CIRCULATORY AND RESPIRATORY SYSTEMS 63 
V. 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 the vertebrates, of two kinds: (1) 
colourless, spheroidal, nucleated, and exhibiting amceboid move- 
ments; 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 discs, 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 the absence of nucleus and relatively small 
size. As arule 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 generalisation there are many exceptions. The 
transverse diameter of the red corpuscles in Man averages zy5q of 
an inch, which is exceptionally large, and only exceeded by the 
Elephant (z7;5), and by 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 7. javanicus and meminna not more than >z475.1 
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 verte- 
brates that the right auriculo-ventricular valve is tendinous like the 
left, consisting of flaps held in their place by fibrous ends (chorde 
tendinie) and arising from projections of 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 dis- 
tinct but rather irregular ossification (0s cordis) is developed in the 
central tendinous portion of the base of the heart. 
Blood-vessels,—The orifices of the aorta and pulmonary artery are 
1G. Gulliver, Proc. Zool. Soc., 1862, p. 91. 
