The Vascular System 203 



two wide, thin-walled pulmonary veins, Fig. 58, 

 V.p.s., V.p.d. They leave the lungs somewhat 

 caudad to their middle region, near the point of 

 entrance of the bronchii and the pulmonary ar- 

 teries, pass mediad in a direction almost at right 

 angles to the long axis of the body, and enter the 

 left auricle at the same point. 



Blood leaves the heart through five large vessels : 

 (i) the pulmonary artery, (2) the two aortic arches, 

 (3) the right subclavian, (4) the primary carotid. 



The pulmonary leaves the small right ventricle 

 as a single stem, which soon branches into two 

 arteries that pass cephalad and laterad to the lungs, 

 along with and close to the main bronchi. The 

 other arteries that carry blood into the systemic 

 circulation are fused at their base to form a sort 

 of conus arteriosus which may be distended in 

 injected specimens until it is larger than the two 

 ventricles together. When opened this conus is 

 found to contain two chambers that lead into the 

 left ventricle; the larger chamber gives origin to 

 the right systemic arch, the right subclavian, and 

 the primary carotid; the smaller chamber is the 

 basal part of the left systemic arch. 



The two vsystemic vessels, Fig. 58, Ao.s, Ao.d, 

 pass, in the usual manner, as two arches to the 

 dorsal region, just posterior to the ventricles, 

 where they form the dorsal aorta in the manner to 

 be described in connection with the arterial system. 



The further course of the primary carotid and 



