308 DEVELOPMENT OF TISSUES OF THE TOOTH. [sect. 142. 



the cranial bones. The smallest of these scales presented distinct lacunse, 

 and had a slightly yellow colour, but were still quite soft and transparent, and 

 passed, at the borders, imperceptibly into a very clear blastema, containing 

 cells. In larger ones the borders were similar, but the middle was darker and 

 firmer ; and, in like manner, every intermediate condition was observed up to 

 that of true bone, without any deposition of calcareous granules. As the fang 

 lengthens, new bony scales like the above make their appearance, and gra- 

 dually coalesce from above downwards into a simple layer, upon which, then, 

 new bone is deposited, in the same manner, from without, to such amount as 

 is necessary to form the entire thickness of the cement. 



The enamel-cuticle must, as we have already seen, be regarded as the original 

 membrana praformativa, which remains during the formation of the tooth, 

 and becomes somewhat thickened. Upon the dentine, the membrana prce- 

 formativa is covered by the deposition of the cement, and is afterwards no 

 longer demonstrable as a special layer* 



If, finally, we cast a glance at the different substances of the tooth, and 

 their position relatively to each other, it is seen that, although agreeing in 

 certain respects, they cannot be brought under one category. Dentine and 

 cement are much more closely related to each other than to enamel ; and the 

 dentine is simply osseous tissue, whose matrix is pure intercellular substance, 

 and whose cells have become transformed into long anastomosing canals. In 

 many cases, also^ the cement, or bone, and dentine are very nearly related. 

 This is especially the case when, on the one hand, the dentine is traversed by 

 numerous Haversian canals, and, on the other, when the cement possesses 

 greatly elongated cells with numerous processes, and also vascular canals, or 

 contains, along with but few lacunse, numerous parallel canals like dentinal 

 tubes ; and it is readily conceivable that the dentinal canals frequently anas- 

 tomose with the bone-cells of the cement. The dentine also agrees very 

 much with the cement, and the bones in general, in its manner of growth ; 

 and the pulp may be compared to the periosteum, and the dentine-cells to 

 the cell-layer furnished by that membrane. The enamel may be most fitly 

 compared with dentine which contains no tubes, such as that which is met 

 with in the outermost layers of the teeth of fishes, and it agrees with the 

 matrix of the dentine, at least in this respect, that it is formed by an excre- 

 tion from cells. When canals occur in the enamel, it has a considerable 

 resemblance to dentine ; but these canals probably have quite a different 

 signification from those in the dentine, namely, that of cavities produced 

 by absorption. The enamel has, for the most part, no analogy with the 

 cement ; still there occurs a homogeneous cement with an indistinct trans- 

 verse striation, which, at least in outward appearance, bears some resem- 

 blance to enamel, and is, perhaps, also nearly related to the latter in its de- 

 velopment. On considering the signification of the parts from which the 

 different substances are formed, it may be inferred, that the dentine, inasmuch 

 as it is formed in the vascular parts of the oral mucous membrane, is a trite 

 production of the mucous membrane, that the enamel is an epithelial structure, 

 and that the cement is a superadded substance furnished by the mucous 

 membrane. 



§ 142. The fully-developed tooth, although a hard structure, is 

 not entirely" devoid of active vitality, as is best shown by the 



