THE TEETH. 71 



isting ; or when with few lacuna, the canaliculi are numerous 

 and parallel, like dentinal canals. This much is certain, that 

 the two substances never become exactly alike and it is probable 

 that their development is always to a certain extent different. 



[The enamel may be best compared with a dentine whose cells 

 are ossified throughout, and which, therefore, presents no canals, 

 like that in the outermost layers of fishes' teeth ; at least the 

 two substances agree in this, that they are entirely composed 

 of elongated cells without any connecting matrix. When 

 canals occur in the enamel, it acquires a very great similarity to 

 dentine ; but these canals probably have a totally different im- 

 port to those ill the dentine, viz. — that of cavities which pro- 

 ceed from absorption. With the cement, the enamel has, in 

 general, no analogy, though there is a kind of homogeneous 

 cement with an indistinct transverse striation which, at least 

 externally, looks somewhat like enamel, but has hardly, like the 

 latter, arisen from elongated cells. If we consider the nature 

 of the parts from which the various substances are developed, 

 the dentine, formed from the vascular part of the mucous mem- 

 brane of the mouth, is a true product of the homologue of the 

 derma (schleimhaut -production) , the enamel an epithelial struc- 

 ture, and the cement an investing substance, afforded by the 

 mucous membrane.] 



§ 143. 



The substance of the perfect tooth, though hard, is by no 

 means incapable of molecular change, as its various diseases 

 best show. The functions of the lacuna and their canaliculi 

 in the bones are here performed by the dentinal canals with 

 their ramifications, the lacuna and canaliculi in the cement, and 

 the fissures between the prisms of the enamel. All these cavi- 

 ties, during life, contain a fluid, derived on the one side, from 

 the vessels of the pulp, on the other, from those of the alveolar 

 periosteum, and permit of changes in the substance, though they 

 may be slow. Nothing definite, however, is known about the 

 latter, but from the circumstance that perfect dentine is not 

 coloured when an animal is fed with madder (Hunter, Flourens, 

 and others ; compare Henle, p. 878), it may be concluded, that 

 they are far less active than in the bones, and perhaps take place 

 in such a manner that the calcareous matters are not at all or 



