CHAPTER IV

THE SKELETON AND TEETH OF MAMMALS

With very rare exceptions, and those only of the latest geological period (Quaternary), the fossil remains of mammals consist only of bones and teeth. The evolutionary changes, so far as these are preserved, are recorded therefore in terms of dental and skeletal modifications. To render these changes intelligible, it is necessary to give some account of the mammalian skeleton and teeth, with no more use of technical language than is unavoidable; ordinary speech does not furnish a sufficient number of terms, nor are most of these sufficiently precise. With the aid of the figures, the reader may easily gain a knowledge of the skeleton which is quite adequate for the discussion of fossil series, which will follow in the subsequent chapters.

I. THE SKELETON

I. The most obvious distinction of the skeletal parts is into _axial_ and _appendicular_ portions, the former comprising the skull, backbone or _vertebral column_, ribs and breast-bone or _sternum_, and the latter including the limb-girdles, limbs and feet. In the axial skeleton only the ribs and certain bones of the skull are paired, but in the appendicular all the bones are in pairs, for the right and left sides respectively.

[Illustration: FIG. 7.—Skull of Wolf (_Canis occidentalis_). _P.Mx._, premaxillary. _Mx._, maxillary. _Na._, nasal. _L._, lachrymal. _Ma._, malar or jugal. _Fr._, frontal. _Pa._, parietal. _Sq._, squamosal. _Zyg._, zygomatic process of squamosal. _O.S._, orbitosphenoid. _Pl._, palatine. _M._, mandible. _cor._, coronoid process of mandible. _m.c._, condyle of mandible. _ang._, angular process of mandible. _p.g._, postglenoid process of squamosal. _Ty._, tympanic (auditory bulla). _mas._, mastoid. _p.oc._, paroccipital process. _con._, occipital condyle. _Ex.O._, exoccipital. _S.O._, supraoccipital.]

The _skull_ is a highly complex structure, made up of many parts, most of which are immovably fixed together, and performing many functions of supreme importance. In the first place, it affords secure lodgement and protection for the brain and higher organs of sense, those of smell, sight and hearing, and second, it carries the teeth and, by its movable jaws, enables these to bite, to take in and masticate food. The portion of the skull which carries the brain, eyes and ears, is called the _cranium_, and the portion in front of this is the face, the boundary between the two being an oblique line drawn immediately in front of the eye-socket (Fig. 7). A great deal of the endless variety in the form of the skull of different mammals depends upon the differing proportions of cranium and face. In the human skull, for example, the cranium is enormously developed and forms a great dome, while the face is shortened almost to the limit of possibility; the skull of the Horse, on the other hand, goes to nearly the opposite extreme of elongation of the facial and shortening of the cranial region. The posterior surface of the skull, or _occiput_, is made up of four bones, which in most adult mammals are fused into a single _occipital_ bone. At the base of the occiput is a large opening, the _foramen magnum_, through which the spinal cord passes to its junction with the brain; and on each side of the opening is a large, smooth, oval prominence, the _occipital condyles_, by means of which the skull is articulated with the neck. The _paroccipital processes_ are bony styles of varying length, which are given off, one on each side external to the condyles. The boundary of the occiput is marked by a ridge, the _occipital crest_, which varies greatly in prominence, but is very well marked in the more primitive forms and tends to disappear in the more highly specialized ones. The roof and much of the sides of the cranium are formed by two pairs of large bones, the _parietals_ behind and the _frontals_ in advance; along the median line of the cranial roof, where the two parietals meet, is usually another ridge, the _sagittal crest_, which joins the occipital crest behind. The sagittal crest also varies greatly in prominence, being in some mammals very high and in others entirely absent, and, like the occipital crest, is a primitive character; as a rule, it is longest and highest in those mammals which have the smallest brain-capacity. As pointed out by Professor Leche, the development of the sagittal crest is conditioned by the relative proportions of the brain-case and the jaws. Powerful jaws and a small brain-case necessitate the presence of the crest, in order to provide sufficient surface of attachment for the temporal muscles, which are important in mastication, while with large brain-case and weak jaws the crest is superfluous. Though the brain-case proper may be quite small, yet it may have its surface enormously increased by great thickening of the cranial bones, as is true of elephants and rhinoceroses, and in them sufficient surface for attachment is afforded to the muscles without the development of a crest.

[Illustration: FIG. 8.—Skull of Wolf, top view. _P.Mx._, premaxillary. _Na._, nasal. _Ma._, malar or jugal. _L._, lachrymal. _Fr._, frontal. _Sq._, squamosal. _Pa._, parietal. _S.O._, supraoccipital.]

[Illustration: FIG. 9.—Skull of Wolf, view of base. _P.Mx._, premaxillary. _Mx._, palatine process of maxillary. _Pl._, palatine. _Fr._, frontal. _Pt._, parietal. _Ma._, malar or jugal. _Sq._, glenoid cavity of squamosal. _B.S._, basisphenoid. _B.O._, basioccipital. _Ty._, tympanic (auditory bulla). _p.oc._, paroccipital process. _con._, occipital condyle. _S.O._, supraoccipital.]

The structure of these cranial bones, more particularly of the parietals, is subject to important changes; in most mammals they are of moderate thickness and have dense layers, or “tables,” forming the outer and inner surfaces and, between these, a layer of spongy bone. In many large mammals, however, especially those which have heavy horns or tusks, the cranial bones become enormously thick and the spongy layer is converted into a series of communicating chambers, or _sinuses_, the partitions between which serve as braces, thus making the bone very strong in proportion to its weight. Sinuses are very generally present in the frontals and communicate by small openings with the nasal passage, even in genera of moderate size and without horns or tusks. The frontals form the roof of the eye-sockets, or _orbits_, and usually there is a projection from each frontal, which marks the hinder border of the orbit and is therefore called the _postorbital_ process. The roof of the facial region is made by the _nasals_, which are commonly long and narrow bones, but vary greatly in form and proportions in different mammals; in those which have a proboscis, like tapirs and elephants, or a much inflated snout, such as the Moose (_Alce_) or the Saiga Antelope (_Saiga tatarica_) the nasals are always very much shortened and otherwise modified in form.

The anterior end of the skull is formed by a pair of rather small bones, the _premaxillaries_, which carry the incisor teeth; they bound the sides of the nasal opening, or _anterior nares_, reaching to the nasals, when the latter are of ordinary length; they also form the front end of the hard or bony palate, which divides the nasal passage from the mouth. The _maxillaries_, or upper jaw-bones, make up nearly all of the facial region on each side and send inward to the median line from each side a bony plate which together constitute the greater part of the hard palate; the remainder of the upper teeth are implanted in the maxillaries. A varying proportion of the hinder part of the hard palate is formed by the _palatines_, which also enclose the _posterior nares_, the opening by which the nasal passage enters the back part of the mouth. The maxillary of each side extends back to the orbit, which it bounds anteriorly and in the antero-superior border of which is the usually small _lachrymal_. The inferior, and more or less of the anterior, border of the orbit is made by the cheek-bone (_malar_ or _jugal_) which may or may not have a postorbital process extending up toward that of the frontal; when the two processes meet, the orbit is completely encircled by bone, but only in monkeys, apes and Man is there a bony plate given off from the inner side of the postorbital processes, which extends to the cranial wall and converts the orbit into a funnel-shaped cavity. For most of its length, the jugal projects freely outward from the side of the skull and extends posteriorly beneath a similar bar of bone, the _zygomatic process_ of the _squamosal_. This process and the jugal together constitute the _zygomatic arch_, which on each side of the skull stands out more or less boldly, and the size and thickness of which are subject to great variation in different mammals, the massiveness of the arch being proportional to the power of the jaws. One of the principal muscles of mastication (the _masseter_) is attached to the zygomatic arch.

