffused and the true zonary type.
In the true Ruminants or Pecora, among the Artiodactyle Ungulates, the villi are aggregated in masses called cotyledons, with bare spaces between. Such a placentation is called “polycotyledonary.” In another modification the villi are collected in a more or less broad band encircling the chorion, leaving a very large portion of the two poles bare, constituting the “zonary placenta,” characteristic of the Carnivora, and also occurring in the Elephant, Hyrax, and Orycteropus. The fact of the form of the placenta of these three last-named animals agreeing together, and with that of the Carnivora, does not, however, necessitate the ascription of zoological affinities, as the same ultimate form may have been attained by different processes of development.
In another form one pole only of the chorion is non-vascular, the placenta assuming a dome or bell shape, as in the Lemurs and the Sloths. The transition from this, by the gradual restriction of the vascular area, is easy to the oval or discoidal form of placenta of the Anteaters, Armadillos, and higher Primates. The discoidal placenta of the Rodents, Insectivores, and Chiroptera, though showing so much superficial resemblance to that of the last-named order as to have led to the inclusion of all these forms in one primary group, is now known to be developed in another manner, not by the concentration of villi from a diffused to a limited area, but by retaining the area to which it was originally restricted in consequence of the large surface of the chorion occupied, as before mentioned, by the umbilical vesicle. To compensate for the smallness of area, the complex or deciduate structure has been developed. Among some Rodents there is evidence to show that the discoidal placenta has been derived from a zonary one, of which distinct vestiges have been detected in the Mouse. We may conclude that, although the characters and arrangement of the fœtal structures may not have that extreme importance which has been attributed to them by some zoologists, they will form, especially when more completely understood, valuable aids in the study of the natural affinities and evolution of the Mammalia.[21]