0.9 indicative of the Acjc of the Animal. 
289 
encircle dlfTerent portions of it. Thus, in the case before us, 
when the tooth is first cut, the gum embraces the crown ; when 
further advanced, it sunounds tlic neck ; still further, and we find 
it around the upper part of the fang. 
The lower portion of a tooth, as has just been explained, is 
called its fang ; marked c in the figure. It is that part which is 
implanted in the socket {alveolus), and by which the tooth is 
mainly held in its situation. The fangs of teeth vary consider- 
ably. In some cnses they are single, in others double, not unfre- 
quently treble. We see these varied forms chiefly in the fangs 
of molar teeth. The incisors and the tushes have single fangs as 
a rule ; occasionally we find, however, the fangs of a tush bifid, 
as in the upper one of the pig. 
The firmness of a tooth is much influenced by the form as well 
as by the number of its fangs. The incisors of the ox are so 
loosely imbedded, that, as I have said, some motion can always 
be detected between the tooth and its socket, and the same thing 
is observed in the sheep. This looseness increases with age from 
the circumstance that the tooth is less deeply imbedded, and also 
that the lower part of the fang is rounder than the upper. This 
is not the case with the pig, the fangs of the incisors being both 
square in form, and deeply inserted in the jaws. The implanta- 
tion of a tooth in its socket, being similar to that of a nail 
in a piece of wood, has been called a union by gompJiosis (from 
yop(,(pos, a nail). 
After all, the division of a tooth into crown, neck, and fang, 
is to a considerable extent arbitrary, as well as the statement of 
the crown being alcove or without the socket and the fang within 
it. Tlu'ee-fourths of the body of a molar tootti of the horse, ox, 
and sheep are often imbedded in the socket. Some of these mo- 
lar teeth are all but fangless, although perhaps they may be from 
one to two inches long, even in an animal as small as the sheep. 
This great length is in fact produced by their body, and such 
teeth cannot strictly speaking be said to have any neck, certainly 
none where they are embraced by the gum. The incisors of the 
horse and the pig, more particularly those of the lower jaw, are 
similar examples of teeUi wanting necks. (See figs. 2 and 45.) 
This observation will likewise apply to the tushes. 
Three kinds of structure unite to form the solid part of a tooth, 
whether it belongs to the division called simple or to that termed 
compound. Tliese structures, which partake more or less of the 
character of bone, arc designated enaviel, dentine, and crust a or 
cement. They vary considerably in hardness and consequently in 
their power to resist attrition. Enamel is by far the hardest, and 
therefore we find it as a kind of cap to a simple tooth, but enter- 
ing more or less deeply into the body and flanking the sides of a 
