THE ENDOSKELETON 697 



visceral arch ossifies, usually though not always, forming two bones on 

 each side, the paired anterior pterygoid and the paired posterior quadrate. 

 These now become the suspensor of the lower jaw. In some forms such 

 as the teleosts and the reptiles, there appears an entire series of ptery- 

 goid bones. 



Outside of the pterygoquadrate a second arch of membrane bone 

 develops to form the functional upper-jaw in all bony vertebrates. This 

 when fully developed consists of: 



Squamosal, underlying the quadrate. 



Quadratojugal, which follows immediately. 



Zygomatic, also called malar or jugal. 



Maxillary. 



Premaxillary, which forms the tip of the jaw. 



Only the maxillary and premaxillary bear teeth. 



When, as in the higher forms of human life, the roof of the skull is 

 not continuous, but openings of various kinds are seen, such openings 

 are known as fossae. The more common and constant are as follows : 



Infratemporal, being the most lateral. 



Supratemporal, which is separated from the foregoing by the 

 squamoso-postorbital bars. 



Posttemporal, lying between the parietal, supratemporal, and occipi- 

 tal bones. 



Temporal, when infra and supra temporal fossae unite. 



Any of the bones mentioned above may fuse or disappear entirely 

 in certain groups, while in others there may be connections quite differ- 

 ent from the usual type. 



The lower jaw is by no means modified as extensively as the upper. 

 Meckel's cartilage, by an ossification, gives rise to two bones in each 

 half of the lower jaw. There is an articular bone (articulare) where the 

 jaw meets with the quadrate, and at the tip where both sides unite the 

 symphysis there may be a mento-Meckelian bone, although this does 

 not occur often. The rest of Meckel's cartilage forms an axis about 

 which the membrane bones of the lower jaw are arranged. These are 

 as follows : 



(1) Dentary, surrounding the Meckelian in front and bearing teeth. 



(2) Splenial, on the inner side behind the dentary and often bear- 

 ing teeth. 



(3) Angulare, on the lower side usually extending back to the 

 hinder end of the jaw. 



(4) Surangulare, lying on the outer posterior part of the jaw. 



(5) Coronoid, on the upper side attach the muscles which close the 

 jaws. 



(6) Goniale, also called antarticular or dermarticulare, lies on the 

 medial and ventral sides of the articulare with which it usually fuses. 



