316 COMPARATIVE ANATOMY 



of the arterial and venous blood. The muscular walls of the ventricle 

 are strongly developed and very compact. This is particularly the 

 case in the left ventricle, on the inner wall of which the papillary 

 muscles are well developed : the left ventricle is partially surrounded 

 by the right, the cavity of the latter having a semilunar transverse 

 section, and its walls being much thinner than those of the former 

 (Fig. 258). 



In both Birds and Mammals the blood from the head and body 

 passes by means of the precavals and postcaval into the right 



Ao. 



\-MKl 



FIG. 257. HEART OF GOOSE (Anser vtilgaris), DISSECTED FROM THE RIGHT SIDE. 



(After Rose.) 



The right atrium and ventricle are cut open, and their walls reflected. S.a, 

 septum atriorum ; L. Vi, limbus Vienssenii a ridge arising from the ventral 

 wall of the right atrium ; the space between this and the septum atriorum is 

 known as the spatium intersepto-valvulare (comp. Figs. 254 and 255). F.a.*, 

 V.a.d, the two sinu-auricular valves, situated at the entrance of the postcaval ;. 

 MK, MK', muscular right atrio-ventricular valve ; Ao, aorta ; V.c.s.d, right 

 precaval ; V.c.c, aperture of coronary vein. 



atrium, as does also that from the walls of the heart through the 

 coronary vein l (Figs. 257, 259, 260, B), and the sinus venosus 

 especially in Mammals is scarcely recognisable (Figs. 257, 230) : 

 the right atrium is separated from the right ventricle by means of 

 a well-developed valve. In Birds (Fig. 257) this valve resembles 

 that of Crocodiles, and is very large and entirely muscular, while in 

 most Mammals it consists of three membranous lappets (tricuspid 



1 Coronary veins are present in most of the lower Vertebrates also (comp. e.g., 

 Fig. 255), and the heart is supplied with arterial blood by coronary arteries, usually 

 arising in Fishes from a hypobranchial artery connected with the efferent branchials 

 or subclavians, and in higher forms from the base of the aorta. 



