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: 
(1) 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 systemic 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 
