72 SPECIAL HISTOLOGY. 



only very slowly renewed. The dentine is undoubtedly best pro- 

 vided with fluid supplies, from its being penetrated by very nu- 

 merous 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 exposed. The enamel, again, is the 

 hardest dental tissue, possesses scarcely any organic matrix 

 and no constant system of canals. Nasmyth's membrane, which 

 is attacked with so much difficulty by chemical reagents, is, 

 very probably, still more impenetrable than the enamel itself, 

 and hence these two substances serve admirably to protect the 

 teeth. The sensibility of the teeth arises from the nerves of 

 their pulp ; they are affected by contact, heat, cold, and chemical 

 agents. Slight mechanical influences can only act by the 

 vibrations which they may communicate to the substance of 

 the tooth and thence to the pulp ; it is therefore the more 

 remarkable that the teeth have a certain sense of locality, so 

 that it is possible to distinguish whether they are touched 

 internally or externally, above or below, on the right or on the 

 left side. The sensibility of the teeth is indeed tolerably 

 delicate, especially on the masticating surface, where the 

 smallest foreign bodies, as hairs, grains of sand, &c, are 

 perceived when these surfaces are rubbed against one another; 

 and as regards its acuteness, it is, in disease at least, excessive, 

 which is sufficiently explained by the considerable number of 

 nerves in the pulp and the readiness with which they may be 

 compressed within their hard receptacle. 



With age the teeth become denser ; the pulp cavity is filled 

 with a kind of irregular dentine and may be totally obliterated, 

 which is, perhaps, the normal cause of their falling out. In cer- 

 tain cases observed by Tomes, the faugs in old age were quite 

 transparent, like horn. 



