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PACHYDERMATA. 
terials will be deposited to form the teeth, viz. 
the bone or ivory (dentine) which will be 
formed by the gelatinous processes coming 
from the bottom of the capsule, and the enamel, 
which will be deposited by the membranous 
septa, and by the general internal surface of 
the capsule and its prolongations, the base only 
excepted.” 
ere is, however, according to Cuvier, a 
very delicate membrane interposed between the 
ivory and the enamel, which, previous to the 
deposit of the ivory, immediately infolds the 
ivory pulp wall, and invests it very closely ; but 
as the ivory pulp transudes the ivory, it is 
ushed inwards, and separated from this mem- 
rane, which then forms a covering common 
both to the ivory and to the pulp that secretes it. 
On the other side the enamel is deposited 
upon the exterior of this membrane by the sur- 
face of the prolongations of the internal lamina 
of the capsule, and by its pressure upon the 
ivory obliterates the intervening membrane, so 
that the latter soon becomes imperceptible in 
the newly formed tooth, or its place is only 
indicated when a section is made, by a fine 
greyish line which separates the enamel from 
the ivory. It is, however, evident that this 
thin membrane is the only bond of union be- 
tween the two substances as they become in- 
durated at the bottom of the capsule, for with- 
out it they would indubitably separate from 
each other. : 
The ivory and the enamel are therefore 
tonjoined by a kind of juxta-position. ‘The 
former is deposited by layers advancing from 
without to within, the internal layer being that 
last formed and also of greatest extent, exactly 
as in the growth of shells; and as its deposition 
commences at the most prominent points of the 
gelatinous, ivory-forming pulp of the tooth, it 
is at these points that the ivory-forming sub- 
Stance is thickest, and goes on becoming thin- 
her as it recedes from them. 
If, therefore, we bring our thoughts to bear 
upon the epoch when the deposition of ivory 
commences, it is easy to conceive how there is 
first formed a little crust of ivory upon each of 
the prominent points of the indented margins 
of the ivory pulp, and as new layers are conti- 
_ Dually within each other, the little crusts are 
changed into conical caps; when the newly 
deposited internal layers have descended as far 
as the bottoms of these indentations, all the 
caps become united into a single transverse 
ers and lastly, when the deposition of ivory 
proceeded as far as the bases of the ivory 
pulps, all the transverse plates will become 
united into a single crown of a tooth, which 
_ would present the same eminences and the 
_ Same depressions as were conspicuous in the 
_ pulp which formed it, if in the mean time other 
substances had not been in progress of deposi- 
_ tion, and partially filled up the intervals be- 
tween them. 
‘The enamel is deposited upon the external 
Surface of the ivory by the internal membrane 
of the capsule, under the form of little fibres, 
or rather of miuute crystals, all disposed per- 
pendicularly to that surface, so as to form, 
869 
during the earlier.periods of its deposition, a 
kind of velvet with a very fine pile. When a 
capsule of a young tooth is opened, the little 
molecules of the future enamel are in fact 
easily perceived adhering to the inner surface 
of the capsule, from which they are easily de- 
tached. Some are even seen floating in the 
fluid that intervenes between the capsule and 
the germ of the tooth. The opinion of Hunter 
that the enamel is only a sediment deposited 
from the fluid interposed between the capsule 
and the tooth is inexact, inasmuch as he does 
not attach sufficient importance to the functions 
of the capsular membrane, from which in rea- 
lity the molecules of enamel proceed ; never- 
theless, it is yery true that these molecules are 
originally situated between this membrane and 
the tooth before they become attached to the 
latter. But to proceed. 
A thick layer of enamel being thus deposited 
around the ivory forming the crown of the 
tooth, partially fills up the intervals by which 
the transverse plates and their indentations 
were formerly separated. The remainder of 
these interspaces now remains to be filled up, 
which is effected by the formation of a third 
substance, called the cementum or crusta pe- 
trosa. This superadded material, which is very 
different in its characters from either of the 
others, is formed by the same membrane and 
the same surface as formerly produced the ena- 
mel. The proof of this is, that the membrane 
in question always remains external to the 
cementum, in precisely the same relation to it 
which it previously had to the enamel, and that 
it continues soft and free as long as the depo- 
sition of the cementum leaves room for it. 
The only change perceptible is in its texture. 
Whilst it continued to secrete enamel, it was 
thin and transparent; but when it begins to 
secrete cementum, it becomes thick, spongy, 
opaque, and of a reddish colour. 
The membranous prolongations of the cap- 
sule of the tooth are retracted towards the top 
and towards the sides of the cavity, in propor- 
tion as the cementum which they deposit fills 
up the spaces between the different lamina of 
the tooth. The summits even of the lamine 
are covered with cementum before the tooth 
begins to be worn. Sometimes, indeed, the 
same prolongation of the capsule is secreting 
cementum ‘near around the top of a lamina, 
whilst it is forming enamel lower down. From 
the same cause, the upper portion of the inter- 
spaces is already filled up with cementum, 
while the lower parts remain separate, under 
which circumstances the lower portion of the 
capsular prolongation becomes separated from 
the upper, and only receives its nourishment 
through its lateral adhesions to the capsule. 
The deposit of enamel commences almost at 
the same time as the formation of the ivory, 
and the secretion of the cementum speedily 
follows; so that the summit of each lamina has 
all the three substances belonging to it com- 
pletely formed long before its base and conti- 
guous lamine are soldered together by their 
upper portions, even before their bases are 
completed. We may likewise add that all 