The squamosal itself is a large plate, which makes up a great part of the side-wall of the cranium and articulates above with the frontal and parietal; it also supports the lower jaw, the articular surface for which is called the _glenoid cavity_. The lower jaw is held in place by the _postglenoid process_, which is a projection, usually a transverse ridge, behind the cavity. Back of the postglenoid process is the entrance to the middle ear, the _auditory meatus_, which may be merely an irregular hole, or a more or less elongated tube. The meatus is an opening into the _tympanic_, a bone which at birth is a mere ring and in some mammals remains permanently in that condition, but as a rule develops into a swollen, olive-shaped _auditory bulla_, which sometimes reaches enormous proportions, especially in nocturnal mammals. The labyrinth of the internal ear is contained in the _periotic_, a very dense bone which is concealed in the interior of the cranium, but in many mammals a portion of it, the _mastoid_, is exposed on the surface between the squamosal and occipital.

The lower jaw-bone (_inferior maxillary_, or _mandible_) is the only freely movable element of the skull; it consists of two halves which meet anteriorly at the chin in a contact of greater or less length, called the _symphysis_. In nearly all young mammals and in many adult forms the two halves of the lower jaw are separate and are held together at the symphysis only by ligaments, while in others, as in Man, they are indistinguishably fused to form a single bone. Each half consists of two portions, a horizontal part or _ramus_ and an _ascending ramus_ or vertical part; the former supports all of the lower teeth, and its length, depth and thickness are very largely conditioned by the number and size of those teeth. The ascending ramus is a broad, rather thin plate, divided at the upper end into two portions, the hinder one of which terminates in the _condyle_, a rounded, usually semicylindrical projection, which fits into the glenoid cavity of the squamosal. The anterior portion of the ascending ramus ends above in the _coronoid process_, which serves for the insertion of the temporal muscle, the upper portion of which is attached to the walls of the cranium and thus, when the muscle is contracted, the jaws are firmly closed; the coronoid process passes inside of the zygomatic arch. The lower jaw is therefore a lever of the third order, in which the power is applied between the weight (_i.e._ the food, the resistance of which is to be overcome) and the fulcrum, which is the condyle. At the postero-inferior end of the ascending ramus is the _angle_, the form of which is characteristically modified in the various mammalian orders and is thus employed for purposes of classification.

The _hyoid arch_ is a U-shaped series of small and slender bones, with an unpaired element closing the arch below; each vertical arm of the U is attached to the tympanic of its own side and the whole forms a flexible support for the tongue, but with no freely movable joint like that between the lower jaw and the squamosal.

The mammalian skull in its primitive form may be thought of as a tube divided into two parts, of which the hinder one is the brain-chamber, or cranial cavity, and the forward one the nasal chamber or passage. With the growth of the brain and consequent enlargement of the cranium, this tubular character is lost; and various modifications of the teeth, jaws and facial region, the development of horns and tusks, bring about the many changes which the skull has undergone.

This brief sketch of the skull-structure is very incomplete, several of its elements having been altogether omitted and only those parts described which are needful in working out the history and descent of the various mammalian groups.

The second portion of the axial skeleton is the backbone, or _vertebral column_, which is made up of a number of separate bones called _vertebræ_. These are so articulated together as to permit the necessary amount of flexibility and yet retain the indispensable degree of strength. The function of the backbone is a twofold one: (1) to afford a firm support to the body and give points of attachment to the limbs, and (2) to carry the spinal cord, or great central axis of the nervous system, in such a manner that it shall be protected against injury, a matter of absolutely vital necessity.

While the vertebræ differ greatly in form and appearance in the various regions of the neck, body and tail, in adaptation to the various degrees of mobility and strength which are required of them, yet they are all constituted upon the same easily recognizable plan. The principal mass of bone in each vertebra is the body, or _centrum_, which is typically a cylinder, or modification of that form, and the two ends of the cylinder are the _faces_, by which the successive vertebræ are in contact with one another. In the living animal, however, the successive centra are not in actual contact, but are separated by disks of _cartilage_ (gristle) which greatly add to the elasticity of the column. From the upper surface of the centrum arises an arch of bone, the _neural arch_, enclosing with the centrum the _neural canal_, through which runs the spinal cord. As already mentioned, the protection of the spinal cord is essential to the life of the animal, yet this protection must be combined with a certain flexibility, both lateral and vertical. Mere contact of the centra, even though these be held in place by ligaments, would not give the column strength to endure, without dislocation, the great muscular stresses which are put upon it. Additional means of articulation between the successive vertebræ are therefore provided, and these vary in size, form and position in different regions of the backbone, in nice adjustment to the amount of motion and degree of strength needed at any particular part of the column. Of these additional means of articulation, which are called the _zygapophyses_, each vertebra has two pairs, an anterior and a posterior pair, placed upon the neural arch. From the summit of the arch arises the _neural spine_, a more or less nearly straight rod or plate of bone, which may be enormously long or extremely short, massive or slender, in accordance with the muscular attachments which must be provided for. Finally, should be mentioned the _transverse processes_, rod-like or plate-like projections of bone, which arise, one on each side of the vertebra, usually from the centrum, less commonly from the neural arch; these also differ greatly in form and size in the various regions of the column. Anatomists distinguish several other processes of the vertebra, but for our purpose it is not necessary to take these into consideration.

[Illustration: FIG. 10.—First dorsal vertebra of Wolf from the front. _cn._, centrum. _r._, facet for the head of the rib. _r′._, facet for the tubercle of the rib. _tr._, transverse process. _pr.z._, anterior zygapophyses. _n.sp._, neural spine.]

Five different regions of the backbone may be distinguished, in each of which the vertebræ are modified in a characteristic way. There is (1) the _cervical_ region, or neck, the vertebræ of which, among mammals (with only one or two exceptions) are always seven in number, however long or short the neck may be; the immoderately long neck of the Giraffe has no more and the almost invisible neck of the Whale has no less, and thus the elongation of the neck is accomplished by lengthening the individual vertebræ and not by increasing their number. (2) Those vertebræ to which ribs are attached are named _dorsal_ or _thoracic_ and can always be recognized by the pits or articular facets which receive the heads of the ribs. (3) Behind the dorsal is the _lumbar_ region, or that of the loins, made up of a number of vertebræ which carry no ribs. The dorso-lumbars are known collectively as the _trunk-vertebræ_ and are generally quite constant in number for a given group of mammals, though often differently divided between the two regions in different members of the same group. In the Artiodactyla, for example, there are very constantly 19 trunk-vertebræ, but the Hippopotamus has 15 dorsals and 4 lumbars, the Reindeer (_Rangifer_) 14 D., 5 L., the Ox (_Bos taurus_) 13 D., 6 L., the Camel (_Camelus dromedarius_) 12 D. and 7 L. (4) Next follows the _sacrum_, which consists of a varying number of coalesced vertebræ. The number of sacral vertebræ varies from 2 to 13, but is usually from 3 to 5. (5) Finally, there are the _caudal_ vertebræ, or those of the tail, which are extremely variable in number and size, depending upon the length and thickness of the tail.

