364 MAMMALIA [ORGANS OF perhaps may be by difference of origin, upon the supposi tion that the former are modifications of the primary branch of mammals from which the Ungulates, and the latter of that from which the Carnivora, are derived. 1 CIRCULATORY, ABSORBENT, RESPIRATORY, AND URINARY SYSTEMS. Blood. The blood of mammals is always red, and during the life of the animal hot, having a nearly uniform temperature, varying within a few degrees on each side of 100 Fahr. The corpuscles are, as usual in vertebrates, of two kinds: (1) colourless, spheroidal, nucleated, and exhibiting amoe boid movements ; while (2) the more numerous, on which depends the characteristic hue of the fluid in which they are suspended, are coloured, non-nucleated, flattened, slightly biconcave disks, with circular outline in all known species except the Camels and Llamas, where they have the elliptical form characteristic of the red corpuscles of nearly all the other vertebrates, though adhering to the mammalian type in absence of nucleus and relatively small size. As a rule they are smaller as well as more numerous than in other classes, but vary considerably in. size in different species, and not always in relation to the magnitude of the animal, a Mouse, for instance, having as large corpuscles as a Horse. Within the limits of any natural group there is, however, very often some such relation, the largest corpuscles being found among the large species and the smallest corpuscles among the small species of the group, but even to this generalization there are many exceptions. The transverse diameter of the red corpuscles in Man averages ^-g^ of an inch, which is exceptionally large, and only exceeded by the Elephant ("2"7To")> an( ^ ^7 some Cetacea and Edentata, They are also generally large in Apes, Rodents, and the Monotremata, and small in the Artiodactyles, least of all in the Chevrotains (Tragulus), being in T. javanicus and meminna not more than r^^ T . 2 Heart. The heart of mammals consists of four distinct cavities, two auricles and two ventricles. Usually the ventricular portion is externally of conical form, with a simple apex, but in the Sirenia it is broad and flattened, and a deep notch separates the apical portion of each ventricle. A tendency to this form is seen in the Cetacea and the Seals. It is characteristic of mammals alone among vertebrates that the right auriculo-ventricular valve is tendinous like the left, consisting of flaps held in their place by fibrous ends (chordae, tendinise) which arise from projections from the muscular walls of the ventricular cavity (musculi papillares). In the Monotremata a transition between this condition and the simple muscular flap of the Sauropsida is observed. In most of the larger Ungulates a distinct but rather irregular ossification is developed in the central tendinous portion of the base of the heart. Blood- The orifices of the aorta and pulmonary artery are each vessels, guarded by three semilunar valves. The aorta is single, and arches over the left bronchial tube. After supplying the tissues of the heart itself with blood by means of the coronary arteries, it gives off large vessels (" carotid ") to the head and ("brachial") to the anterior extremities. The mode in which these vessels arise from the aorta varies much in different mammals, and the study of their disposition affords some guide to classification. In nearly all cases the right brachial and carotid have a common origin (called " innominate artery " in anthropotomy). The other two vessels may come off from this, as is the rule in Ungulates, 1 See Lectures on the Comparative Anatomy of the Organs of Digestion of the Mammalia," Medical Times and Gazette, Feb. -Dec. 1872. 2 G, Gulliver, Proc. Zool. Soc., 1862, p. 91. the common trunk constituting the "anterior aorta" of veterinary anatomy ; or they may be detached in various degrees, both arising separately from the aorta, as in Man, or the left carotid from the innominate and the left brachial from the aorta, a very common arrangement, or the last two from a common second or left innominate as in some Bats and Insectivores. The aorta, after giving off the in tercostal arteries, passes through the diaphragm into the abdomen, and, after supplying the viscera of that cavity by means of the gastric, hepatic, splenic, mesenteric, renal, and spermatic vessels, gives off in the lumbar region a large branch (iliac) to each of the hinder extremities, which also supplies the pelvic visc era, and is continued onwards in the middle line, greatly diminished in size, along the under surface of the tail. In certain mammals, arterial plexuses, called retia mirabilia, formed by the breaking up of the vessel into an immense number of small trunks, which may run in a straight course parallel to one another (as in the limbs of Sloths and Slow Lemurs), or form a closely packed network, as in the intracranial plexuses of Ruminants, or a sponge-like mass of convoluted vessels, as in the intercostals of Cetaceans, are peculiarities of the vascular system the meaning of which is not in all cases clearly understood. In the Cetacea they are obviously receptacles for contain ing a large quantity of oxygenated blood available during the prolonged immersion, with consequent absence of respiration, to which these animals are subject. The vessels which return the blood to the heart from the head and upper extremities usually unite, as in Man, to form the single vena cava superior or precaval vein, but in some Insectivores, Cliiroptcra, and Rodents, and in the Elephant, and all Marsupials and Monotremes, the two superior caval veins enter the right auricle without uniting, as in birds. In Seals and some other diving mammals there is a large venous sinus or dilatation of the inferior cava immediately below the diaphragm. In the Cetacea the purpose of this is supplied by the immense abdominal venous plexuses. As a rule the veins of mammals are furnished with valves, but these are said to be altogether wanting in the Cetacea, and in the superior and inferior cava, subclavian and iliac veins, the veins of the liver (both portal and hepatic), heart, lungs, kidneys, brain, and spinal chord of other mammals. Many of the veins within the cranium are included in spaces formed by the separation of the laminse of the dura mater, and do not admit of being dilated beyond a certain size ; these are termed sinuses. The portal circulation in mammals is limited to the liver, the portal vein being formed by the superior and inferior mesenteric, the splenic, the gastro- epiploio, and the pancreatic veins. The kidney is supplied solely by arterial blood, and its veins empty their contents only into the inferior cava. The absorbent or lymphatic system of vessels is very Lym- completely developed in the Mammalia. Its ramifications phatic extend through all the soft tissues of the body, and con- vessels - vey a colourless fluid called lymph, containing nucleated corpuscles, and also, during the process of digestion, the chyle, a milky fluid taken up by the lymphatics (here called lacteals) of the small intestine, and pour them into the general vascular system, where they mix with the venous blood. The lymphatic vessels of the hinder extremities, as well as those from the intestinal canal, unite in the abdomen to form the " thoracic duct," the hinder end or commencement of which has a dilatation called the receptaculum chyli. The duct, which is of irregular size and sometimes double, often dividing and uniting again in its course, or even becoming plexiform, passes forwards close to the bodies of the thoracic vertebra?, and empties itself, by an orifice guarded by a valve, into the great left brachio-
cephalic vein, having previously received the lymphatics