314 



COMPARATIVE ANATOMY 



septum, which may be incomplete, as in Lizards (Fig. 253, B), 

 Snakes, and Chelonians, or complete, as in Crocodiles. 



The conns arteriosus now becomes practically absorbed into 

 the ventricular portion of the heart, and each aortic root may be 

 made up at its origin of two arches, anastomosing with one another 

 (Lacerta, Fig. 243, A), or of one only (certain Lizards, Snakes, 

 Chelonians, and Crocodiles, Figs. 253, B, 255), from which the 

 carotid artery arises directly. The left and right aortic arches cross 

 at their base, so that the left arises on the right side, and vice 

 versa. 1 The most posterior arterial arch gives rise to the pul- 

 monary artery (comp. Fig. 243, D). 



The blood from the right ventricle passes into the pulmonary 

 artery as well as into the left aortic arch, and, according as the septum 



A n.s- 



L.v 



Ao.aU. 



FIG. -254. HEART OF Cydodus boddaertei. From the dorsal side. (After Rose). 

 The sinus venosus is almost entirely absorbed into the right atrium. 



D.CU, D.C.d, precaval veins; V.c.i, postcaval vein; V'.j.d, jugular, V.s.d, sub- 

 clavian, and V.G.d. posterior cardinal vein of the right side. L.v, pulmonary 

 vein ; P.s, P.d, pulmonary arteries ; An.*, An, innominate arteries ; Ao.aM, 

 dorsal aorta ; Sp.i, spatium intersepto-valvulare (comp. Fig. 257). 



ventriculorum is complete or incomplete, is either entirely venous 

 (Crocodiles) or mixed (other Reptiles, Fig. 253, c). 



The valves of the heart have undergone a considerable reduction 

 in Reptiles : at the origin both of the aorta and of the pulmonary 

 artery there is only a single row ; this is also the case in all other 

 Amniota. In Crocodiles the right atrio-ventricular aperture is 

 guarded by a large muscular valve on the right (outer) side of the 

 aperture. 



The sinus venosus, which even in the Amphibia especially 

 Anura shows indications of becoming sunk into the right atrium, 

 is now usually no longer recognisable as a distinct chamber ex- 



1 A small aperture of communication between the two aortic roots, the foramen 

 Panizzce, exists in Crocodiles. 



