CHAPTER XXVIII

BRANCH CHORDATA (_Continued_). CLASS MAMMALIA: THE MAMMALS

THE MOUSE (_Mus musculus_)

TECHNICAL NOTE.--It is best to catch specimens alive in a good trap. A live trap well baited and placed in some old granary should furnish plenty for class use. White mice can often be obtained at "bird-stores." When mice are not procurable, use rats. A rat is perhaps preferable on account of its size, but all essential structures can readily be made out in the mouse. Specimens should be killed by chloroform as described for the toad, p. 5.

=Structure= (fig. 147).--Compare the external characters of the mouse with those of the toad and sparrow. The mouse, unlike the other vertebrates so far studied, is thickly covered with _hair_ all over its body except on the tip of the nose and the soles of the feet. Where are the _nostrils_ placed? What are the large leaf-like expansions called _pinnæ_ situated just back of the eyes? Pull open the _mouth_ and note the large _incisor teeth_ on the upper and lower jaws. Cut one corner of the mouth back and observe the large flat-topped _molar teeth_ on both jaws. How does the attachment of the large fleshy _tongue_ differ from the condition in the toad? The toad's tongue is for snapping up insects, whereas in the mouse this organ serves to move food about in the mouth. On the tongue are numerous small _taste-papillæ_. Notice the long hairs, "feelers," on each side of the nose. Note the similarity between the front paws and our own hands; each has four fingers with a small rudimentary thumb on the inner side of the paw. How does the hind foot of the mouse differ from the foot of man? Posteriorly the body is terminated by a long _tail_. At the root of the tail is a small aperture, the _anus_, and just below, or ventral to it, is the opening from the kidneys and reproductive organs.

TECHNICAL NOTE.--Place the mouse on its back in a dissecting-pan and cut through the skin from anus to the lower jaw. Extend the legs, pin down each foot and pin out the cut edges of the skin. Now carefully cut forward through the body-wall from the anal region and on through the breast-bones and ribs. Pin each side out.

Near the hindmost pair of _ribs_ note a sheet of muscles, the _diaphragm_, which extends across the body-cavity, dividing it into an anterior portion, the _thoracic cavity_, and a posterior, the _abdominal cavity_. What are the most conspicuous organs in the thoracic cavity? Leading anteriorly to the mouth-cavity is a long tube, the _trachea_, composed of a series of cartilaginous parts of rings placed end to end. Note at its anterior end the _glottis_ and _epiglottis_. Insert a blowpipe into the glottis and inflate the _lungs_, which will fill all the otherwise unfilled space in the thoracic cavity. The abdominal cavity contains the _viscera_ suspended in a fold of the lining membrane, as in the other vertebrates studied. Note lying against the diaphragm a large, red, glandular structure, the _liver_. Separate the two large lobes of the liver and expose the opalescent _gall-bladder_. By passing a canula into this and ligaturing, the _cystic duct_ may be injected. Beneath the liver is a large loop-shaped expansion of the alimentary canal, the _stomach_. Arising from the right end of the stomach is the narrow _duodenum_, which gradually merges into the very much convoluted _small intestine_, or _ileum_, which is followed by the _large intestine_, or _colon_, the last part of which is a straight tube, the _rectum_. The small intestine occupies most of the space in the peritoneal cavity. Within the loop of the pylorus will be found an irregular pinkish mass of tissue, the _pancreas_. Beneath the stomach on the left side of the body lies a very dark glandular mass not much unlike the liver but altogether detached from it. This structure is the _spleen_, a ductless gland.

