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 

 imoscles 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 



\-Mk: 



Fi(i. 2j7. — Hbakt of Goose (Aiiser vulgaris), dissected from the right side. 



(After Biise.) 



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

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

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

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

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

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

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



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

 coronary vein''- (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 



^ Coronari) rehu 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 hypobrauchial artery connected with the efferent branchials 

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



