490 SPECIAL HISTOLOGY. 



0*16-0'2 of a line. At this period it contains vessels in its outer third 

 and its network is metamorphosed into true connective tissue (Mikr. 

 Anat. ii. Fig. 211). On the inner side of the spongy tissue of the 

 enamel organ, lies the so-called enamel memhrane^ 7nemhrana adaman- 

 tince (Raschkow), a true cylinder epithelium, of which it need only be 

 said that its cells measure 0'012 of a line in length, and 0-002 of a line 

 in breadth, are finely granular and delicate, and possess nuclei fre- 

 quently situated at the ends of the cells. 



The development of the dental tissues has, hitherto, always been re- 

 garded as a very difficult subject. The simplest relations are presented 

 in the enamel, where there can be no doubt whatever that the enamel- 

 cells become, by their complete ossification, the enamel-fibres. As soon 

 as only a small portion of the cells has become ossified (without the 

 previous deposit of calcareous matter in a granular form), a little scale 

 of enamel is recognizable upon the somewhat larger cap of dentine, 

 Avhich has also just been produced. The deposition of calcareous mat- 

 ter in the cells constantly advances outwards, until at last they are en- 

 tirely converted into enamel fibres, and extends at the same time to 

 other cells, so that the layer of enamel increases in width. During this 

 process, the enamel membrane does not disappear in the locality in 

 which ossification commenced, but retains, there and elsewhere, the 

 same thickness, so long as the deposition of enamel continues : its ossi- 

 fying part, therefore, must be replaced by the incessant development of 

 new substance, which takes place apparently, not by the apposition of 

 new cells, but by the continual growth of the old ones. The enamel 

 organ has assuredly some very important relation to the development 

 of the enamel ; probably serving by the abundance of albumen and of 

 mucus in its meshes, as a storehouse, out of which the enamel membrane, 

 distant as it is from bloodvessels, is enabled to draw the materials for 

 its increase. In fact, the spongy tissue is seen to decrease more and 

 more during the development of the enamel, and finally, when the 

 enamel is complete, to disappear. 



In the development of the dentine, as in that of the enamel, it is not 

 the whole pulp which shares in the process, but only its most external, 

 epithelium-like layer of cells, wdiich appears to maintain a constant 

 thickness by the elongation of the original cells, accompanied by a con- 

 tinual multiplication of their nuclei (Mikr. Anat. ii. 2, p. 103 et seq.) 

 I by no means intend to assert that one and the same cell suffices for the 

 whole duration of the development of the dentine, although this is not 

 at all inconceivable; indeed, I consider it possible that the dental cells 

 are from time to time replaced by others, which are formed upon their 

 inner surface ; but what I deny is, that the whole pulp is simply changed 

 progressively, from without inwards, into dentinal cells and ossified, and 

 I am of opinion that, like the spongy tissue of the enamel organ, the 



