60 



COMPARATIVE ANATOMY. 



arises first, and is distinguished from the other or post-oral arches 

 as the mandibular arch. The post-oral arches only, function as gill- 

 bearers in the adult fish : even the first of these, the hyoid, which 



FIG. 46. DIAGRAM SHOWING THE RELATIONS OF THE EMBRYONIC VISCERAL 



SKELETON. 



N, nasal capsule ; A, eye ; 0, auditory capsule ; Tr, trabeculse, which, from being 

 bent down anteriorly (t), again extend forwards in a line with the vertebral 

 axis ; M, Meckel's cartilage ; Qu, quadrate ; Hy, hyoid arch ; B, B, branchial 

 arches, between which, are seen the gill-clefts ; S, spiracle ; Co, Co, basi-hyoid 

 and basi-branchials. 



is supplied by the facial nerve, becomes modified from those 

 lying behind it: the latter, or branchial arches proper, are 

 supplied by the glossopharyngeal and vagus. Nevertheless, 



FIG. 47. SEMI-DIAGRAMMATIC FIGURE OF AN ELASMOBRANCH SKULL, SHOWING 

 THE RELATIONS OF THE SEGMENTAL CRANIAL NERVES. 



N, nasal capsule ; A, eye ; 0, auditory capsule ; Tr, trabecula ; Q and PQ, quadrate 

 and palatopterygoid, which are bound to the trabecula by ligament at t ; M, 

 Meckel's cartilage ; L, L l , labial cartilages ; H, hyomandibular ; K, hyoid arch ; 

 a to e, branchial arches, between which the gill-clefts (/ to V) are seen ; S, 

 spiracle ; C, notochord ; W, W, vertebrae ; V, trigeminal nerve, and 1, 2, 3, its 

 three main divisions : Up 1 , its palatine branch ; VII, facial nerve ; Up, its 

 palatine branch ; IX, glossopharyngeal ; X, vagus. 



everything goes to prove that formerly a time existed in which all 

 the visceral arches must have borne gills, and in the embryos of 

 Elasmobranchs they even still do so. 



Originally unsegmented in most cases, the individual arches 

 may become broken up into different (as many as four) pieces, of 



