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 
« 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 
_ _ the interior of a little mem- 
Fig. 89.— Development op Teeth. , , . , . , 
a, the gum; b, the lower jaw; c, angle of the branOUS Sac, Which IS lodged 
jaw; d, dental capsules. in tlie thickness of the 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, 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 d which in Man constitutes nearly its whole 
v ^ structure; this conversion takes place first 
at its highest points, d, d. The crown or 
upper portion of the tooth receives a 
b — 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. 
