Processes of the capsule descend from its summit into the interspaces of the dentinal pulp-plates, and consequently resemble them in form ; but they adhere not only by their base to the surface of the capsule next the mouth, but also by their lateral margins to the sides of the capsule, and thus resemble partition-walls, confining each plate of the dentinal pulp to its proper chamber ; the margin of the partition opposite its attached base is free in the interspace of the origins of the dentinal pulp plates. The enamel organ, which Cuvier appears to have recognised under the name of the internal layer of the capsule, is distinguishable by its light blue sub-transparent colour and usual microscopic texture, adhering to the free surface of the partitions formed by the true inner layer of the capsule. Although the enamel-pulp be in close contact with the dentinal pulp prior to the commencement of the formation of the tooth, one may readily conceive a vacuity between them, which is continued uninter ruptedly, in many foldings, between all the gelatinous plates of the dentinal pulp, and the partitions formed by the combined enamel pulp and the folds of the capsule. According to the excretion-view, this delicate apparatus must have been immediately subjected to the violence of being compressed in the unyielding bony box, by the deposition of the dense matters of the tooth in the hypothetical vacuity between the enamel and dentinal pulps; a process of absorption must have been con ceived to be set on foot immediately that the altered condition of the gelatinous secreting organs took place ; and, according to Cuvier's hypothesis, the secreting function must be supposed to have proceeded, without any ir regularity or interruption, while the process of absorption was superinduced in the same part to relieve it from the effects of pressure produced by its own secretion.
The formation of the dentine commences immediately beneath the niembrana propria of the pulp : a part which Cuvier distinctly recognised, and which he accurately traced as preserving its relative situation between the dentine and enamel throughout the whole formation of the dentine, and discernible in the completed tooth " as a very fine greyish line, which separates the enamel from the in ternal substance" or dentine.
The calcification and conversion of the cells of the dentinal pulp commence as usual at the peripheral parts of the lamelliform pro cesses furthest from the attached base. It may readily be conceived, therefore, that, at the commencement, there is formed a little cap upon each of the processes into which time edges of the pulp-plates are divided. As the centripetal calcification proceeds the caps are converted into horn-shaped cones ; when it has reached the bottom of the notches of the edge of the pulp-plate all the cones be come united together into a single transverse plate ; and, the process of conversion having reached the base of the pulp-plate, these plates coalesce to form a common base to the crown of the tooth, which would then present the same eminences and notches that charac terised the gelatinous pulp, if, during the period of conversion, other substances had not been formed upon the surface and in the interspaces of the pulp-plates.
Coincident, however, with the formation of the dentine, is the deposition of the hard ening salts of the enamel in the extremely slender prismatic cells, which are for the most part vertical to the plane of the inner surface of the folds of the capsule to which they are attached ; these cells or moulds give a sub transparent bluish tint to the enamel pulp.
The true inner part of the capsule forms those thick transverse folds or partitions which support the enamel organ, and with it fill the interspaces of the dentinal pulps. With re gard to the formation of the cement, Cuvier, after cling the opinion of Tenon —that it was the result of ossification of the internal layer of the capsule, and that of Blake — that it was a deposition from the opposite surface of the capsule to that which had deposited the enamel, states his own conviction to be that the cement is produced by the same layer and by the same surface as that which has produced the enamel. The proof alleged is, that so long as any space remains between the cement and the external capsule, that space is found to contain a soft internal layer of the capsule with a free surface next the cement. The phenomena could not, in fact, be otherwise explained according to the "ex cretion theory of dental development. To the obvious objection that the same part is made, in this explanation, to secrete two different products, Cuvier replies, that it un dergoes a change of tissue : " Whilst it yielded enamel only it was thin and transparent ; to give cement it becomes thick, spongy, and of a reddish colour."* The external characters of the enamel organ and cement-forming cap sule are correctly defined ; only, the one, in stead of being converted into the other, is in fact changed into its supposed transudation : the enamel fibres being formed, and properly disposed in the direction in which their chief strength is to lie, by the assimilative proper ties of the pre-arranged elongated prismatic non-nucleated cells, which take from the sur rounding plasma the required salts and com pact them in their interior.
Whilst this process is on foot, and before the enamel fibres are firm in their position, the capsule begins to undergo that change which results in the formation of the thick cement ; the calcifying process commences from several points, and proceeds centrifu gally, radiating therefrom, and differing from the ossification of bone chiefly in the number of these centres, which, though close to the new-formed enamel, are in the substance of the inner vascular surface of the capsular folds. The cells arrange themselves in con centric layers around the vessels, and act like those of the enamel pulp in receiving into their interior the bone-salts in a clear and compact state; during this process they be come confluent with each other, their pt Uni tive distinctness being indicated only by their persistent granular nuclei, which now form the radiated Purkinjian corpuscles. The in terspaces of the concentric series of confluent cells become filled with the calcareous salts in a rather more opaque state, and the con version of the capsule into cement goes on, according to the processes more particularly described in the Introduction to my " Odon tography," until a continuous stratum is formed in close connection with the layer of enamel.