224 STRUCTURE OF THE VERTEBRATES 



After the afferent branchials break into capillaries in the gills 

 they re-collect as efferent branchial arteries. The four pairs of 

 efferent branchials of Squalus form complete loops around each 

 gill pouch, and then enter the paired dorsal aortae which forms a 

 single vessel posterior to the pharynx. 



From the first efferent branchials a pair of common carotid 

 arteries passes forward, and these divide to form the internal 

 and external carotids. These vessels supply the head and brain. 

 Posterior to the carotids branches pass from the aortae to the 

 mouth and esophagus; and posterior to these the large suh- 

 clavians go to the pectoral fins. The hypobranchial arteries sup- 

 ply the muscles of the gills, and from them the coronary arteries 

 pass to the heart and pericardium. 



The body muscles are supplied b}" the small, metameric parietal 

 arteries. The larger branches of the dorsal aorta, in order from 

 anterior to posterior, are: (1) coeliac which divides into several 

 branches, and goes to the liver, stomach, duodenum, pancreas, 

 and parts of the ileum; (2) the anterior mesenteric artery to the 

 left side of the intestine; (3) the gastro-splcnic to the spleen and 

 parts of the pancreas and stomach; (4) the posterior mesenteric 

 supplies the rectal gland, rectum and cloaca; (5) the renal arter- 

 ies to the kidneys; and (6) the iliac arteries which supply the 

 pelvic fins. The dorsal aorta is continued into the tail as the 

 caudal artery. In the female the oviducal arteries arise directly 

 from the dorsal aorta. 



Blood is returned to the heart through the veins, which will 