We must next consider briefly some of the structural features which characterize the vertebræ of the different regions. (1) The length of the neck varies greatly in different mammals and, up to a certain point, flexibility increases with length, but, as the number of 7 cervicals is almost universally constant among mammals and the lengthening of the neck is accomplished by an elongation of the individual vertebræ, a point is eventually reached, where greater length is accompanied by a diminution of mobility. For instance, in the Giraffe the movements of the neck are rather stiff and awkward, in striking contrast to the graceful flexibility of the Swan’s neck, which has 23 vertebræ, more than three times as many.

[Illustration: FIG. 11.—Atlas of Wolf, anterior end and left side. _cot._, anterior cotyles. _n.c._, neural canal. _n.a._, neural arch. _tr._, transverse process. _v.a._, posterior opening of the canal for the vertebral artery.]

The first two cervical vertebræ are especially and peculiarly modified, in order to support the skull and give to it the necessary degree of mobility upon the neck. The first vertebra, or _atlas_, is hardly more than a ring of bone with a pair of oval, cuplike depressions (_anterior cotyles_) upon the anterior face (superior in Man) into which are fitted the occipital condyles of the skull. By the rolling of the condyles upon the atlas is effected the nodding movement of the head, upward and downward, but not from side to side; this latter movement is accomplished by the partial rotation of skull and atlas together upon the second vertebra in a manner presently to be explained. On the hinder aspect are two articular surfaces (_posterior cotyles_) in shape like the anterior pair, but very much less concave, which are in contact with corresponding surfaces on the second vertebra. The neural arch of the atlas is broad and low and the neural canal is apparently much too large for the spinal cord, but, in fact, only a part of the circular opening belongs to the neural canal. In life, the opening is divided by a transverse ligament into an upper portion, the true neural canal, and a lower portion, which lodges a projection from the second vertebra. The atlas usually has no neural spine and never a prominent one; the transverse processes are broad, wing-like plates and each is perforated by a small canal, which transmits the vertebral artery.

[Illustration: FIG. 12.—Axis of Wolf, left side. _od.p._ odontoid process. _cot._, anterior cotyles. _n.a._, neural arch. _n.sp._, neural spine. _pt.z._, posterior zygapophyses. _tr._, transverse process. _v.a′._, anterior opening of canal for the vertebral artery. _v.a″._, posterior opening of the same.]

The second vertebra, or _axis_, is a little more like the ordinary vertebra, having a definite and usually elongate centrum, on the anterior end of which are the two articular surfaces for the atlas. Between these is a prominent projection, the _odontoid process_, which fits into the ring of the atlas and has a special articulation with the lower bar of that ring. In most mammals the odontoid process is a bluntly conical peg, varying merely in length and thickness, but in many long-necked forms the peg is converted into a semicylindrical spout, convex on the lower side and concave above. The neural spine of the axis is almost always a relatively large, hatchet-shaped plate, which is most developed in the carnivorous forms, and the transverse processes are commonly slender rods.

The five succeeding cervical vertebræ are much alike, though each one has a certain individuality, by which its place in the series may readily be determined. The centrum has a convex anterior and concave posterior face, which in long-necked animals form regular “ball and socket” joints; neural spines are frequently wanting and, when present, are almost always short and slender; the zygapophyses are very prominent and are carried on projections which extend before and behind the neural arch; the transverse processes are long, thin plates and, except in the seventh cervical, are usually pierced by the canal for the vertebral artery, but in a few forms (_e.g._ the camels) this canal pierces the neural arch.

(2) The dorsal or thoracic vertebræ have more or less cylindrical centra, with nearly flat faces, and on the centra, for the most part at their ends, are the concave facets for the rib-heads. The transverse processes are short and rod-like and most of them articulate with the tubercles of the ribs. The zygapophyses are smaller than in the cervical region, less prominent and less oblique; the anterior pair, on the front of the neural arch, face upward and the posterior pair downward. The neural spines are very much longer than those of the neck and those of the anterior dorsals are often of relatively enormous length, diminishing toward the hinder part of the region.

[Illustration: FIG. 13.—Fifth cervical vertebra of Wolf, left side. _tr._, transverse process. _v.a″._, posterior opening of canal for the vertebral artery. _pr.z._ and _pt.z._, anterior and posterior zygapophyses. _n.sp._, neural spine.]

[Illustration: FIG. 14.—First dorsal vertebra of Wolf, left side. _c._, centrum. _r._, anterior rib-facet. _r″._, posterior rib-facet. _tr._, transverse process. _pr.z_. _pt.z._, anterior and posterior zygapophyses. _n.sp._, neural spine.]

(3) The lumbar vertebræ are almost always heavier and larger than those of the dorsal region; they carry no ribs and their neural spines and transverse processes are broad and plate-like and the latter are far larger and more prominent than those of the dorsals. As an especial degree of strength is frequently called for in the loins, together with a greater flexibility than is needed in the dorsal region, the modes of articulation between the successive vertebræ are more complex, sometimes, as in the Edentata, most elaborately so. Taking the dorso-lumbars, or trunk-vertebræ, as a single series, we may note that in a few mammals (_e.g._ the elephants) all the neural spines have a backward slope, but in the great majority of forms this backward inclination ceases near the hinder end of the dorsal region, where there is one vertebra with erect spine, while behind this point the spines slope forward.

[Illustration: FIG. 15.—Third lumbar vertebra of Wolf, front end and left side. _tr._, transverse process. _cn._, centrum. _pr.z._ and _pt.z._, anterior and posterior zygapophyses. _n.sp._, neural spine.]

(4) The sacral vertebræ, varying from 2 to 13 in number, are fused together solidly into one piece, the combined centra forming a heavy mass and the neural canals a continuous tube, while the neural spines are united into a ridge. As a rule, only the first two vertebræ of the sacrum are in contact with the hip-bones, to support which they have developed special processes, the remainder of the mass projecting freely backward.

[Illustration: FIG. 16.—Sacrum of Wolf, upper side. _I_, _II_, _III_, first, second and third sacral vertebræ. _pl._, surface for attachment to hip-bone.]

[Illustration: FIG. 17.—Caudal vertebræ of Wolf, from anterior and middle parts of the tail. Letters as in Fig. 15.]

(5) The caudal vertebræ vary greatly, in accordance with the length and thickness of the tail. In an animal with well-developed tail several of the anterior caudals have the parts and processes of a typical vertebra, centrum, neural arch and spine, zygapophyses and transverse processes. Posteriorly, these gradually diminish, until only the centrum is left, with low knobs or ridges, which are the remnants of the various processes. A varying number of long, cylindrical centra, diminishing backward in length and diameter, complete the caudal region and the vertebral column. In some mammals, _chevron bones_ are attached to the under side of the anterior and middle caudals; these are forked, Y-shaped bones, which form a canal for the transmission of the great blood-vessels of the tail.

