COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 235 



caval enters the liver from behind to the right of the hepatic portal vein. At this 

 point the serosa of the liver is fused to the pleuroperitoneal membrane over the 

 ventral face of the lung. This fusion should be broken and the postcaval vein 

 freed. On following it posteriorly it will be found to swerve toward the median 

 line where it runs alongside a large artery (dorsal aorta). The postcaval may 

 be traced to the posterior end of the pleuroperitoneal cavity. Its relations there 

 will be described later. 



Add the left hepatic and postcaval veins to your drawing of the precaval vein. 



5. The pulmonary veins. — A pulmonary vein passes from each lung to the 

 left auricle of the heart. It is situated posterior to the bronchus where it should 

 be identified. Follow it toward the heart. It passes dorsal to the precaval 

 vein. The right pulmonary runs in the dorsal wall of the pericardial sac anterior 

 to the sinus venosus and joins the left vein at the entrance of both into the left 

 auricle. The point of entrance is near the left precaval vein. 



6. The aortic arches and their branches. — From the ventricle three large 

 arterial trunks extend forward. Together they constitute the ventral aorta which 

 must be conceived of as having split into three trunks. Clean away the connec- 

 tive tissue from these arteries and separate them from each other. The trunk 

 farthest to the left is the pulmonary artery; the vessel next to it is the 

 left aorta; the third and right-hand trunk is the right aorta, but it is concealed 

 from view by the large branch, the brachiocephalic (innominate) artery which- 

 it gives off immediately on leaving the heart. Note the small coronary arteries 

 springing from the base of the brachiocephalic artery and branching over the 

 surface of the heart. The brachiocephalic artery lies in the median line and 

 forks at once into large branches. In the angle of the fork lies a reddish body, 

 the thyroid gland. 



a) The branches of the brachiocephalic artery: We shall follow this vessel first; 

 it divides at once into four trunks; the large medial ones are the right and left 

 subclavian arteries, the much smaller lateral ones are the right and left carotid 

 arteries. Clean away connective tissue from these vessels and follow their 

 courses. The two subclavians embrace the thyroid gland between their bases 

 and supply small thyroid arteries into this gland. Each subclavian next gives 

 off branches to the ventral side of the neck and to the trachea, of which the chief 

 one is the ventral cervical artery, a vessel arising from the subclavian about one- 

 half inch beyond the thyroid gland and branching profusely into the esophagus, 

 trachea, muscles of the neck, and thymus gland. The thymus gland is a yellowish 

 mass lateral to the ventral cervical artery and receiving branches from it. The 

 subclavian artery, now named the axillary, turns laterally and passes to the inner 

 surface of the pectoral girdle, where it gives off some branches into the pectoral 

 and shoulder muscles and the brachial artery to the fore limb. The axillary 

 artery then turns sharply posteriorly and about one inch beyond the turn 

 gives off the small dorsal cervical artery to the neck and a scapular artery 



