PACHYDEBMATA. 



869 



tL-rials 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 interpdsed 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 separated from this mem- " 

 brane, 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 

 conjoined 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, when 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 lo 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 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 very 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 superadcled 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 lamina? 

 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 laminae are soldered together by their 

 upper portions, even before their bases are 

 completed. \Ye may likewise add that all 