Note dorsally of the trachea a long tube passing through the diaphragm and connecting the mouth with the stomach. What is this tube? Note the _Eustachian tubes_ extending from the mouth to the ears. The median part of the roof of the mouth is the _palate_, hard in front, soft behind. A pair of small bodies at the sides of the soft palate near its hinder end are the _tonsils_. At the posterior angle of the lower jaw are glandular bodies, the _sub-maxillary glands_, which lead by a short _duct_ anteriorly to open on the floor of the mouth. On the sides of the neck just below the ears are pink or yellowish bodies, the _parotid glands_, opening anteriorly in the sides of the mouth-cavity. These two sets of glands are collectively known as the _salivary glands_, the function of which is to secrete the saliva. Push apart the sub-maxillary glands and note below them overlying the trachea on either side two dark-red lobes connected by a band of tissue. These constitute the _thyroid gland_, another of the so-called ductless glands. Within the thoracic cavity anterior to the heart note a mass of pinkish tissue, the _thymus gland_. Observe the large _masseter muscles_, which cover the jaws. What is their function? On either side of the neck lies a large blood-vessel, the _external jugular vein_, which collects blood from the head and carries it down to the heart. Note the large _pectoral muscles_ which cover the breast and extend out into the arms, and which are so strong and highly developed in the sparrow. The head is supported by large muscles which run down the back of the neck to the ribs. Others are attached to the ribs, which they raise and lower. These movements, together with the contraction of the diaphragm, cause the expansion and contraction of the thoracic cavity whereby the lungs are regularly filled and emptied. Note that the abdomen is covered by a double layer of muscular tissue, the outer part made up of the _external oblique muscles_, the inner by the _internal oblique muscles_.

[Illustration: FIG. 148.--Diagram of the circulation of the blood in a mammal; _a_, auricles; _l_, lung; _lv_, liver; _p_, portal vein bringing blood from the intestine; _v_, ventricles; the arrows show the direction of the current; the shaded vessels carry venous blood, the others arterial blood. (From Kingsley.)]

Examine the _heart_. How many _auricles_ has it? The _ventricles_ in the mouse, as in the bird, are entirely separated, forming two complete compartments, a _right_ and a _left ventricle_. The blood flowing from the veins of the body is collected in the right auricle, thence it passes into the right ventricle, whence it is conveyed to the lungs; returning it flows through the left auricle into the left ventricle, whence it is forced through the arteries of the body. For a study of the circulatory system in mammals (fig. 148), a rat or a rabbit should be injected by the teacher and an advanced text-book, as Parker's "Zootomy" or Marshall and Hurst's "Practical Zoology," used as a guide. A sheep's heart is very good to cut open for a class demonstration.

Make a drawing of the organs observed thus far in the dissection.

The _kidneys_ in the mouse are situated in the dorsal region next to the backbone. They consist of two bean-shaped smooth glands. From them a pair of ducts, the _ureters_, can be traced down to a median thin-walled muscular sac, the _bladder_. The bladder opens to the exterior of the body by means of a short tube, the _urethra_. Cut open a kidney longitudinally and examine the cut surfaces.

The two egg-glands of the female mouse lie in the median portion of the abdominal cavity, somewhat below the kidneys, and from the vicinity of each runs an egg-tube. These tubes meet below the bladder, and open to the exterior of the body through the aperture noted below the anus. In the posterior parts of these tubes lie until birth the developing embryos.

TECHNICAL NOTE.--For a study of the nervous system place the specimen ventral side down and cut through the skull with the bone-cutters or heavy scissors, exposing the brain and spinal cord.

Note the large _brain_ (fig. 149), composed of small _optic lobes_, large _cerebrum_, _cerebellum_, and _medulla oblongata_, followed by the long _spinal cord_. Note the nerves arising from the brain and spinal cord.

[Illustration: FIG. 149.--Diagram of brains of vertebrates; _Olf. L._, olfactory lobes; _Cbr._, cerebrum; _Md. Br._, midbrain (optic lobes); _Cbl._, cerebellum; _Med. Ob._, medulla oblongata; _Sp. Cd._, spinal cord. (From specimens.)]

For a careful dissection of the mammalian nervous system a larger mammal, such as a cat or dog or rabbit, should be used. For guide use a text-book such as, for the dog, Howell's "Dissection of the Dog"; for the cat, Reighard and Jennings' "Anatomy of the Cat"; and for the rabbit, Parker's "Zootomy" or Marshall and Hurst's "Practical Zoology." Make a good preparation of the brain and preserve it for future use in some fluid like Fischer's fluid (see page 453).

TECHNICAL NOTE.--Prepare a well-cleaned skeleton by boiling a specimen in a soap solution and thoroughly cleansing it (see p. 452).

Note the very compact _skeleton_ of the mouse. Note the closely sutured _skull_. How many _cervical_ or _neck vertebræ_ are there? The _ribs_ are attached to the _thoracic vertebræ_. How many pairs of ribs? The bony thorax supports the _shoulder-girdle_ and bones of the fore legs. The thorax is followed by a series of ribless vertebræ, the _lumbar vertebræ_, which in the posterior region of the body fuse with the _pelvic girdle_ supporting the hind limbs. The body vertebræ are succeeded by the very much smaller _caudal vertebræ_. Compare the skeleton of the mouse with that of the bird; also with that of the toad. For directions for a detailed study of the skeleton see in Parker's "Zootomy" an account of the skeleton of the rabbit, pp. 263-286.