[Illustration: FIG. 18.—Ribs of Wolf from anterior and middle parts of the thorax. _cp._, head, _t._, tubercle.]

The _ribs_, which are movably attached to the backbone, together with the dorsal vertebræ and breast-bone, compose the _thorax_, or chest. The articulation with the vertebræ is by means of a rounded head; in most cases the head has two distinct facets, the pit being formed half on the hinder border of one dorsal vertebra and half on the front border of the next succeeding one, but posteriorly the pit is often shifted, so as to be on a single vertebra. A second articulation is by means of the _tubercle_, a smooth projecting facet on the convexity of the rib’s curvature and near the head; the tubercle articulates with the transverse process of its vertebra. The ribs, in general, are curved bars of bone, which in small mammals generally and in the clawed orders are slender and rod-like, while in the hoofed mammals they are broader, thinner and more plate-like, especially the anterior ones. The number of pairs of ribs is most commonly 13, but ranges among existing mammals from 9 in certain whales to 24 in the Two-toed Sloth (_Cholœpus didactylus_). The complex curvature of the ribs, outward and backward, is such that, when they are drawn forward (in Man upward) by muscular action, the cavity of the thorax is enlarged and air is drawn into the lungs, and when they are allowed to fall back, the cavity is diminished and the air expelled.

Below, a varying number of the ribs are connected by the cartilages in which they terminate with the breast-bone (_sternum_); sometimes these cartilages are ossified and then form the _sternal ribs_, but there is always a flexible joint between the latter and the true ribs. In certain edentates, notably the anteaters and the extinct †ground-sloths, these sternal ribs, at their lower ends, are provided with head and tubercle, for articulation with the sternum.

The _sternum_, or breast-bone, is made up of a number of distinct segments, usually broad and flat, but often cylindrical, which may unite, but far more commonly remain separate throughout life. The number, size and form of these segments often give useful characters in classification. The first segment, or _manubrium_, has quite a different shape from the succeeding ones and is considerably longer.

[Illustration: FIG. 19.—Sternum and rib-cartilages of Wolf, lower side. _P.S._, manubrium. _X.S._, xiphisternum.]

II. The appendicular skeleton consists of the limb-girdles and the bones of the limbs and feet. The limb-girdles are the means of attaching the movable limbs to the body, so as to combine the necessary mobility with strength. The anterior, or _shoulder-girdle_, has no direct articulation with the vertebral column, but is held in place by muscles; it is made up of the shoulder-blade and collar-bone, though very many mammals have lost the latter.

[Illustration: FIG. 20.—Left scapula of Wolf. _gl._, glenoid cavity. _c._, coracoid. _ac._, acromion. _sp._ spine.]

[Illustration: FIG. 21.—Left scapula of Horse. This figure is much more reduced than Fig. 20.]

[Illustration: FIG. 22.—Left scapula of Man in position of walking on all fours. Letters as in Fig. 20.]

The shoulder-blade, or _scapula_, is a broad, thin, plate-like bone, which contracts below to a much narrower neck, ending in a concave articular surface, the _glenoid cavity_, for the head of the upper arm-bone, the two together making the shoulder-joint. On the outer side the blade is divided into two parts by a prominent ridge, the _spine_, which typically ends below in a more or less conspicuous projection, the _acromion_, which may, however, be absent, its prominence being, generally speaking, correlated with the presence of the collar bone. A hook-like process, the _coracoid_, rises from the antero-internal side of the glenoid cavity and varies greatly in size in the different groups of mammals; though it usually appears to be merely a process of the scapula, with which it is indistinguishably fused, yet its development shows it to be a separate element and in the lowest mammals (Prototheria), as in the reptiles and lower vertebrates generally, it is a large and important part of the shoulder-girdle and articulates with the sternum.

The collar-bone, or _clavicle_, is a complexly curved bar, which, when present and fully developed, extends from the forward end of the sternum to the acromion, the projecting lower end of the scapular spine, supporting and strengthening the shoulder-joint. In many mammalian orders, notably all existing hoofed animals, the clavicle has become superfluous and is lost more or less completely; it may be said, in general, that the clavicle is developed in proportion to the freedom of motion of the shoulder-joint and to the power of rotation of the hand upon the arm. In arboreal animals, such as monkeys, in which the hand rotates freely and the arm moves in any direction on the shoulder, the clavicle is large and fully developed, as it also is in Man. Many burrowing mammals (_e.g._ the moles) have very stout clavicles.

[Illustration: FIG. 23.—Left clavicle of Man, front side.]

[Illustration: FIG. 24.—Left hip-bone of Wolf. _Il._, ilium. _Is._, ischium. _P._, pubis. _ac._, acetabulum.]

The posterior, or _pelvic_, girdle is composed on each side of a very large, irregularly shaped bone, which is firmly attached to one or more of the coalesced vertebræ which form the sacrum and thus affords a solid support to the hind leg. Each half of the _pelvis_, or hip-bone, is made up of three elements, called respectively the _ilium_, _ischium_ and _pubis_, which are separate in the very young animal, indistinguishably fused in the adult. The three elements unite in a deep, hemispherical pit, the _acetabulum_, which receives the head of the thigh-bone, a perfect example of the “ball and socket joint.” In the inferior median line the two pubes meet and may become coalesced, in a _symphysis_, the length of which differs in various mammals. The pelvis and sacrum together form a short, wide tube, the diameter of which is normally greater in the female skeleton than in the male.

The limbs are each divided into three segments, which in the anterior extremity are the arm, fore-arm and hand (or fore foot) and in the posterior extremity are the thigh, leg and foot (or hind foot), and there is a general correspondence between the structure of these segments in the fore and hind legs, however great the superficial difference. The bones of the limbs, as distinguished from those of the feet, are the _long bones_ and, except in a few very large and heavy mammals, are essentially hollow cylinders, thus affording the maximum strength for a given weight of bone; the cavity of a long bone contains the marrow and hence is called the _medullary cavity_. In the young mammal each of the long bones consists of three parts, the _shaft_, which makes up much the greater part of the length, and at each end a bony cap, the _epiphysis_. Growth takes place by the intercalation of new material between the shaft and the epiphyses; when the three parts unite, growth ceases and the animal is adult.

[Illustration: FIG. 25.—Left humerus of Wolf, from the front and outer sides, the latter somewhat oblique. _h._, head. _int.t._, internal tuberosity. _ext.t._, external tuberosity. _bc._, bicipital groove. _dt._, deltoid ridge. _sh._, shaft. _s._, supinator ridge. _int. epi._, internal epicondyle. _s.f._, anconeal foramen. _tr._, trochlea. _tr′._, trochlea, posterior side. _ext. epi._, external epicondyle. _a.f._, anconeal fossa.]

[Illustration: FIG. 26.—Left humerus of Horse, front side. _i.t._, internal tuberosity. _ex.t._, external tuberosity. _bc._, outer part of bicipital groove. _dt._, deltoid ridge. _s._, supinator ridge. _tr._, trochlea.]

