230 THE DOG-FISH 



the ventral surface of the skull, unites with it^ 

 fellow to form a median vessel, which passes 

 through a foramen in the skull into the cranial 

 cavity, and then divides into right and left arteries 

 supplying the brain. 



_^^ 



^ b. The hyoidean artery arises from the anterior limb 

 of the first loop near its ventral end, and runs, 

 forwards and outwards along the anterior surface 

 of the cerato-hyal. It passes round the dorsal 

 border of the spiracle, giving branches to the 

 pseudobranch, and then runs forwards along the 

 sides of the mouth. 



3. The dorsal aorta and its branches. 



The aorta is a median vessel, formed by the union 

 of the efferent branchial vessels of the two sides. The 

 anterior pair of these vessels unite opposite the level of 

 the inner openings of the fourth gill-clefts, and the suc- 

 ceeding ones open in pairs into the aorta so formed. 



The dorsal aorta so formed runs back the whole 

 length of the body, lying in the trunk just below the 

 vertebral colurnn, and in the tail, where it becomes 

 the caudal artery, in the haemal arches of the ver- 

 tebra. It gives off branches supplying the whole of 

 the body behind the head. 



To dissect the dorsal aorta and its branches, inject with 

 plaster of Paris from the caudal artery as explained above 

 (p. 228). The anterior part of its course should be dissected 

 between the kidneys ; the posterior part, ivhich is deeply placed 

 between and behind the kidneys, is best seen by dissecting one 

 of the kidneys from the body -wall and turning it inwards. 



The principal branches of the dorsal aorta are as 

 follows. 



i. The subclavian arteries are a pair of vessels, 

 which arise from the dorsal surface of the 

 aorta between the third and fourth pairs of 

 efferent branchial vessels, and run outwards 