TECHNICAL NOTE.--For the study of the eye (fig. 150) the teacher should obtain the eye of some large mammal, as the ox or sheep, with which to make a class demonstration. The eye of a rabbit or cat can of course be used. For an account of the vertebrate eye see Parker and Haswell's "Text-book of Zoology," Vol. II. pp. 103-107. For a study of the ear use a bird or mammal, and see pp. 107-110 of the same book.

[Illustration: FIG. 150.--Diagram of vertebrate eye; _c_, choroid; _i_, iris; _l_, lens; _n_, optic nerve; _r_, retina; _s_, sclerotic. (From Kingsley.)]

=Life-history and habits.=--The house-mouse is not a native of North America, but was introduced into this country from Europe, to which, in turn, it came from Asia, its original habitat. The mouse came to this country in the vessels of early explorers. Similarly the brown and black rats, now so abundant all over North America, and members of the same genus as the mouse, were introduced from Europe. Accompanying man in his travels the mouse has spread from Asia until it is now to be found over the whole world.

The habits of mice are well known; their fondness for living in our homes and outbuildings makes them familiar acquaintances. Their food is varied; they seem to thrive best, however, on a vegetable diet. Grains and nuts are favorite foods. The house-cat is their greatest enemy, but man takes advantage of their instinct to go into holes by constructing traps with funnel or tunnel entrances which, baited with cheese or other favorite food, are fatally attractive. In climbing, mice are aided by the tail. Their strong hind legs enable them to stand erect, and even to take several steps in this posture. They can swim readily, although naturally they rarely take to water. Their special senses are keen, the senses of hearing and taste being unusually well developed. Their "singing," which has been the subject of much discussion, seems to be actually a voluntary and normal performance which, however, hardly deserves to be called singing, but rather a slightly varied peeping or whistling.

The mouse is a prolific mammal, producing from four to six times a year broods of from four to eight young. The mouse makes a cosy nest of straw, bits of paper, feathers, wool or other soft materials, and in this the young are born. The newly born mice are very small and are blind and helpless. They are odd little creatures, being naked and almost transparent. They grow rapidly, being covered with hair in a week, although not opening their eyes for about two weeks. A day or two after their eyes are open they begin to leave the nest, and hunt for food for themselves.

OTHER MAMMALS

The mammals constitute the highest group of animals, including man, the monkeys and apes, the quadrupeds, the bird-like bats and fish-like seals and whales; in all about 2500 species. They are found everywhere except on a few small South Sea islands. Only a few species, however, have a world-wide distribution. The name Mammalia is derived from the mammary or milk glands with which the females are provided and by the secretion of which the young of this class, born free in all but a few of the lowest forms, are nourished for some time after birth. In size mammals range from the tiny pigmy-shrew and harvest mouse, which can climb a stem of wheat, to the great sulphur-bottom whale of the Pacific Ocean, which attains a length of a hundred feet and a weight of many tons. Mammals differ from fishes and batrachians and agree with reptiles and birds in never having external gills; they differ from reptiles and agree with birds in being warm-blooded and in having a heart with two distinct ventricles and a complete double circulation; finally, they differ from both reptiles and birds in having the skin more or less clothed with hair, the lungs freely suspended in a thoracic cavity separated from the abdominal by a muscular partition, the diaphragm, and in the possession by the females of mammary glands. In economic uses to man mammals are the most important of all animals. They furnish the greater portion of the animal food of many human races, likewise a large amount of their clothing. Horses, asses, oxen, camels, reindeer, elephants, and llamas are beasts of burden and draught; swine, sheep, cattle, and goats furnish flesh, and the two latter milk for food; the wool of sheep, the furs of the carnivores, and the leather of cattle, horses, and others are used for clothing, while the bones and horns of various mammals serve various purposes.

[Illustration: FIG. 147.--Dissection of the Mouse, _Mus musculus_.]

