332 FISHES CHAP. 



branchial arteries of the two sides the dorsal aorta is formed. It 

 may be pointed out that the anterior efferent vessel of each arch, 

 which is usually larger than the posterior one, is to be regarded 

 as the primary efferent artery of the corresponding holobranch, 

 and as such is directly continuous with an epibranchial artery, 

 the posterior efferent artery being a secondary vessel which opens 

 not into the primary trunk of its own branchial arch, but into 

 that of the succeeding arch. 1 The principal arteries which 

 supply the various parts of the head with blood are derived 

 from the first efferent branchial vessel. From the ventral end 

 of this artery a mandibular artery is given off, which subdivides 

 into branches for the muscles of the lower jaw as well as into 

 nutrient vessels for the hyoidean hemibranch. At about the middle 

 of its length the same artery gives off an afferent pseudobranchial 

 or hyoidean artery, to the spiracular or mandibular pseudobranch. 

 From the latter organ the blood is collected by an anterior 

 carotid artery which, after giving off an ophthalmic branch to 

 the eye, perforates the orbital wall and enters the cranial cavity, 

 where it is joined by an anastomotic trunk from the posterior 

 carotid of the opposite side ; finally, the anterior carotid divides 

 into anterior and posterior cerebral arteries for the brain. The 

 third and last of the cephalic arteries is the posterior carotid ; 

 this artery arises from the dorsal extremity of the first efferent 

 branchial vessel, and, on entering the orbit, gives off the anosto- 

 motic trunk previously mentioned. The latter vessel enters the 

 cranial cavity, and, after crossing its fellow, joins the anterior 

 carotid of the opposite side, as described above. The main trunk 

 is then continued forwards in the orbit, and its various branches 

 eventually supply the eye-muscles, the mandibular adductor muscle, 

 and some other parts of the head. 



It is worthy of note that the median dorsal aorta is prolonged 

 forwards in front of the first pair of epibranchial arteries as a 

 slender median vessel (a.d.a), which ultimately divides into two 

 branches, each branch uniting with the posterior carotid of its 

 side. 



A remarkable system of arteries for the supply of nutrient 

 blood to the gills and heart has its origin in the following 



1 Chlamydoselachus is more primitive in this respect, and has but a single 

 efferent vessel for the two hemibranchs of each arch, which corresponds with the 

 more anterior of the two in Mustelus (Ayers, op. cit.). 



