THE ELASMOBRANCH FISHES 227 



the skull close below the posterior margin of the orbit. The 

 common carotid artery divides into external and internal 

 carotids. The former is given off immediately in front of 

 the union of the common carotid with the bifurcation of the 

 dorsal aorta. It passes through a small foramen in the hinder 

 floor of the orbit, runs forward below the maxillary branch 

 of the fifth nerve, and breaks up into branches, supplying 

 the upper jaw and snout. The internal carotid is a continua- 

 tion of the course of the common carotid to the middle line. 

 Here it passes into the cranial cavity by a median foramen, 

 crosses the internal carotid of the opposite side, unites with 

 the opposite hyoidean artery, and breaks up into branches, 

 supplying the brain. 



Posteriorly, the dorsal aorta is continued backward as a 

 large vessel lying close beneath the vertebral column ; in the 

 caudal region it is enclosed in the hsmal arches of the 

 vertebrae. In its course it gives off arteries to the limbs, 

 viscera, and myotomes, and diminishes correspondingly in 

 size towards the tail. The most important of these arteries 

 are : The subclavian arteries, arising just in front of the 

 union of the fourth efferent branchials with the dorsal aorta ; 

 they supply the pectoral girdle and pectoral fins. The coeliac 

 artery is a large median vessel arising from the ventral surface 

 of the aorta, a short distance behind the junction of the 

 last efferent branchial arteries. After a short course in the 

 mesentery, it divides into two branches, one of which supplies 

 the anterior end of the stomach and the liver, the other the 

 anterior end of the intestine and the pancreas. 



The anterior mesenteric and the lieno-gastric (fig. 51) 

 arteries arise close together from the dorsal aorta about an 

 inch and a half behind the coeliac artery. The former 

 supplies the intestine, the latter, arising just behind the 

 anterior mesenteric, crosses it in its course, and is distributed 

 to the posterior bend of the stomach and to the spleen. 

 The posterior mesenteric artery is a small median vessel 

 which leaves the dorsal aorta some little way behind the 

 lieno-gastric and runs in the meso-rectum to the rectal gland. 

 The above are the more conspicuous branches of the dorsal 

 aorta. In addition there are smaller vessels — e.g. the inter- 

 costal arteries, a pair of which are given off in each trunk 

 segment to the myotomes ; the pelvic arteries, 'supplying the 



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