250 THE DOG-FISH. 



b. The hyoidean artery arises from the anterior limb 

 of the first loop about the middle of its length, 

 in line with the connecting vessels between the 

 successive branchial loops, and runs forwards to 

 the anterior wall of the spiracle, where it gives 

 branches to the pseudobranch. It then con- 

 tinues its course forwards and inwards across 

 the floor of the orbit, crossing ventral to the ex- 

 ternal carotid artery, and, entering the skull by 

 a small hole, ^ inch in front of the inter-orbital 

 foramen, joins with the internal carotid artery of 

 the opposite side, and divides into branches 

 supplying the brain. 



3. The dorsal aorta and its branches. 



The anterior pair of efferent branchial vessels 

 unite about the level of the inner openings of the 

 fourth gill-clefts, forming the dorsal aorta, into 

 which the hinder vessels open in pairs. 



In front of the point of union of the first efferent 

 branchial vessels the aorta is continued forwards 

 beneath the skull as a slender vessel : after a short 

 course this divides into right and left branches which, 

 running outwards, join the carotid arteries just before 

 their division into external and internal carotids. 



The dorsal aorta runs backwards along the whole 

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

 vertebral column, and in the tail, where it becomes 

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

 tebrae. It gives off branches supplying the whole of 

 the body behind the head. 



The anterior part of the aorta should be dissected between 

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

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

 kidjieys from the body-wall and turning it inivards. The 

 posterior part is, Iwtvever, best left until the excretory system 

 is dissected. 



