Mammalia: Visceral Systems 



623 



B I CUSP I D 

 VALVE 



Fig. 175. Diagram (ventral view) of the chambers of the mammalian heart 

 and their associated vessels and valves. The walls of the ventricles are shown in 

 black; those of the auricles are stippled. The direction of flow of blood is indicated 

 by arrows. (After Jammes. Courtesy, Neat and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



In adult modern reptiles the ventral heart is connected with the 

 dorsal aorta by two arteries which bend dorsalward, one on the right 

 and one on the left, to join in the dorsal aorta (Fig. 476). This pair of 

 vessels develops in the region of the fourth visceral arch of the 

 embryo and therefore corresponds to the fourth of the several aortic 

 arches of a fish. The relation of the two aortic trunks to the ventricles 

 is extraordinary. Just anterior to their connection with the heart, the 

 two vessels cross one another so that the one which passes upward on 

 the right side connects with the left ventricle, and the vessel on the 

 left conies from the right ventricle along with the main pulmonary 

 trunk. This left vessel therefore carries "impure" blood. The ventricle 

 in most reptiles has only an incomplete partition between right and 

 left chambers. Accordingly, some mingling of aerated and nonaerated 

 blood occurs in the ventricles. Even so, the right aortic trunk carries 