[Illustration: FIG. 27.—Left humerus of Man, front side. Letters as in Fig. 25.]

The superior segment of the fore limb has a single bone, the _humerus_, the upper end of which is the rounded, convex _head_, which fits into the glenoid cavity of the shoulder-blade, forming the joint of the shoulder; in front of the head are two prominent and sometimes very large projections for muscular attachment, the _external_ and _internal tuberosities_, separated by a groove, in which play the two tendons of the biceps muscle and is therefore called the _bicipital groove_. In a few mammals, such as the Horse, Camel and Giraffe, the groove is divided into two by a median tubercle or ridge. From the external tuberosity there generally passes down the front face of the shaft a rough and sometimes very prominent ridge, the _deltoid crest_, to which is attached the powerful _deltoid_ muscle. At the lower end of the humerus is the _trochlea_, an irregular half-cylinder, for articulation with the two bones of the fore-arm and varying in form according to the relative sizes of those bones. On each side of the trochlea is frequently a rough prominence, the _epicondyle_, and above the inner one is, in many mammals, a perforation, the _epicondylar foramen_, for the passage of a nerve. Extending up the shaft from the outer epicondyle is a rough crest, the _supinator ridge_, to which is attached one of the muscles that rotate the hand and is conspicuously developed in those mammals which have the power of more or less free rotation and especially in burrowers. On the posterior face of the humerus, just above the trochlea, is a large, deep pit, the _anconeal fossa_.

[Illustration: FIG. 28.—Left fore-arm bones of Wolf, front side. _R._, radius. _U._, ulna. _ol._, olecranon. _h._, head of radius.]

[Illustration: FIG. 29.—Left fore-arm bones of Man, front side. Letters as in Fig. 28. The small object at the right of each figure is the head of the radius, seen from above.]

The two bones of the fore-arm, the _radius_ and _ulna_, are, in most mammals, entirely separate from each other, but in certain of the more highly specialized hoofed animals are immovably coössified. Primitively, the two bones were of nearly equal size, but in most of the mammalian orders there is a more or less well-defined tendency for the radius to enlarge at the expense of the ulna. These bones are normally crossed, the radius being external at the upper end and passing in front of the ulna to the inner side of the arm. The radius varies considerably in form in accordance with the uses to which the hand is put; if the capacity of rotation is retained, the upper end, or head, of the radius is small, circular or disk-like, covering little of the humeral trochlea, but when the head of the radius is broadened to cover the whole width of the humerus, then all power of rotation is lost. (Cf. Figs. 28 and 29.) As a rule, the radius broadens downward and covers two-thirds or more of the breadth of the wrist-bones.

[Illustration: FIG. 30.—Coössified bones of left fore-arm of Horse, front side. For most of its length, the ulna is concealed by the radius.]

[Illustration: FIG. 31.—Left fore-arm bones of the Tapir (_Tapirus terrestris_). _R._, radius. _U._, ulna. _h._, head of radius. _h′._, sigmoid notch of ulna. _ol._, olecranon. N.B. This figure is on a much larger scale than Fig. 30.]

The ulna is longer than the radius, its upper end being extended into a heavy process, the _olecranon_, or _anconeal process_, into which is inserted the tendon of the great triceps muscle, the contraction of which straightens the arm; this process is the bony projection at the back of the elbow-joint. Below the olecranon is a semicircular articular concavity, which embraces the humeral trochlea and its upper angle fits into the anconeal fossa of the humerus. The ulna contracts and grows more slender downwards and its lower end covers but one of the wrist-bones. While in the more primitive mammals, and in those which retain the power of rotating the hand, the ulna has nearly or quite the same thickness as the radius, it is often much more slender and in the more highly specialized of the hoofed animals, such as the horses, camels and true ruminants, the radius carries the entire weight and the ulna has become very slender, more or less of its middle portion is lost and the two ends are coössified with the radius, so that the fore-arm appears to have but a single bone. The reverse process of enlarging the ulna and reducing the radius is very rare and practically confined to the elephant tribe.

[Illustration: FIG. 32.—Left manus of Wolf, front side. _SL._, scapho-lunar. _Py._, pyramidal. _Pis._, pisiform. _Tm._, trapezium. _Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. I-V_, first to fifth metacarpals. _Ph. 1_, first phalanx. _Ph. 2_, second phalanx. _Ung._, ungual phalanx. _I_, first digit, or pollex. _II-V_, second to fifth digits.]

[Illustration: FIG. 33.—Left manus of Man. _S._, scaphoid. _L._, lunar. _Py._, pyramidal (pisiform not shown). _Tm._, trapezium. _Td._, trapezoid. _M._, magnum. _Un._, unciform.]

The fore foot, or hand, for which the term _manus_ may be conveniently employed, is divisible into three parts, corresponding in ourselves to the wrist, back and palm of the hand, and the fingers. The bones of the wrist constitute the _carpus_, those of the back and palm the _metacarpus_, and those of the fingers the _phalanges_.

The carpus consists primitively of nine distinct bones, though one of these, as will be shown later, is not a true carpal. These bones are of a rounded, subangular shape, closely appressed together, with very little movement between them, and are arranged in two transverse rows. The upper row contains four bones, which enumerating from the inner side are the _scaphoid_, _lunar_, _pyramidal_ (or cuneiform) and _pisiform_. The scaphoid and lunar support the radius, while the ulna rests upon the pyramidal. The pisiform, though very constantly present, is not a true carpal, but an ossification in the tendon of one of the flexor muscles, which close the fingers; it projects more or less prominently backward and articulates with the ulna and pyramidal. The second row is also made up of four bones, which, from within outward, are the _trapezium_, _trapezoid_, _magnum_ and _unciform_. The relations of the two rows vary much in different mammals and the arrangement may be serial or alternating; thus, the scaphoid rests upon the trapezium and trapezoid and usually covers part of the magnum; the lunar may rest upon the magnum only, but very much more frequently is equally supported by the magnum and unciform and the pyramidal by the latter only. The ninth carpal is the _central_, which, when present and distinct, is a small bone, wedged in between the two rows. Few existing mammals have a separate central, which, though present in the embryo, has coalesced with the scaphoid in the great majority of forms. In the more advanced and differentiated mammals the number of carpals may be considerably reduced by the coössification of certain elements or the complete suppression and loss of others. In all existing Carnivora and a few other mammals the scaphoid and lunar are united in a compound element, the _scapho-lunar_ (or, more accurately, the scapho-lunar-central); hoofed animals with a diminished number of toes generally lose the trapezium, and other combinations occur. The second row of carpals carries the metacarpals, and primitively the trapezium, trapezoid and magnum are attached each to one metacarpal and the unciform has two.

The metacarpus consists typically of five members, a number which is never exceeded in any normal terrestrial mammal; the members are numbered from the inner side, beginning with the thumb or _pollex_, from I to V. Many mammals have fewer than five metacarpals, which may number four, three, two or only one; the third is never lost, but any or all of the others may be suppressed, and functionless rudiments of them may long persist as splints or nodules. The metacarpals are elongate, relatively slender and of more or less cylindrical shape; but the form varies considerably in different groups, according to the way in which the hand is used. When employed for grasping, as in many arboreal animals and pre-eminently in Man, the pollex is frequently opposable to the other fingers and enjoys much freedom of motion. In the camels and true ruminants the third and fourth metacarpals are coössified to form a _cannon-bone_ (_see_ Fig. 43, p. 91), but the marrow cavities and the joints for the phalanges remain separate.