=Body form and structure.=--The mammalian body varies greatly. Its variety of form and general organization is explained by the facts that, although most of the species live on the surface of the earth, some are burrowers in the ground, some flyers in the air, and some swimmers in the water. Mammals never have more than two pairs of limbs; in most cases both pairs are well developed and adapted for terrestrial progression. In the aerial bats the fore limbs are modified into organs of flight; among the aquatic seals, sea-lions, walruses, and whales both sets are modified to be swimming flippers or paddles. In many of these aquatic forms the hind limbs are greatly reduced or even completely wanting.

Most mammals are externally clothed with hair, which is a peculiarly modified epidermal process. Each hair, usually cylindrical, is composed of two parts, a central pith containing air, and an outer more solid cortex; each hair rises from a short papilla sunk at the bottom of a follicle lying in the true skin. In some mammals the hairs assume the form of spines or "quills," as in the porcupine. The hairy coat is virtually wanting in whales and is very sparse in certain other forms, the elephant, for example, which has its skin greatly thickened. The claws of beasts of prey, the hooves of the hoofed mammals, and the outer horny sheaths of the hollow-horned ruminants are all epidermal structures.

The bones of mammals are firmer than those of other vertebrates, containing a larger proportion of salts of lime. Among the different forms the spinal column varies largely in the number of vertebræ, this variation being chiefly due to differences in length of tail. Apart from the caudal vertebræ their usual number is about thirty. The mammalian skull is very firm and rigid, all the bones composing it, excepting the lower jaw, the tiny auditory ossicles, and the slender bones of the hyoid arch, being immovably articulated together. The correspondence between the bones of the two sets of limbs is very apparent. The number of digits varies in different mammals, and also in the fore and hind limbs of a single species. Among the Ungulates the reduction in the number of digits is especially noticeable; the forefoot of a pig has four digits, that of the cow two, and that of the horse one. The two short "splint" bones in the horse are remnants of lost digits. The teeth are important structures in mammals, being used not only for tearing and masticating food, but as weapons of offence and defence. A tooth consists of an inner soft pulp (in old teeth the pulp may become converted into bone-like material) surrounded by hard white dentine or ivory, which is covered by a thin layer of enamel, the hardest tissue known in the animal body. A hard cement sometimes covers as a thin layer the outer surface of the root, and may also cover the enamel of the crown. The teeth in most forms are of three groups: (_a_) the incisors, with sharp cutting edges and simple roots, situated in the centre of the jaw; (_b_) the canines, often conical and sharp-pointed, next to the incisors; (_c_) next the molars, broad and flat-topped for grinding, and divided into premolars and true molars. There is great variety in the character and arrangement of these structures in mammals, their variations being much used in classification. The number and arrangement of the teeth is expressed by a dental formula, as, for example, in the case of man

2--2 1--1 2--2 3--3 _i_----, _c_----, _p_----, _m_---- = 32. 2--2 1--1 2--2 3--3

[Illustration: FIG. 151.--A group of Rocky Mountain sheep, or "big horns," _Ovis canadensis_, including males, females and young. (Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche, University of Kansas.)]

The mouth is bounded by fleshy lips. On the floor of the mouth is the tongue, which bears the taste-buds or papillæ, the organs of taste. The œsophagus is always a simple straight tube, but the stomach varies greatly, being usually simple, but sometimes, as in the ruminants and whales, divided into several distinct chambers. The intestine in vegetarian mammals is very long, being in a cow twenty times the length of the body. In the carnivores it is comparatively short--in a tiger, for example, but two or three times the length of the body.

The blood of mammals is warm, having a temperature of from 35° C. to 40° C. (95° F. to 104° F.). It is red in color, owing to the reddish-yellow, circular, non-nucleated blood-corpuscles. The circulation is double, the heart being composed of two distinct auricles and two distinct ventricles. Air is taken in through the nostrils or mouth and carried through the windpipe (trachea) and a pair of bronchi to the lungs, where it gives up its oxygen to the blood, from which it takes up carbonic-acid gas in turn. At the upper end of the trachea is the larynx or voice-box, consisting of several cartilages attaching by one end to the vocal cords and by the other to muscles. By the alteration of the relative position of these cartilages the cords can be tightened or relaxed, brought together or moved apart, as required to modulate the tone and volume of the voice.

