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 amoeboid 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, Avith 

 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 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 generalisation there are many exceptions. The 

 transverse diameter of the red corpuscles in Man averages -32V0 of 

 an inch, which is exceptionally large, and only exceeded by the 

 Elephant { z fa s ), 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 T. javanicus and meminna not more than x^stts- 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 (chordce 

 tendinice) and arising from projections of the muscular walls of 

 the ventricular cavity (riiusculi papUlares). 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 (os 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 



1 G. Gulliver, Proc. Zool. Soc., 1862, p. 91. 



