Motor System — Muscles and Skeleton 125 



roofed by a pair of dermal bones, the nasals, each of which joins the 

 anterior end of a frontal bone. 



Certain cranial bones are related to the ear or to the eye. The 

 otic capsule usually ossifies to form three bones, an epiotic above, 

 a prootic in front, and an opisthotic behind (Fig. 120, 12, 10, 11). 

 In some cases, one or two more bones develop. These otic bones often 

 become more or less fused together. They may all fuse into a single 

 periotic (or petrosal) bone. This otic group is situated between the 

 occipital arch and the parietal arch, therefore locally separating the 

 two arches which are contiguous above and below the otic region. 

 A usually large dermal bone, the squamosal (Fig. 120, 13), is devel- 

 oped in close relation to the otic bones and also to the joint between 

 the upper and lower jaws. 



In the region of the orbit appear several dermal bones (Fig. 120, 

 14-18) forming in part the wall of the orbit or else serving to extend 

 the cranial roof laterally so that it overarches the orbit, giving added 

 protection to the eye. In some fishes the outer rim of the orbit is com- 

 pletely encircled by circumorbital bones. In some reptiles these 

 orbital bones form, over the orbit, an anteroposterior arch consisting 

 of a postfrontal, a postorbital, one to three or four supraorbitals, 

 and a prefrontal. A dermal lacrimal bone (Fig. 120, 18) makes part 

 of the anterior wall of the orbit. 



The floor of the cranium is reinforced by dermal bones. The 

 cranial floor is also the roof of the mouth-cavity. This cavity is lined 

 by a membrane which is continuous with the external skin and has 

 the same general structure as the skin. Its deeper layer produces 

 dermal bone. A median dermal parasphenoid, extending forward 

 from the region of the cartilaginous sphenoid bones, usually consti- 

 tutes an important part of the roof of the mouth. Anterior to the 

 parasphenoid is a pair of dermal bones commonly called vomers. Just 

 here there is some confusion in names. In mammals a median bone in 

 the anterior part of the roof of the mouth has long been known as the 

 "'vomer." There is, however, some evidence that it corresponds to 

 the parasphenoid of other vertebrates. If this should prove true, giving 

 mammals priority on the name "vomer," then the parasphenoid of 

 vertebrates other than mammals should be (but is not) called " vomer," 

 and the two dermal bones just in front of it could be (and frequently 

 are) called "prevomers." 



Bones of Visceral Skeleton 



The mandibular arch produces some cartilage bones but the 

 definitive jaws consist mainly of dermal bones. The palato-pterygo- 

 quadrate cartilage of some fishes ossifies to form an anterior palatine 



