REPTILIA 567 



modification are apparent in the pulmonary circulation. The system 

 consists of a heart, arteries, capillaries, and veins, forming a continu- 

 ous or closed set of vessels throughout the body. 



The heart lies at the extreme anterior end of the body cavity and 

 is enclosed in a thin- walled sac, the pericardium. The heart consists 

 of a dorsal sinus venosus, a right and a left auricle, located anteriorly, 

 and a posteiior ventricle. The auricles are thin-walled chambers 

 which communicate with the ventricle by means of the auriculoven- 

 tricidar aperture. This aperture is guarded by an auriculoventricular 

 valve which consists of two flaps. The thin vertical inter auricular 

 septum separates the auricles. The ventricle is a thick, cone-shaped, 

 muscular chamber. The base is directed anteriorly and is connected 

 to the auricles at the auriculoventricular aperture. The thick, spongy 

 walls of the ventricle leave only a small cavity within its chamber just 

 below the auriculoventricular aperture. It is completely divided into 

 tw^o chambers by the perforated interventricular septum only w^hen in 

 a state of contraction. 



Three arterial trunks arise from the ventral surface of the ven- 

 tricle. The pidmonary artery arises from the right portion of the 

 ventricle. Its aperture is separated from that of the right aorta by 

 means of a muscular partition. 



The pulmonary artery is visible for only a short distance. It origi- 

 nates from the right side of the ventricle, passes anteriorly, then 

 turns dorsal to the heart. It soon divides into right and left trunks 

 supplying the respective lungs. 



The right aortic arch arises from the left half of the ventricle and 

 at first is dorsal to the other two arterial trunks. Soon the pulmonary 

 artery assumes the dorsal position and the right aortic arch becomes 

 ventral and passes to the right and anteriorly between the auricles. 

 At the anterior end of the heart it forks to form a Y. The branch 

 passing to the left is the left common carotid artery. The right 

 branch forks again. The anterior branch is the right common carotid 

 artery, and the other is the continuation of the right aortic arch, 

 which gives off the two subclavian arteries. 



The left aortic arch is the most ventral of the three arterial trunks 

 and it also arises from the left side of the ventricle. The right aortic 

 arch is in close relation to the left arch. The left aortic arch then 

 passes laterally and diagonally across the pulmonary artery and the 



