as indicative of the Acje of the Animal. 
303 
which are thi-ce standing behind the temporary in each row : the 
first of these is developed from a papilla which rises in the 
lengthened primitive groove, behind the last temporary molar ; 
and from cavities of reserve, with a slight modification of the 
plan, the two last are subsequently formed. Thus the additional 
permanent teeth of mammals are, like the temporary and their 
successors, productions from tlie membrane of the mouth, as 
had previously been seen to be the case with the teeth of fishes, 
&c. The implantation of the teeth in bony sockets in animals 
is only to give them a greater hold of the jaw, the better to 
serve their important offices. 
Passing now from this general description of the formation of 
the teeth to the structures of which they consist, let us consider 
first the Dentine, the substance making up the bulk of a tooth. 
All observers agree that Dentine is a product of the parts enter- 
ing into the composition of the papilla which rises in the dental 
groove, but they differ materially in their explanation of the 
process of its formation. When fully develojied, the papilla is 
chiefly composed of numerous microscopic cells, held together 
by a netw(nk of very delicate fibres, and receiving its blood from 
vessels which enter at its base.* 
It is sufficient to state that recent observations have fully 
proved that the Dentine is first produced upon the apex of the 
papilla, and that from this point it extends downwards upon its 
sides, and thus encloses it as with a cap. This may be the better 
understood by supposing one's finger to be covered with a thumb- 
stall, and looking to the finger as the papilla and the thumbstall 
as the dentine. The papilla, thus capped by dentine, is now 
called the tooth pulp. This layer of Dentine is at first very thin, 
but by fresh depositions added from the pulp within, its thickness 
is daily increased. Hence a gradual decrease in the size both of 
the pulp and of the cavity in Avhich it is placed. The ultimate 
consequences of this diminution, with regard to the nutrition of 
the tooth as derived from the pulp, has been already shown. 
The continuous lessening of the cavity is nicely seen in a trans- 
verse section of the fang of an incisor of the ox. Even to the 
unassisted eye a preparation of this description shows concentric 
lines, one within the other, marking fresh deposits of dentine. 
* The precise manner in which the dentinal tubes and the inter-tubular structure 
is formed out of these elements of the papilla, is still a question for future inves- 
tigators. It may be that Schwan's view of the tubes being produced by the 
elongation of the cells and their union endways, and of the intertubular structure 
becoming solidified by a deposition of earthy materials, is correct. To describe 
the minutia? of the process, however, as given by other authorities equal with 
Scliwan, but who differ from his views, would be scarcely suited to a work of this 
kind were I to attempt their explanation, and which, for the above reason, I shall 
refrain from doing. 
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