THE SKULL 



69 



cartilaginous skull (chondrocranium) itself, which is thus more or 

 less completely replaced by an osteocranimn. In general the 

 higher the systematic position of the animal, the less extensive 

 are the cartilaginous constituents and the more important the 

 bony. 



I. The Visceral Skeleton. 



The primarily cartilaginous visceral arches encircle the anterior 

 section of the alimentary canal, lying embedded in the inner part 

 of the walls of the throat (Figs. 52 and 53) and usually becoming 

 ossified latter. They are always present in a greater number (up 

 to as many as nine) in forms which 

 possess gills than in higher types 

 (Amniota), in which they gradually be- 

 come reduced, and may undergo a 

 change of function, certain of them in 

 some cases taking on definite relations 

 to the auditory organ and larynx. 



The most anterior arch, serving as 

 a support for the walls of the mouth 

 and receiving its nerve supply from the 

 trigeminal, arises first, and is distin- 

 guished from the other or post-oral 

 arches as the mandibular arch. The 

 post-oral arches only function as gill- 

 bearers in the Anamnia : even the first 

 of these, the hyoid, which 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. All 

 the visceral arches must originally, how- 

 ever, have borne gills. 



Primarily unsegmented, the indi- 

 vidual post-oral arches may become 

 broken up into as many as four pieces, 

 of which the uppermost becomes inserted 

 under the base of the skull, while the 

 lowermost is connected with its fellow by a median basal piece 

 (Fig. 52). 



The mandibular arch also undergoes segmentation, and becomes 

 divided into a short proximal piece, the quadrate, and a long distal 

 mandibular or Meckel's cartilage (Fig. 53). The quadrate grows 

 out ant-feriorly into a process, the palatoqimdrate or palatofterygoid, 

 which usually becomes fixed to the base of the skvill and forms 

 the primary upper jaw, Meckel's cartilage forming the lower jaw. 



ifiG. 52. — Diagrammatic 

 Tkansvbrse Section of a 

 STILL Later Stage in the 

 Development of the 

 Primordial Skull. 



C, notochord ; Tr, trabeculae, 

 which enclose the brain 

 (C) ventrally and later- 

 ally; 0, auditory capsule ; 

 RH, the cavity of the 

 pharynx, enclosed by the 

 visceral skeleton ; 1 to 4, 

 the individual elements 

 composing each visceral 

 arch, which is united 

 with its fellow by a basal 

 piece (Cp). 



