298 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



admixture of arterial and venous blood can occur. In the birds 

 and mammals (fig. 317, F) there is complete internal separation 

 of the two sides of the heart, though externally it usually shows but 

 slight signs of the division. As a result of this division blood must 

 pass twice through the heart (once through the venous, once through 

 the arterial half) in order to make a complete circuit of the body. 

 Venous blood enters the right atrium, passes to the right ventricle, 

 by which it is forced to the lungs (pulmonary or respiratory circu- 

 lation). Returning to the heart by the pulmonary veins, it passes 



through the left atrium and ventricle 

 and thence through the systemic circula- 

 tion, by which all parts of the body are 

 supplied. 



When first formed, the heart lies close 

 behind the mandibular artery (first 

 aortic arch to be described below), but 

 as other vessels are formed it is forced 

 farther back into a position, in the lower 

 vertebrates, ventral to and a little behind 

 the pharynx, but in the adult tetrapoda 

 it is carried back, as a result of unequal 

 growth, even into the thorax, the ex- 

 treme of migration being seen in the 

 giraffe and the long-necked birds. 



Although all of the blood of the body 

 passes through the heart at short in- 

 tervals, this is not sufficient for the 

 nourishment of that organ. Therefore 



Fig. 318. — Dorsal and ventral 

 views of the heart of Amia, after 

 Parker and Davis, a, auricle; cd, 

 dorsal coronary artery; cv, ventral 

 coronary artery; cIV, commissure 

 from fourth arch; h, hypobranchial 

 artery; s, sinus venosus; v, ventri- 

 cle; va, ventral aorta. The auricle 

 has been removed from the dorsal 

 view leaving the atrio-ventricular 

 canal visible. 



its muscles are usually supplied with 



•'• 



blood through coronary arteries (fig. 



318) which arise from the aortic arches 

 and run back along the truncus arteriosus to reach the atrium and 

 ventricle. 



THE ARTERIES 



Aorta and Aortic Arches. — Theventraj^aOTta is the trunk in front 

 of the pericardium, extending from the truncus arteriosus to the 

 mandibular artery (first aortic arch). It runs, not through a cavity, 

 but between muscles and through connective tissue. The mandibu- 

 lar arteries pass upward on either side of the pharynx until they 

 reach its dorsal surface. With development, the ventral aorta 



