VASCULAR SYSTEM 



315 



ternally (Figs. 254-256). It becomes partially divided into two 

 portions by a septum ; and the left precaval, opening on the left of 



1).C.S 



Md. 



n.cd.- 



VaA- 



Fic. 25.3. 



Fic. 256. 



Fig. 255. — Heakt of a YonNG Crocodilun nilotkus. From the dorsal side. 



(After Rose). 



Tr.cc, common carotid ; S.s, S.d, subclavian arteries ; A.s and A.d, left and right 

 aortic arches ; A.m, mesenteric artery ; L. V.h, H. V.h, left and right atria ; 

 V.c.c, coronary vein. Other letters as in Fig. 244. 



Fig. 256. — Heart of C'rorodilns nihtirus. From the right side. (After Rose). 

 Part of the wall of the right atrium is removed. 



O.a.v, atrio-ventricular aperture ; Va.d and Va.s, the two sinu-auricular valves, 

 the white line between which is the margin of the sinu-atrial septum. Other 

 letters as in Figs. 244 and 245. 



this septum, may appear to enter the right atrium independently 

 {e.g., Snakes,) The pulmonary veins unite into a single trunk 

 before entering the left atrium. 



Birds and Mammals. — In these Classes, the atrial and ventri- 

 cular septa are always complete, and there is no longer any mixture 



