166 



DEVELOPMENT OF TEETH. 



Mastication. 



174. The act of Mastication, or the mechanical division of 

 the alimentary matter, is effected in most of the higher animals, 

 by the Teeth ; which are implanted in the jaws, and are so fixed 

 as to act against one another, with a cutting, crushing, or 

 a grinding power, according 



to the nature of the food on 

 which they have to operate. 

 The manner in which they 

 are formed is worthy of 

 note. In Man, who may be 

 taken as a fair example, 

 c each tooth is developed in 



6 _. the interior of a little mem- 



Fig. 89. DEVELOPMENT OF TEETH. , , . , . , n 



a, the gum ; b, the lower jaw ; c, angle of the DrailOUS SRC, which IS lodged 

 jaw ; d, dental capsules. in the tl^knegg O f the j aw . 



bone ; as seen in the accompanying figure, which represents 

 half the lower jaw of a very young infant, from which the 

 outside has been removed. This sac, which is named the 

 dental capsule (a, fig. 90), is composed of two membranes, 

 abundantly furnished with blood-vessels ; and it encloses in 

 its interior a little bud-like protuberance, b, in which ramify 

 a great number of nervous filaments and minute vessels, c. 

 The matter composing this little body, which is termed the 

 pulp, is gradually converted into the dentine ( 54) of the tooth, 

 d a which in Man constitutes nearly its whole 



structure ; this conversion takes place first 

 at its highest points, d, d. The crown or 

 upper portion of the tooth receives a 

 covering of enamel ( 54). Gradually the 

 process of conversion extends more and 

 more to the interior of the pulp ; and at 

 last the whole is changed into dentine, 

 with the exception of a small portion 

 that still remains, occupying what is termed the cavity of 

 the tooth, which is frequently laid open by decay of its 

 external wall. The fang of the tooth, which is the part 

 last formed, receives an envelope of cementum ( 54), which 

 invests it up to the part at which the enamel begins. As the 



Fig. 90. DENTAL 

 CAPSULE. 