The phalanges in land mammals never exceed three in each digit, except the pollex, which, when present and fully developed, has but two. The phalanges are usually slender in proportion to their length, but in very heavy hoofed animals they are short and massive. The terminal joint is the _ungual phalanx_, which carries the nail, claw, or hoof, its shape varying accordingly.

[Illustration: FIG. 34.—Left femur of Wolf, front side. _h._, head. _gt.tr._, great trochanter. _tr. 2_, second trochanter. _int. con._, internal condyle. _r.g._, rotular groove, _ext. con._, external condyle.]

[Illustration: FIG. 35.—Left femur of Horse. _tr. 3_, third trochanter. Other letters as in Fig. 34, than which this drawing is very much more reduced.]

The hind leg is constituted in very much the same manner as the fore, but with certain well-marked and constant differences. The thigh-bone, or _femur_, is usually the longest and stoutest of the limb-bones and in very large animals may be extremely massive. At the upper end is the hemispherical _head_, which is set upon a distinct _neck_ and projects inward and upward, fitting into the acetabulum of the hip-bone. Nearly all land mammals have a small pit on the head of the femur, in which is inserted one end of the _round ligament_, while the other end is attached in a corresponding depression in the floor of the acetabulum. This ligament helps to hold the thigh-bone firmly in place and yet allows the necessary freedom of movement. On the outer side of the upper end of the femur is a large, roughened protuberance, which often rises higher than the head and is called the _great trochanter_; another, the _second_ or _lesser trochanter_, is a small, more or less conical prominence on the inner side of the shaft, below the head. These two processes are well-nigh universal among land mammals; and in a few of the orders occurs the _third trochanter_, which arises from the outer side of the shaft, usually at or above the middle of its length. Though comparatively rare in the modern world, the third trochanter is an important feature, and the early members of most, if not all, of the mammalian orders possessed it. The shaft of the femur is elongate and, except in certain very bulky mammals, of nearly cylindrical shape. The lower end of the bone is thick and heavy and bears on the posterior side two large, rounded prominences, the _condyles_, which articulate with the shin-bone to form the knee-joint. On the anterior side is a broad, shallow groove, the _rotular groove_, in which glides the _patella_, or knee-cap. The patella is a large ossification, of varying shape, in the tendon common to the four great extensor muscles of the thigh, the action of which is to straighten the leg.

[Illustration: FIG. 36.—Left femur of Wolf, inside of lower end. _ext. con._, external condyle. _int. con._, internal condyle. _r.g._, rotular groove. Above, are two views of the left patella, anterior and internal sides.]

The lower leg, like the fore-arm, has two bones, which, however, are always parallel, never crossed, and have no power of rotation. Of these, the inner one is the shin-bone, or _tibia_, which is always the larger and alone enters into the knee-joint. The external bone is the _fibula_, which is almost entirely suppressed in certain highly specialized forms, such as the horses and ruminants, the tibia carrying the whole weight. The upper end of the tibia is enlarged and extends over that of the fibula; it has two slightly concave surfaces for articulation with the condyles of the femur, the approximate edges of which are raised into a bifid _spine_. The upper part of the shaft is triangular, with one edge directed forward, and the superior end of this edge is roughened and thickened to form the _cnemial crest_, to which is attached the patellar ligament. The middle portion of the shaft is rounded and the lower end broad and usually divided by a ridge into two grooves or concavities for the ankle-bone; from the inner side of this end projects downward a tongue-like process, the _internal malleolus_, which prevents inward dislocation of the ankle.

The fibula is relatively stoutest in the less advanced mammals and is usually straight and slender, with enlarged ends, the lower one forming the _external malleolus_, which serves to prevent outward dislocation of the ankle. In many forms the fibula is coössified with the tibia at both ends, and in the most highly specialized hoofed animals, such as the horses, camels and true ruminants, the bone has apparently disappeared. The young animal, however, shows that the ends of the fibula have been retained and the shaft completely lost; the upper end is in the adult firmly fused with the tibia and, in the horses, the lower end is also, but this remains separate in the ruminants and camels, forming the _malleolar bone_, which is wedged in between the tibia and the heel-bone. Because of its importance in holding the ankle-bone in place, this lower end of the fibula is never lost in any land mammal.

[Illustration: FIG. 37.—Bones of left lower leg of Wolf, front side. _T._, tibia. _F._, fibula. _sp._ spine of tibia. _cn._ cnemial crest. _i.m._, internal malleolus. _e.m._, external malleolus.]

[Illustration: FIG. 38.—Bones of left lower leg of Horse (much more reduced). _cn._ cnemial crest. _F._, lower end of fibula, coössified with tibia. Other letters as in Fig. 37.]

[Illustration: FIG. 39.—Bones of lower leg, left side, of Tapir. _T._, tibia. _F._, fibula. _sp._, spine of tibia. _cn._, cnemial crest. _i.m._, internal malleolus. _e.m._, external malleolus. N.B. This figure is on a much larger scale than Fig. 38.]

[Illustration: FIG. 40.—Left pes of Wolf, front side. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Ch._, cuboid. _Cn. 1_, _Cn. 2_, _Cn. 3_, internal, middle and external cuneiforms. _Mt. I_, rudimentary first metatarsal. _Mt. II-V_, second to fifth metatarsals. _Ph. 1_, first phalanx. _Ph. 2_, second phalanx. _Ung._, ungual phalanx. _I_, rudimentary hallux. _II-V_, second to fifth digits.]

[Illustration: FIG. 41.—Left pes of Man. Note the large size of _Mt. I_, the metatarsal of the first digit, or hallux. Letters as in Fig. 40, except _Cb._, cuboid.]

The hind foot, or _pes_, like the manus, is clearly divisible into three parts, the bones of which are called respectively the _tarsus_, _metatarsus_ and _phalanges_, and the correspondence in structure between manus and pes is close and obvious. The tarsus consists typically of seven bones, which are tightly packed and rarely permit any movement between them. The upper row of the tarsus consists of two bones, which are peculiarly modified to form the ankle-joint and heel; on the inner side is the ankle-bone, or _astragalus_, the shape of which is highly characteristic of the various mammalian orders. The upper or posterior portion of the astragalus, according to the position of the foot, is a pulley which glides upon the lower end of the tibia and is held firmly in place by the internal and the external malleolus. Below the pulley-like surface the astragalus usually contracts to a narrow neck, which ends in a flat or convex head. The astragalus is supported behind (or beneath) by the heel-bone, or _calcaneum_, which is elongate and extends well above (or behind) the remainder of the tarsus; it frequently has a distinct articulation with the fibula, but more commonly is not in contact with that bone. The astragalus rests upon the _navicular_, which is moulded to fit its head and corresponds in position to the central of the carpus, but, unlike that carpal, it is a very important element and is never suppressed or lost in any land mammal. The navicular, in turn, rests upon three bones of the second row, which are called respectively the _internal_, _middle_ and _external cuneiform_, which correspond to the trapezium, trapezoid and magnum of the carpus and to which are attached the three inner metatarsals, one to each. Finally, the _cuboid_, the external element of the second row, is a large bone, which supports the calcaneum and often part of the astragalus and to which the fourth and fifth metatarsals are attached; it is the equivalent of the unciform in the manus. The number of tarsals is more constant than that of the carpals, but some suppressions and coössifications do occur.

