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." There 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 pushed inwards, and sepamted from this mem brane, which then forms a covering cornmon 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 the); 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 ner 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 nually 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 piece ; and lastly, v.-hen the deposition of ivory has 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 minute crystals, all disposed per pendicularly to that surface, so as to form, 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 enaniel 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 lIunter 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 rernains to be filled up, which is effected by the formation of a third substance, called the eementum or erasta pe irosa. 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 cemenlwn, 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.