516 ANATOMY OF VERTEBRATES. 



chus the blood-discs are circular and average ^^Voth inch in 

 diam. : being larger in proportion than in Man, though less than 

 in the Sloths. The numerous and insignificant gradations of size 

 of Mammalian blood-discs between the two extremes noted in 

 clxxix" have been recorded, decimally, in ccxxxix, vol. i. p. 84. 



§ 347. Heart of Mammalia. — In Mammals, as in other Hsema- 

 tothermals, the venous and arterial parts of the vascular system 

 have no communication, beyond the heart, save at the peripheral 

 capillaries. 



The right auricle is less definitely divided into ' sinus ' and 

 f auricle' proper than in Birds, and the intervening valves, always 

 less efficient against reflux from the auricle into the sinus, 

 gradually disappear. The right auriculo-ventricular valve re- 

 sembles in structure the left, as being membranous and attached 

 by tendinous threads to muscle. Other differences between the 

 circulating systems of the two hot-blooded classes are shown by 

 blood-vessels. 



The heart, with its bag, or pericardium, is exclusively located 

 in the thorax, and in many Mammals is more or less separated by 

 a lobe of the lung, fig. 308, n, from the diaphragm, q. 



A. Heart of Lyencephala. — In the Ornithorhynchus, fig. 308, 

 a. b, c, it presents a rounded oblong, scarcely conical, form ; it is 

 situated in the middle of the fore part of the chest, parallel with 

 the axis of the cavity, inclosed in a thin subtransparent but strong 

 pericardium. The right auricle, b, is larger and longer than the 

 left ; its appendix is free and is slightly bifid. It receives the 

 venous blood by three great veins ; the left precaval, f descend- 

 ing behind the left auricle, c, to join the termination of the post- 

 caval, h ; to the right of which the coronary vein also terminates 

 in the auricle. The right precaval, e } is joined to the left by a 

 transverse branch, g. There is a deep but closed fossa ovalis 

 near the upper extremity of the septum of the auricles ; in- 

 dicating that the intra-uterine existence of the young was of 

 longer duration than in the Marsupials. The right ventricle, a, 

 is capacious, with thin parietes. The tricuspid valve consists of 

 two membranous and two fleshy portions : the smallest of the 

 latter is situated nearest the origin of the pulmonary artery, and 

 corresponds with the lesser fleshy valve in the heart of certain 

 Birds (vol. ii. p. 188, fig. 92, m) : it is attached to the whole of the 

 side of the first or adjoining membranous portion. The second 

 fleshy portion answers to the larger muscular valve (ib. fig. 92, /). 

 The two edges of the lower half of the second fleshy portion of 

 the valve in the Ornithorhynchus are free ; but those of the 





