Motor System — Muscles and Skeleton 127 



RELATIONS OF JAWS TO CRANIUM 



Comparison of various vertebrates reveals radical differences in 

 the relations of the jaws to the cranium. In the sharklike fishes, the 

 upper jaw is somewhat loosely suspended from the under side of the 

 cranium. The attachment may be merely by ligaments at the anterior 

 and posterior ends of the jaw, the hyoid arch being quite independent 

 of the mandibular arch (Fig. 116). In many sharks, however, the 

 hyoid arch becomes attached to the mandibular arch at the joint 

 between upper and lower jaws, the lower end of the hyomandibula 

 being especially tightly bound to the jaw and its upper end strongly 

 attached to the otic region of the cranium. The hyomandibula thus 

 becomes a suspensor of the jaws, while the more ventral portion of 

 the hyoid arch is loosely swung behind the lower jaw (Fig. 113). In 

 the bony fishes the hyomandibular cartilage usually ossifies to form 

 two bones, a dorsal one which retains the name hyomandibula, and 

 a ventral symplectic (Fig. 120, 37, 38). These two hyomandibular 

 bones serve as a suspensor for the jaws much as the single cartilaginous 

 hyomandibula does in sharks. The dorsal end of the upper bone is 

 joined to the otic part of the cranium and the lower end of the sym- 

 plectic attaches to the quadrate bone. Thus the series of bones of the 

 upper jaw and the hyoid suspensor, as viewed laterally, are arranged 

 in the form of the letter V, the upper ends of the arms of the Y being 

 attached to the ventral side of the cranium, and the quadrate bone, 

 to which the lower jaw articulates, forming the point of the V (Fig. 

 120). 



In all vertebrates other than fishes, the hyoid structures of the 

 adult have nothing to do with the jaws. The V loses its posterior arm. 

 It is as if, in the absence of the hyoid suspensor, the upper jaw had 

 rotated bodily upward — its anterior attachment to the cranium being 

 the pivot of rotation — to the level of the floor of the cranium. It arrives 

 in this new location in such a way that the bones of the primary 

 palato-pterygo-quadrate series are nearer the median plane and 

 those of the secondary dermal series are lateral to them (Fig. 121). 

 The members of the first series thus come to be adjacent to bones of 

 the floor of the cranium and acquire rigid (usually) attachment to 

 certain of them — commonly to the vomer (prevomer), parasphenoid, 

 and basisphenoid — thereby becoming bones of the roof of the mouth. 

 The bones of the second series lie along the outer border of the roof 

 of the mouth. The prcmaxilla and maxilla commonly become so much 

 broadened as to add to the extent of the anterior region of the roof 

 of the mouth. Another feature of the new arrangement is the sepa- 



