REVIEWS. 
575 
during life. In this respect fishes differ greatly from mam- 
mals, as in them there are but two sets, the temporary and 
the permanent. If torn away, fresh teeth soon arise from the 
membrane of the mouth of the fish. Many other illustrations? 
in various animals, showing that the teeth belong to the 
mucous membrane, as an internal skin, and not to the 
skeleton, or bony parts of the frame, might given; such, how- 
ever, are not required. 
From this general history and formation of the teeth, our 
author proceeds to make some general remarks on the three 
kinds of teeth of the mammalian class , viz., incisors, tushes, and 
molars ; in the course of which, we come to the 
“ 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. These 
structures, which partake more or less of the character of bone, are designated 
enamel , dentine , and crusta or cement. They vary considerably in hardness 
and consequently in their power to resist attrition, a circumstance which has 
been previously alluded to. Enamel is by far the hardest of the three, and 
therefore we find it existing as a kind of cap to a simple tooth, but entering 
more or less deeply into the body and flanking the sides of a compound one . 51 
“ The central portion of a tooth is hollowed out to receive the pulp from 
which the organ chiefly derives its nourishment. The size of this cavity de- 
pends upon the age of the tooth. It is always large in a young tooth, becom- 
ing gradually smaller as age advances. The pulp cavity is bounded by the 
dentine, which makes up the chief part of the organ.” 
“To proceed to the structure of the dentine. This substance makes up 
the great bulk of both the body and fangs of a tooth. It is that which gives 
form and size to the organ, and upon which its hardness mainly depends. 
In the interior of the dentine we meet with a cavity of large size compared 
with the dimensions of the tooth, and more particularly of one recently 
formed. From containing a red and pulpy mass, composed chiefly of blood- 
vessels and nerves, interposed with cells and filamentous tissue, designated 
the tooth pulp, this cavity has been called the pulp cavity. It is from the 
pulp that the tooth receives sensation as well as its greatest supply of fluids, 
which are derived from the blood, for its nourishment. Entering the cavity 
from below and being bounded on all sides with dentine, the pulp is secured 
from injury. Thus rude pressure can be borne by a tooth without the pulp 
being damaged : a simple but effectual provision to maintain the vitality of 
the organ.” 
“ I proceed to speak of the crusta, the third constituent of a tooth. This 
substance approaches nearer to ordinary bone in the arrangement of its com- 
ponent parts than does either the dentine or enamel. Lt also resembles 
bone both in its chemical composition aud in its density, and hence it is fre- 
quently designated the bone of the tooth. The proportionate quantity of 
crusta to the other constituents depends upon the kind rather than upon the 
size of the tooth. For example, in a simple tooth very little is present, but 
on the contrary in a compound one, a good deal of crusta exists. This dif- 
ference arises from the circumstance that the crusta, in a simple tooth, is 
