STRUCTURE OF THE VERTEBRATES 241 



liver to the heart. As the liver tubules develop, the right omphalo- 

 mesenteric is interrupted and breaks into capillaries. This con- 

 dition persists for a short time, and then the left vein breaks into 

 capillaries and the sj'stem is completed. 



The earliest homologue of the hepatic portal is found in 

 Amphioxus, where the subintestinal vein breaks into small ves- 

 sels which re-unite before entering the ventral aorta. In the 

 elasmobranch fishes the system is fully developed, and with 

 minor changes remains in all the classes of vertebrates. Blood 

 from the liver is carried to the heart by the two hepatic veins, 

 which belong with the systemic vessels. 



9. The Human Blood System. Blood leaves the heart through 

 the aorta, which turns dorsally from right to left. A few inches 

 from its base is given off the innominate artery which gives rise 

 to the right common carotid and the right subclavian. Next is 

 given off the left common carotid, and then the left subclavian. 

 The development of the aortic arches will make the relationship 

 clear. The external branch of the carotids passes to the muscles 

 of the neck and face; the internal branch goes to the brain and 

 its coverings. At the base of the brain the two internal carotids 

 anastomose forming a circle of vessels. 



The aorta passes posteriorly along the ventral side of the verte- 

 brae, giving off the metameric parietal arteries to the ribs and 

 body muscles and several esophageal branches. Next are the 

 phrenic arteries to the diaphragm which separates the pleural 

 cavities from the abdominal cavity. The large coeliac axis is 

 next, going to the viscera, and then in order: (1) the superior 

 mesenteric; (2) the paired suprarenal (adrenal) vessels; (3) the 

 paired renals; (4) the paired spermatic (genital) arteries to the 

 gonads; (5) the inferior mesenteric to the colon and rectum; and 

 (6) the sacral artery. Immediately posterior to the last the aorta 

 divides to form the two large iliac arteries which supply the 

 pelvic limbs. The numerous branches of the latter need not be 

 discussed. 



The external jugular vein, after making connections with the 

 internal jugular, enters the subclavian. The internal jugular 

 fuses with the subclavian to form the innominate. The conditions 

 are approximately the same on both sides. The two innominate 



