Fig. 43.—Lepus cuniculus. Ventral view of the vascular system. The heart is somewhat displaced towards the left of the subject; the arteries of the right and the veins of the left side are in great measure removed. a.epg, internal mammary artery; a.f, anterior facial vein; a.m, anterior mesenteric artery; a.ph, anterior phrenic vein; az.v, azygos vein; br, brachial artery; c.il.a, common iliac artery; c.il.v, common iliac vein; cœ, coeliac artery; d.ao, dorsal aorta: e.c, external carotid artery; e.il.a, external iliac artery; e.il.v, external iliac vein; e.ju, external jugular vein; fm.a, femoral artery; fm.v, femoral vein; h.v, hepatic veins; i.c, internal carotid artery; i.cs, intercostal vessels; i.il.a, internal iliac artery; i.il.v, internal iliac vein; i.ju, internal jugular vein; i.l, iliolumbar artery and vein; in, innominate artery; l.au, left auricle; l.c.c; left common carotid artery; l.pr.c, left pre-caval vein; l.v, left ventricle; m.sc, median sacral artery; p.a, pulmonary artery; p.epg, epigastric artery and vein; p.f, posterior facial vein; p.m, posterior mesenteric artery; p.ph, posterior phrenic veins; pt.c, post-caval vein; p.v, pulmonary vein; r, renal artery and vein; r.au, right auricle; r.c.c, right common carotid artery; r.prc, right pre-caval vein; r.v, right ventricle; s.cl.a, right subclavian artery; s.cl.v, subclavian vein; spm, spermatic artery; s.vs, vesical artery; ut, uterine artery and vein; vr, vertebral artery. (From Parker's Zootomy.)
There are also features in the system of arteries and veins which are eminently distinctive of mammals. In the first place, the aorta leaving the heart and conveying blood to the body is only a half arch, and bends to the left side as seen in Fig. 43. The right and left halves are present in reptiles, and meet behind the heart. In the bird the right half alone has remained. This fact, therefore, shows that the mammal cannot have been derived from a bird-like ancestor, but that both must have independently come from an ancestor with both halves of the aortic arch present, of which one half has disappeared in one group, and the other half in the other. It is an interesting fact, too, to notice that the four
cavities of the mammal's heart, which fourfold division it shares with birds alone, do not exactly correspond compartment for compartment with those of the bird's heart, at least in so far as concerns the ventricles. For the reptilian heart is provided with only one ventricle, and therefore the division of that cavity must have been independently accomplished in mammals and in birds.
There are two features in the venous system which distinguish all the Mammalia (with the exception of Echidna in one of these points) from vertebrates standing lower in the series. The hepatic portal system is limited to a vein which conveys to the liver blood derived from the alimentary tract; in no mammal except in Echidna is there any representative of the anterior abdominal vein of lower vertebrates. In that animal there is such a vein, which apparently arises from a capillary network upon the bladder and passes up, supported by a membrane, along the ventral wall of the abdomen to the liver, thus emptying blood into that organ exactly as does the anterior abdominal vein of the frog. In no mammal is there any trace of a renal portal system. The kidneys derive their blood from the renal arteries only.
Many mammals have two superior venae cavae; this is the case, for instance, in the Elephant and the Rodents and other types lying comparatively far down in the series. In most if not in all mammals there are considerable remains of one of the posterior cardinals, in the form of the azygos vein, which opens into the vena cava superior or pre-caval vein, i.e. the superior cardinal just before the latter debouches into the heart. This one posterior cardinal is usually on the right side; but it may be on the left side, for instance in Trichosurus vulpecula. In Halmaturus bennettii there are two azygos veins, one left and one right, of which the left is rather the larger.[[34]]
Urinary Organs.—The kidneys in the Mammalia have a compact form, which contrasts with the somewhat diffuse and vaguely-outlined kidneys of the Sauropsida. In mammals the organ is as a rule of that peculiar shape which is called "kidney-shaped"; a depression termed the hilum, which receives the ducts of the glands, indenting the border of an otherwise oval-shaped gland. In some few mammals the kidney is broken up
into lobules; this is the case with the Whales, the Bears, the Oxen, and a few other forms. A curious fact about the kidneys of the Mammalia is their very general asymmetry of position. One of them usually lies in a more advanced position than the other. The ureters lead from the kidneys to the urinary bladder, which in its form and relations is quite distinctive of the Mammalia. The bladder is formed out of the remains of the allantois, and is therefore not the exact homologue of the bladder of the frog, which is the equivalent of the entire sac which grows out of the cloaca in the mammal, and is the foetal allantois. The ureters open into the bladder in the higher Mammalia, but lower down in the urino-genital passage in the more primitive mammals.
The Body Cavity.—The Mammalia differ from all other living vertebrates by the arrangement of the body cavity in which lie the viscera. That cavity is divided into two by a partly muscular and partly tendinous partition, the diaphragm. No other vertebrate has this precise disposition of the coelom. The diaphragm lies usually transversely to the longitudinal axis of the body, but gets a much more oblique arrangement in the Cetacea and the Sirenia, whose needs demand a more expanded chamber for the lungs. For in front of the diaphragm lie the lungs and heart; behind it the stomach, liver, intestines, and the organs of reproduction and excretion. The diaphragm is used in respiration; when its muscles contract, the surface directed toward the pleural cavity becomes less convex, and the cavity of the lungs is thus increased, allowing them to expand under the pressure of the entering air.