The long bones of the pes constitute the metatarsus, which is the counterpart of the metacarpus. There are never more than five metatarsals in any normal mammal, but there may be any number less than five, down to a single one. In form and size the metatarsals of any given mammal are usually so like the metacarpals, that it requires some experience to distinguish them, but when either manus or pes is especially adapted to some particular kind of work, there may be very decided differences between metatarsals and metacarpals. For example, the burrowing forefoot of the moles is very different from the hind foot, which has undergone but little modification, and even more striking is the difference between the wing of a bat and its foot. Many other instances of a less extreme divergence might be enumerated, but when manus and pes are used only for locomotion, as in nearly all hoofed animals and many other mammals, the metacarpals and metatarsals are very similar. When there is a difference in number, it is the general rule that there are fewer metatarsals; an instance of this is found in the tapirs, which have four toes in the front foot and three in the hind. Forms which have a cannon-bone in the manus have it also in the pes, and some, like the peccaries and the jumping rodents called jerboas, have it only in the pes. The first (or inner) metatarsal, that of the great toe, or _hallux_, is sometimes opposable to the others, as in the monkeys, apes and lemurs.

The word _metapodial_ is a useful general term which includes both metacarpals and metatarsals. A metapodial with its phalanges is a _digit_, a term often employed because of the ambiguity which arises in the use of the words “fingers” and “toes,” and is applicable to both fore and hind feet.

Normally, the phalanges of the pes are so like those of the manus as to require no particular description; and only when the two pairs of extremities are specialized for entirely different functions, is there any notable divergence between the phalanges of manus and pes.

[Illustration: FIG. 42.—Left pes of Black Bear (_Ursus americanus_), showing the plantigrade gait. _T._, tibia. _F._, fibula. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 3_, external cuneiform. _Cb._, cuboid. _Mt. V._, fifth metatarsal.]

[Illustration: FIG. 43.—Left pes of Patagonian Deer (_Hippocamelus bisulcus_), showing the unguligrade gait. _T._, tibia. _F._, lower end of fibula (malleolar bone). _Cal._, calcaneum. _As._, astragalus. _N.Cb._, coössified cuboid and navicular. _Mt. III_, _Mt. IV_, cannon-bone, formed by the coössification of the third and fourth metatarsals. _V._, Rudimentary fifth digit.]

Before leaving the subject of the skeleton, it will be well to explain the terms used in describing the gait and manner of using the feet. When the entire sole of the foot is in contact with the ground and weight is thrown upon the heel-bone, or calcaneum, the gait is said to be _plantigrade_ and is exemplified in Man, bears, raccoons and many other mammals. The Dog is _digitigrade_, that is to say, the feet in the standing position are nearly erect and the wrist and heel are raised high above the ground; the weight is borne upon ball-like pads, one under the phalanges of each functional digit and one under the metapodials. The digitigrade gait is found not only in all the dogs and cats, but in many other Carnivora and in the camels and llamas, as well. Transitions between the plantigrade and digitigrade gait are so numerous and gradual, that such terms as _semi-plantigrade_ and _semi-digitigrade_ are sometimes necessary. An animal is said to be _unguligrade_ when the weight is carried entirely upon the hoofs and is used only of hoofed animals; examples are the horses, pigs, deer, antelopes, oxen, etc. The so-called “knee” of a horse is really his wrist and the “hock” is the heel, so that the feet make nearly half the apparent length of the legs. Certain very large and massive animals, such as the rhinoceroses and elephants, are unguligrade in a modified sense; the foot is a heavy column, seemingly a part of the leg, and the weight is borne upon a great pad of elastic tissue, with the hoofs disposed around its periphery. A very peculiar mode of locomotion is exemplified by certain of the Edentata, in the forefoot of the existing Ant Bear (_Myrmecophaga jubata_) and in both extremities of some of the later representatives of the extinct †ground-sloths, or †Gravigrada. Here the weight is carried upon the outer edge of the foot, the palm and sole being turned inward. No term has been suggested for this very exceptional gait, which is a modified form of plantigradism.

II. THE TEETH

It was pointed out in Chapter II (p. 38) that very often the teeth are all that remains to us of extinct genera and species of mammals, and it may be further noted that the teeth are very characteristic and often suffice to fix the systematic position of a genus. Since, therefore, the teeth play such an uncommonly important part as fossils and are so pre-eminently useful to the palæontologist, it is necessary to give some general account of them.

Among the mammals the teeth display a very great variety of size and form in accordance with the manner in which they are used. Primarily, the function of the teeth is to seize and masticate food, and the kind of food habitually eaten by any animal is well indicated by the form of its teeth. The beasts of prey have teeth adapted for shearing flesh and crushing bones; plant-feeders have teeth fitted for cropping plants and triturating vegetable tissues; insect-eaters have teeth with numerous sharp-pointed cusps, or it may be, no teeth at all, swallowing without mastication the insects which they capture, etc. Among animals that have similar diet there is very great difference in the form and elaborateness of the grinding apparatus and it is often possible to follow out the steps of evolutionary change, by which, from simple beginnings, a high degree of complexity has been attained. In addition to the uses of the teeth as organs of mastication, they frequently serve as weapons of offence or defence. In the flesh-eaters which capture living prey they are formidable offensive weapons, and the fangs of the Lion or the Wolf are instances familiar to every one; but the tusks of the elephants or the hippopotamuses have nothing to do with the taking of prey. Several Old World deer, which have no antlers or very small ones, possess scimitar-like upper tusks, with which they are able to defend themselves very effectually.

In the lower vertebrates, such as reptiles and fishes, the number of teeth is usually indefinite and they continue to be shed and replaced, as needed, throughout life; but in each species of mammal, aside from abnormalities, the number is fixed and constant. Mammalian teeth are very generally divisible into four categories: (1) the _incisors_, or front teeth, which in the upper jaw are inserted in the premaxillary bones, (2) the _canines_, or eye-teeth, which are never more than one on each side of each jaw, or four in all, (3) the _premolars_, called in Man the bicuspids, the anterior grinding teeth which have predecessors in the milk-series and (4) the _molars_, the posterior grinding teeth which have no such predecessors.

[Illustration: FIG. 44.—Dentition of Wolf, left side. _i. 3_, third incisor. _C._, canine. _p. 1_, first premolar. _p. 4_, fourth premolar. _m. 1_, first molar.]

