Page:The American Cyclopædia (1879) Volume XI.djvu/85

 MAMMALIA 77 loticed chiefly in the limbs ; in the mole, for instance, the flexors of the arm, the great pec- toral, and the latiasimus dorsi are very large ; the herbivora and pachyderms require mas- sive muscles,- and the agile carnivora compact g and energetic ones ; the muscles of the ears 'are specially developed in the herbivora, and those of the nose in the hog ; the glutceus max- imus, the largest of all in man, is much small- er in the monkeys, and very small in the low- er mammals ; the nates in the horse are com- posed principally of the glutceus medius ; the muscles of the calf, so characteristic of man, are small in all below him, and the short mus- cles of the human hand are absent in the low- er mammals ; those of the wings in bats are arranged somewhat as in birds, and those of cetaceans as in fishes. A muscle remarkably developed in many mammals, but rudimen- tary in man, is the cutaneous layer, the pan- niculus carnosus, of which the human analogue is the platysma my aides of the sides of the neck and face ; we notice its action in the horse when a fly or any irritating object touch- es the skin, in the erection of the quills of the porcupine, and in the coiling of the body of the armadillo and hedgehog. The minute coccygeal muscles of man are represented by numerous and powerful ones in the pre- hensile tail of certain monkeys, in the strong trowel of the beaver, and in the fluke of the whale, analogous to the human multifidus %pin<K, In man and mammals the heart is composed of two distinct halves, each divided into two cavities, an auricle and a ventricle ; the course of the blood is from the left ventri- cle to the aorta and over the body, pure arte- rial; then traversing the systemic capillaries it enters into the veins, and is carried to the right auricle ; thence it passes to the right ven- tricle, and thence by the pulmonary artery to the lungs, in whose capillaries it becomes pu- rified by the oxygen of the respired air, and is returned by the pulmonary veins to the left auricle, whence it enters the left ventricle to be distributed as before. Here, therefore, the blood passes twice through the .heart and through two systems of capillaries before com- pleting its circle ; hence the circulation is called double, and it is also complete, as the whole mass of the blood is purified in the lungs before it is sent over the body. Before birth, when the lungs are impervious, the auricles communicate directly, and one or more vessels pass from the right ventricle to the aorta, conveying the blood over the body without sending it to the lungs ; but when respiration begins these communica- tions between the arterial and venous systems are closed. In the dugong the two ventricles are separated by a deep cleft ; in some mam- mals the right auricle receives three venae cavas ; the apex is not inclined to the left, as in man, ex- cept in some monkeys, and in some hoofed ani- mals two small flat bones are imbedded in the substance of the left ventricle. In cetaceans there is a plexiform arrangement of the arte- ries of the walls of the chest, allowing an ac- cumulation of blood in them, to be used as re- quired during prolonged submersion ; in many ruminants the internal carotid forms a rete mirabile, or network of vessels, at the entrance of the skull, doubtless to prevent injury to the brain from too great force of the blood while the head is in a dependent position; in the slow-moving sloths the arteries of the limbs communicate very freely, rendering compres- sion during their climbing impossible except in a few vessels at a time. A similar disposition prevails in the venous system ; in the seal and otter, as in the ducks, the inferior cava is di- lated into a receptacle which holds the blood while they are under water, and only permits it to pass on to the lungs when they come to the surface; in the porpoise tortuous sinuses receive the intercostal veins, and in the foot of the horse a fine network is distributed on the front of the coffin bone. The heart is composed of muscular fibres, each cavity hav- ing its own, arranged in a spiral manner from the point to the base ; the course of the blood is directed from the auricles to the ventricles by the mitral valve on the left side and the tricuspid on the right, kept in place by tendi- nous cords attached to fleshy columns, and the entrances of the aorta and pulmonary artery are guarded each by three semilunar valves which prevent regurgitation. The lungs of mammals are almost always in pairs, and hang freely in the chest suspended by the straight windpipe, and enclosed within the serous cavity lined by the pleura ; the air tubes are distributed to all their parts, and the pulmonary cells are minute- ly subdivided and do not communicate with any other air cells in the body as they do iri birds. The windpipe varies much in length, in the number of its rings (which are from 14 to 78), and in their completeness ; the cartilages do not generally form a complete circle, being membranous posteriorly, and in the whales the membranous portion is said to be in front. The mechanism of the mammalian respiration has been described under DIAPHRAGM, the muscular partition which separates the thoracic and ab- dominal cavities in this class. The voice, under the control of the will, is produced by the pas- sage of air from the lungs over certain organs in the larynx or upper portion of the wind- pipe ; in man the larynx is a short and wide tube, suspended as it were from the hyoid bone, formed of cartilaginous plates, called the thyroid, cricoid, and two arytenoid cartilages ; the prominence commonly called " Adam's apple " is the anterior surface of the thyroid cartilage. The mucous membrane forms two lateral folds from before backward, like the lips of a buttonhole, the vocal cords or liga- ments ; above these are two other folds, be- tween which and the vocal cords is a cavity on each side, the ventricle of the larynx ; the space between these four folds is the glottis, which is covered above, during the passage of food or drink, by a fibre-cartilaginous tongue,