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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 Z'eeth ; 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, 
.¢ each tooth is developed in 
Fig. 89.—DEVELopMENY or TEETH the interior of a little mem- 7 
a, the gum; 6, the lower jaw; c, angle of the branous Sac, which is lodged 
jaw; d, dental capsules. in the thickness of the j jaw-— 
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, 6, in which ramify ~ 
a great number of nervous filaments and thinute vessels, ¢. 
The matter composing this little body, which is termed the 
Pulp, 3 is gradually converted into the dentine (§ 54) of the tooth, 
a Which in Man constitutes nearly its whole a 
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 antl | 
- more to the interior of the pulp; and at 7} 
Fig. 90.—Dewrar _ last the whole is changed into dentine, | 
eabialane with the exception of a small portion | 
that still remains, occupying what is termed the cavity of 7} 
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 — 
