72 



THE BIOLOGY OF THE FROG 



CHAP. 



but of small size and uniform structure. With the excep- 

 tion of the two patches of vomerine teeth, they are confined 

 to the upper jaw. The jaw teeth rest against the dental 

 processes of the maxillary and premaxillary bones, to which 

 they are attached by cement substance. They are embedded 

 in the mucous membrane of the mouth, 

 ^ ^ beyond which they project only for a short 



distance. Each tooth is approximately cy- 

 lindrical in form, tapering shghdy toward the 

 upper end, which is somewhat incurved. 

 'Fhe basal portion of the tooth which is fas- 

 tened to the jaw is called the roo^. Upon 

 this rests the crown, which is separated from 

 the root by a transverse furrow. In the 

 center is a cavity filled with the pulp, which 

 is a very vascular tissue in which the cells 

 iyodontoblasts) are situated that produce 

 new material for the growth of the tooth. 

 The greater portion of the crown is com- 

 posed of substance called dentine which 

 forms a hard calcareous wall, traversed by 

 numerous fine branching canals which lead 

 from the pulp cavity. The upper half of the 

 crown is coated with a very hard, resistent layer of enaniel 

 which is considerably thickened over the tip. The enamel 

 shows a stratified structure, but it does not contain the verti- 

 cal prisms found in higher forms. Outside of the enamel 

 there is a thin, resistent membrane, the ciiticula dcntis. The 

 root of the tooth is composed of a substance resembling 

 bone. 



The teeth of the frog are not used for mastication, but only 

 for holding prey, which is the primitive function of teeth 

 among vertebrate animals. There is a continual replace- 



FIG. 9. — Teeth of 

 the bull frog. A, 

 view of inner 

 face ; B, lateral 

 face ; c, crown ; 

 r, root ; /, sec- 

 tion of lower 

 jaw ; /, pulp 

 cavity. 



