THE ELASMOBRANCH FISHES 225 



The remaining demi-branchs are supplied by three pairs of 

 vessels, one pair arising from the middle, and two pairs arising 

 close together from the hind end of the cardiac aorta (fig. 54). 

 These vessels carrying blood to the gills are known as the 

 afferent branchial arteries. They break up into capillaries in 

 the demi-branchs, and the blood is carried away from these 

 latter structures by a system of efferent branchial arteries. 

 whose arrangement is shown in fig. 55. 



The blood from the capillaries of the demi-branchs is 

 collected into vessels having the form of loops, four of which 

 are complete, and surround the inner margins of the first, 

 second, third, and fourth gill-slits. But as the fifth gill-slit 

 has only one demi-branch on its anterior side, the loop be- 

 longing to this slit is incomplete — i.e. is a half-loop, running 

 along its anterior face. The loops communicate with one 

 another by short horizontal vessels placed about the middle 

 of their lengths, and a vessel given off from the corresponding 

 position of the anterior limb of the first loop runs dorsal to 

 the hyomandibular cartilage, turns inwards to pass along the 

 anterior wall of the spiracle, runs across the floor of the orbit, 

 and enters the cranium by a foramen situated just in front of 

 the inter-orbital canal. This vessel {hy.a in fig. 55) is known 

 as the hyoidean, or sometimes as the anterior carotid artery. 



The half-loop belonging to the fifth gill-slit has no special 

 efferent vessel connected with it. All the blood from the 

 demi-branch of this gill-slit passes by way of the horizontal 

 vessel into the loop surrounding the fourth gill-slit. But the 

 upper ends of the loops surrounding the first four gill-slits are 

 continued into four efferent branchial arteries which run 

 upwards, inwards, and backwards, in close connection with 

 the pharyngo-branchial cartilages, to unite with their fellows 

 of the opposite side in a large median vessel, the dorsal aorta. 

 The dorsal aorta is continued both forwards and backwards 

 from the points of union- of the efferent branchial arteries. 

 Anteriorly, it is continued along the lower side of the cranial 

 floor as a small median vessel which bifurcates opposite the 

 first branchial arches, and its branches curve outwards from 

 the middle linCj and unite on either side with the common 

 carotid artery, a vessel springing from the inner and anterior 

 end of the loop surrounding the first gill-cleft,, and curving 

 inwards and forwards along the under surface of the floor of 



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