Elementary Biology. 



FIG. 131. SECTION OF A PORTION OF DEVELOP 

 i.xG TOOTH. (Quain.) 



The teeth, 1 already described as present en the upper 

 jaw and palate, next claim some attention. They are by 

 no means so distinct and elaborate in structure as those of 

 the higher animals, such as those of the rabbit or dog ; yet 

 the several layers distinguishable in the teeth of those latter 

 forms are to be seen even in the frog's teeth. Each tooth 

 consists essentially of a papilla of the dermis, capped by 

 epidermis, which has become considerably modified. In 

 the frog these teeth are constantly being renewed as the 

 older teeth became worn away. A typical tooth, say of a 

 dog, consists of a crown and neck above the level Ot the 



jaw, and a fang sunk 

 in a cavity, or alve- 

 olus, in the jaw itself! 

 In the frog the tooth 

 is simply united to a 

 process of the bone 

 beneath. The sur- 

 face of the exposed 

 portion is covered by 

 an exceedingly hard 

 dense substance 

 termed enamel, 

 covering a hard but 

 sensitive core of dentine a substance not unlike bone 

 which again contains in its interior a papilla of submucous 

 tissues. The enamel consists of long closely packed prisms 

 of phosphate of lime, while the dentine is partly organic, 

 being composed of long branching tubules, imbedded, 



1 The student is recommended to study the figures of the histology 

 of the frog in Howes' Atlas of Biology. The figures introduced in the 

 text are mainly taken from QuaMs Anatomy , and illustrate the histo- 

 logical structure of the mammal. Good woodcuts of the microscopic 

 structure of the lower vertebrata are still a desideratum, but those em- 

 ployed in the text, as being in most points applicable, may serve the 

 purpose in the present instance. 



;, ou'er layer of dentine, fully calcified ; b, un- 

 calcitied layer ; , dentine-forming cells ; d, 

 pulp. 



