VASCULAR SYSTEM 



315 



ternally (Figs. 254256). It becomes partially divided into two 

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



D.Cti. 



FIG. 256. 



From the dorsal side. 



FIG. 255. 



FIG. 255. HEART OF A YOUNG Crocodilu* niloticus. 



(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, fi. V.h, left and right atria ; 

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



FIG. 256. HEART OF Crocodilus niloticus. 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 



