576 
REVIEWS. 
met with chiefly on its fang, while in the other variety it not only covers the 
fang but dips deeply with the enamel into the interior of the organ.” 
“ If teeth die in their sockets, they would greatly impair the health of the 
animal, from his incapability of properly masticating the food, as also from 
the suffering he would endure. The diseases of the teeth of animals and the 
causes leading to them has certainly not as yet received all the attention 
which the importance of the subject merits. Many cases might be cited to 
show this, but on the present occasion I must refrain from adding to these 
observations, although they may have a very practical bearing. 
“ To return to the crusta. On the fangs of teeth recently cut, but little 
of this substance is met with, when compared with that existing on old teeth. 
As age advances, however, the crusta increases. Hereafter it will be shown 
how this and the other structures are originally produced. It may, never- 
theless, be now asked liow the increased quantity upon an old tooth is ac- 
counted for ? Is the crusta always added to from the original source of its 
production, or can it be otherwise augmented ? I answer that it frequently 
receives an addition altogether independent of its original source.” 
“From the explanation of the structure of a tooth, I proceed to speak of 
the manner the teeth are formed, confining, for obvious reasons, my remarks 
to those animals which are the chief subjects of these pages. The develop- 
ment of teeth has of late years been studied with much advantage, and we 
are now enabled to describe the successive stages of the process with far 
greater confidence than formerly. In a work of this kind it is not required 
that I should enter very minutely into this subject, but merely give a general 
outline of it, so that the reader may the better understand how a second set 
of teeth, the permanent, spring up to supply the place of the temporary, after 
they have served their purpose.” 
“ With regard to the formation of the permanent molar teeth, which are 
three standing behind the temporary in each row : the first of these is de- 
veloped from a papilla which rises in the lengthened primitive groove, behind 
the last, temporary molar ; and from cavities of reserve , with a slight modifi- 
cation of the plan, the two last are subsequently formed. 
“ We have thus an explanation of the fact that the additional permanent 
teeth of mammals are, like the temporary and their successors, productions 
from the membrane of the mouth, as had previously been seen to be the case 
with the teeth of fishes, &c. The implantation of the teeth iu bony sockets 
in animats is only to give them a greater hold of the jaw, the better to serve 
their important offices. 
“ To pass from this general description of the formation of the teeth to the 
structures of which they consist. First, of the dentine, the substance making 
up the bulk of a tooth. 
“ In concluding this portion of my subject, I will therefore merely ob- 
serve — 1st, That it appears to me that the dentine is formed beneath the 
original membranous covering of the papilla; 2ndly, That this membrane is 
the true producing organ of the enamel, and identical with both the per- 
formative and the adamantine membranes, these being, not two, but one ; 
3rdly, That the pulpy mass lying external to the enamel membrane is the 
matrix of the crusta ; and 4thly, That the capsule becomes the periodental 
membrane or covering to the tooth, and the 'periosteal lining of the bony 
socket in which it is placed — these being also but one. 
“ This view of the question of the formation of a tooth has at least sim- 
plicity for its basis ; for, after all, it is little more than a layer of mucous 
membrane, which is reflected inwards, changed partly in the arrangement of 
its primitive elements, and then reflected outwards again. That portion of 
the mucous membrane of the mouth which originally flanked the sides of the 