The kidneys of mammals are more compact and definite in form than those of other vertebrates. In all mammals except the Monotremes they discharge their product through the paired ureters into a bladder, whence the urine passes from the body by a single median urethra. Mammary glands, secreting the milk by which the young are nourished during the first period of their existence after birth, are present in both sexes in all mammals, though usually functional in the female only.

[Illustration: FIG. 152.--A group of moose, _Alce americana_, showing male, female, and young. (Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche, University of Kansas.)]

The nervous system and the organs of special sense reach their highest development in the mammals. In them the brain is distinguished by its large size, and by the special preponderance of the forebrain or cerebral hemispheres over the mid- and hind-brain. Man's brain is many times larger than that of all other known mammals of equal bulk of body, and three times as large as that of the largest-brained ape. In man and the higher mammals the surface of the forebrain is thrown into many convolutions; among the lowest the surface is smooth. Of the organs of special sense, those of touch consist of free nerve-endings or minute tactile corpuscles in the skin. The tactile sense is especially acute in certain regions, as the lips and end of the snout in animals like hogs, the fingers in man, and the under surface of the tail in certain monkeys. All the other sense-organs are situated on the head. The organs of taste are certain so-called taste-buds located in the mucous membrane covering certain papillæ on the surface of the tongue. The organ of smell, absent only in certain whales, consists of a ramification of the olfactory nerves over a moist mucous membrane in the nose. The ears of mammals are more highly developed than those of other vertebrates both in respect to the greater complexity of the inner part and the size of the outer part. A large outer ear for collecting the sound-waves is present in all but a few mammals. A tympanic membrane separates it from the middle ear in which is a chain of three tiny bones leading from the tympanum to the inner ear, composed of the three semicircular canals and the spiral cochlea. The eyes (fig. 150) have the structure characteristic of the vertebrate eye, consisting of a movable eyeball composed of parts through which the rays of light are admitted, regulated, and concentrated upon the sensitive expansion, retina, of the optic nerve lining the posterior part of the ball. The eye is protected by two movable lids. In almost all mammals below the Primates there is a third lid, the nictitating membrane. In some burrowing rodents and others the eye is quite vestigial and even concealed beneath the skin.

=Development and life-history.=--All mammals except the Monotremes give birth to free young. The two genera of Monotremes produce their young from eggs hatched outside the body; _Tachyglossus_ lays one egg which it carries in an external pouch, while _Ornithorhynchus_ deposits two eggs in its burrow. The embryo of other mammals develops in the lower portion of the egg-tube, to the walls of which it is intimately connected by a membrane called the placenta. (In the kangaroos and opossums, Marsupialia, there is no placenta.) Through this placenta blood-vessels extend from the body of the mother to the embryo, the young developing mammal thus deriving its nourishment directly from the parent.

The duration of gestation (embryonic or prenatal development in the mother's body) varies from three weeks with the mouse, eight weeks with the cat, nine months with the stag, to twenty months with the elephant. Like the birds, the young of some mammals, the carnivores for example, are helpless at birth, while those of others, as the hoofed mammals, are very soon able to run about. But all are nourished for a longer or shorter time by the milk secreted by the mammary gland of the mother.

=Habits, instinct, and reason.=--Despite the wonderful examples of instinct and intelligence shown by many insects and by the other vertebrates, especially the birds, it is among mammals that we find the highest development of these qualities and of reason. In the wary and patient hunting for prey by the carnivora, in the gregarious and altruistic habits of the herding hoofed mammals, in the highly developed and affectionate care of the young shown by most mammals, and in the loyal friendship and self-sacrifice of dogs and horses in their relations to man, we see the culmination among animals of the development of the functions of the nervous system. In the characteristics of intelligence and reason man of course stands immensely superior to all other animals, but both intelligence and reason are too often shown by many of the other mammals not to make us aware that man's mental powers differ only in degree, not in kind, from those of other animals.

Pure instinct is hereditary, and purely instinctive actions are common to all the individuals of a species. Those actions which the individual could not learn by teaching, imitation, or experience are instinctive. The accurate pecking at food by chicks just hatched from an incubator is purely instinctive. Purely instinctive also is the laying of eggs by a butterfly on a certain species of plant which may have to be sought for over wide acres, so that the caterpillars when hatched shall find themselves on their own special food-plant. Yet the butterfly never ate of this plant and will never see its young. Such elaborate instincts as these have been developed from the simplest manifestations of sensation and nervous function, just as the complex structures of the body have been developed from simple structures (see