It is customary and convenient to express the numbers and kinds of teeth of a given mammalian species by means of a “dental formula”; for example, in Man the formula is: _i_ 2/2, _c_ 1/1, _p_ 2/2, _m_ 3/3, × 2 = 32; the reason for the multiplication by two is that the figures deal only with one side of the mouth and must be doubled to give the sum total. Just because, however, the two sides are alike, it is usual to take the doubling for granted. Written out in full, the formula means that Man has two incisors, one canine, two premolars and three molars on each side of each jaw, the horizontal line indicating the division between upper and lower teeth. The number of teeth is frequently not the same in the upper and lower jaws; for instance, the formula for the Sheep is: _i_ 0/3, _c_ 0/1, _p_ 3/3, _m_ 3/3, × 2 = 32; the total is the same as in Man, but the arrangement is entirely different. The meaning is that in the Sheep there are no upper incisors or canines, but three incisors and a canine are present in each half of the lower jaw, with three premolars and three molars on each side above and below. The Dog gives still another formula: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 2/3, × 2 = 42. What is called the typical formula for the higher terrestrial mammals above the grade of the marsupials and which is but rarely exceeded, is _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 = 44, though most existing mammals have fewer teeth than this. Compared with the typical formula, the Dog has lost but two teeth, the third upper molar on each side, while Man and the Sheep have each lost twelve.

As every one knows from his own experience, mammals normally have two sets of teeth, the first, temporary, or milk-dentition, in the young animal, and the second, or permanent dentition, in the adult. The milk-dentition, when fully developed, consists of incisors, canines and premolars, which usually agree in number, though often not in form, with the permanent teeth which replace them in the adult. The milk-teeth are frequently more conservative than the permanent ones and retain ancestral characters which have disappeared in the adult series, thus affording welcome information as to lines of descent and steps of evolutionary change. While there can be little doubt that the development of more than one dentition, or set of teeth, is the primitive condition among mammals and was derived by inheritance from their lower vertebrate ancestors, in which there was an indefinite succession of teeth; yet there are many mammals in which the milk-dentition is greatly reduced or altogether lost. In some, the milk-teeth are shed and replaced before birth, in others only the germs of the milk-teeth are formed and never cut the gum, while in others again all traces of the temporary series have vanished. This complete loss of the milk-teeth, like the presence of a great number of simple and similar teeth in the dolphins and porpoises, or the total absence of teeth, as in the anteaters and whalebone whales, is a secondary and derivative condition, never a primitive one.

[Illustration: FIG. 44_A_.—First upper molar, right side of Deer (_Odocoileus_). On the left, the masticating surface; heavy black line, enamel. On the right, external side, showing crown and roots. _Brachyodont._]

[Illustration: FIG. 45.—First upper molar, left side, of a fossil horse (_Equus sp._). On the right, external side. On the left, the grinding surface, showing two stages of wear. Heavy black line, enamel; white, dentine; shaded, cement. _Hypsodont_, roots not yet formed.]

The structure of mammalian teeth varies greatly, from the simplest slender cones to enormous and highly complicated apparatus, and, in order to comprehend the significance of these differences, we must look a little more closely into the materials of which the teeth are constructed and the manner in which those materials are combined. In all primitive mammals and in many of the higher and more advanced ones (including Man) a tooth is composed of the _crown_, or portion which is exposed above the gum, and the _roots_, one or more in number, by means of which the tooth is firmly inserted in the jaw-bone; the roots are at least partly formed before the tooth comes into use. Such a tooth is said to be short or low-crowned, or _brachyodont_. In many plant-feeders, such as horses, cattle, elephants, beavers, etc., the teeth continue to grow in height for a long time and do not form roots until late in life, or perhaps not at all. Such teeth are said to be long- or high-crowned, or _hypsodont_, and in very many instances the development of brachyodont into hypsodont teeth may be followed through every step of the change. The advantage of the change is obvious in lengthening the animal’s life, especially in those which feed upon abrasive substances, like grass, for the growth of the teeth long continues to make up for the loss through wear. Serious trouble has often been caused for captive elephants by giving them too soft food, so that the growth of the teeth is not properly balanced by abrasion. Still another category of teeth is the _rootless_, which are of simple form, like those of an armadillo, and grow throughout life, never forming roots. The chisel-like, or _scalpriform_ incisors of the rodents do not cease to grow while the animal lives; they are kept of constant length by continual use, and the arrangement of harder and softer tissue is such that the sharp edge is maintained; through accident or malformation it sometimes happens that the upper and lower teeth fail to meet, then the continued growth causes them to form curved hoops in the mouth, locking the jaws and bringing death by starvation to the unfortunate animal.

[Illustration: FIG. 46.—Dentition of Beaver (_Castor canadensis_). _m. 3_, last molar. _p. 4_, last premolar. _i._, scalpriform incisors; enamel face black, dentine in vertical lines.]

The typical mammalian tooth is composed of three kinds of tissue, all differing in structure and hardness and called respectively (1) dentine, (2) enamel, (3) cement. (1) The _dentine_, or ivory, is the indispensable tissue of the tooth; the other kinds may be absent, but never the dentine. Chemically, it is like bone, but the microscope shows that its structure is quite different from that of true bone, being composed of an immense number of fine tubules, which radiate from the “pulp-cavity,” or chamber which contains the blood-vessels and nerves, these entering the tooth through the canals of the roots. The tubules of the dentine lodge excessively fine fibrillæ of the nerve and that is why the cutting into a live tooth is so painful an operation. (2) The _enamel_, which is the hardest of all animal tissues, has a polished and shining appearance and is arranged in a mosaic of microscopic prisms, closely packed together, which in most mammals are solid, but in the marsupials, with some exceptions, are tubular. The enamel normally covers the entire crown of the tooth, but does not extend upon the roots, where its superior hardness would be of no advantage. In several instances, always as a secondary specialization, the enamel does not cover the whole crown, but is arranged in vertical bands, it may be on one side only, or at intervals around the tooth. The scalpriform incisors of the rodents, already alluded to, have the enamel band on the front face of the tooth; the softer dentine behind wears away more rapidly, keeping the cutting surface bevelled, like the edge of a chisel, while the hard enamel forms the sharp edge. In some instances the enamel is absent altogether and the teeth are composed entirely of dentine, as in the elephant tusk. In all the Edentata, such as sloths and armadillos, both living and extinct, that have any teeth at all, the teeth have no enamel, but in some of the fossil forms the place of the missing enamel is taken by a harder dentine and thus the effect of differential hardness is secured.

[Illustration: FIG. 47.—Section through a lower molar of the Indian Elephant (_Elephas maximus_). Enamel, heavy black; dentine, white; cement, horizontal lines.]

(3) The _cement_ is simply bone, both chemically and in microscopic structure; it is not quite so hard as dentine, but it is less affected by the fluids of the mouth and the juices of the food. In the brachyodont or low-crowned tooth, such as a human molar, the cement merely forms a sheath over the roots and does not appear upon the crown, but in many hypsodont teeth, those of horses and elephants, for example, the cement completely encases the entire tooth in a thick layer, filling up all the depressions and irregularities of the enamel surface and making a freshly erupted and unworn tooth look like a shapeless lump. When the cement and the enamel covering are partially worn through, the masticating surface is made up of three distinct substances, each having a different degree of hardness and thus, through unequal wear, the grinding surface is always kept rough and therefore efficient. Not all hypsodont teeth have the cement covering, but in such teeth the differing degrees of hardness of enamel and dentine suffice to keep a rough surface, though not so effectively.