494 SPECIAL HISTOLOGY. 



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 to- 

 tally different import to those in the dentine, viz. that of cavities 

 which proceed 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 some- 

 what 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 membrane of the mouth, is a true product of the homologue 

 of the derma (schleimhaut-productiori), the enamel an epithelial structure, 

 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 lacunce and their canaliculi in the bones are here 

 performed by the dentinal canals with their ramifications, the lacunce and 

 canaliculi in the cement, and the fissures between the prisms of the 

 enamel. All these cavities, during life, contain a fluid, derived on the 

 one side, from the vessel 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 colored wh'en 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 only very slowly renewed. The dentine is un- 

 doubtedly best provided with fluid supplies, from its being penetrated by 

 very numerous and frequently anastomosing canals. We can as little 

 .suppose any regular circulation in it as in the bones ; but it may be 

 .assumed that a certain movement takes place, proportionate to the 

 :amount of the exudative and absorptive processes in the pulp, of the 

 waste in the tooth itself, and of the supply afforded to the enamel and 

 >cement and probably given off from the latter tissues externally. Though 

 the enamel is not impermeable, it permits of the passage of fluids with 

 difficulty, as is best shown by the circumstance that the nerves of the 

 dental pulp are not affected by acids, so long as the coating of enamel 

 is entire, but readily enough, when, as in the incisors, the dentine is ex- 

 tposed. The enamel, again, is the hardest dental tissue, possesses scarcely 

 -any organic matrix and no constant systems of canals. Nasmyth's 



