FORMATION OF THE ENAMEL. 



799 



Fig. 557. 



portion becomes calcified, or by the union of a series of successively formed cells 

 arranged vertically to the surface. During its formation the enamel is soft and 

 chalky, and can easily be separated into its com- 

 ponent prisms. Afterwards the membranous por- 

 tion of it is nearly all obliterated, and the nuclei 

 entirely disappear, or, according to Tomes, elongate 



Fig. 557. A SECTION THROUGH THE ENAMEL ORGAN 

 AND DENTAL SAC FROM THE TOOTH OP A CHILD AT 

 BIRTH (from Kolliker). sp 



a, outer dense layer of the dental sac ; 6, inner 

 looser texture of the same with capillary blood-vessels 

 and a somewhat denser layer towards the enamel 

 organ ; c, spongy substance ; d, inner cells ; and 

 e, outer cellular layer of the enamel-organ. 



B, four cells of the enamel-membrane. S f2 



into a very fine central canal in each fibre. It is 

 observed by Huxley that, if the pulp be treated 

 with acetic acid, a voluminous, transparent mem- 

 brane is raised from the whole surface in large folds, 

 and that the ends of the enamel fibres are to be 

 seen beneath it. The membrane is from -reV-a^h 

 to -r^jth of an inch in thickness ; is clear, trans- 

 parent, and exhibits little ridges bounding oval or 

 quadrangular spaces ; and is, according to him, 

 continuous with the membrana praeformativa. 

 Huxley, therefore, considers that the enamel appears 

 between the dentine and the preformative mem- 

 brane, and that the enamel-organ takes no part 

 in its formation. Tomes confirms the observation 

 of Huxley with regard to the separability of this 

 apparent membrane by acetic acid ; but, upon 

 closer examination, finds that it may be split into 

 columns, which are, in conformity with his view of 

 the structure of enamel, sheaths containing nuclei. 

 Tomes, further, believes that these sheaths may be 

 seen to pass through the membrane, which Huxley 

 describes as limiting them superficially ; and that, 

 consequently, it is not, as Huxley imagines, the 

 membrana praeformativa. "Waldeyer holds that the 

 membrane described by Huxley between the 

 enamel and the enamel-organ is only a layer of the 

 most recently formed enamel, as he finds it pos- 

 sible always to detect enamel-cells with the ends 

 partially calcified. He returns, therefore, to 

 Schwann's original view, that the formation of the 

 enamel-columns is due to the direct calcification of 

 the enamel-cell?* (Henle's Bericht, &c. for 1864, 

 p. 81, and op. cit.) 



The Cement appears to be formed simultaneously 

 with the dentine of the fang by the periodontal 

 membrane. 



Eruption of the temporary teeth. At the 

 time of birth the crowns of the anterior milk- 

 teeth, still enclosed in their sacs, are com- 

 pleted within the jaw, and their fangs begin 



to be formed. Their appearance through the gums follows a regular order, 

 but the period at which each pair of teeth is cut varies within certain 



