SECT. 142.] VITAL PROPERTIES OF ITS TISSUES. 309 



different diseases to which it is subject. The functions of the 

 lacunae of the bones and their canaliculi, are performed here by 

 the dentinal canals and their ramifications, the lacunae and canali- 

 culi in the cement, and the spaces between the prisms of the 

 enamel. All these spaces, during life, contain a fluid which, on the 

 one hand, comes from the vessels of the pulp, and, on the other, 

 from those of the alveolar periosteum, and furnishes the conditions 

 necessary for a change of material, slow though it be. It is, for 

 the present, impossible to state exactly how the change of substance 

 is effected; still, from the circumstance that the fully-developed 

 dentine is not coloured by madder {Hunter, Flourens, and others; 

 also Ilenle, p. 878), we may conclude, at least, that the change is 

 much less energetic than in bones, and, perhaps, takes place in such 

 a manner that the calcareous earths are not at all, or only very slowly, 

 renewed. The dentine is, doubtless, best provided with a supply of 

 juices, since it is traversed by very numerous, anastomosing canals; 

 still a regular circulation of these fluids is no more admissible in 

 dentine than in bone ; we must rather assume that the movement 

 of fluid takes place sometimes in one direction, and sometimes in 

 another, according to the degree of exsudation and absorption by 

 the pulp, and of the waste of material in the tooth itself; according 

 also to the amount of nutritive matter bestowed on the enamel and 

 cement, and perhaps, too, of that which is given off by these 

 tissues outwardly. The enamel is, indeed, not impermeable; but it 

 allows fluid to pass through it with difficulty, which may be best 

 understood from the fact, that the nerves of the pulp are not 

 affected by acids so long as the enamel-covering is complete, but 

 readily so, when, as upon the incisors, the dentine is exposed. The 

 enamel is undoubtedly the hardest of the dental substances, almost 

 destitute of an organic basis, and without a constant system of 

 canals. Still more impenetrable than the enamel, is, perhaps, the 

 enamel-cuticle, which, also, is with great difficulty attacked by 

 chemical agents; and these two substances are, accordingly, 

 excellently adapted as protecting coverings for the teeth. The 

 teeth acquire sensibility through the nerves of their pulp, and 

 they are sensible both to simple contact, and to heat and cold, as 

 well as to chemical influences. Mechanical impressions of low 

 intensity can operate only when propagated to the pulp by vibra- 

 tions of the dental substance; and it is, therefore, the more sur- 

 prising, that the teeth have a certain sensibility for locality, so that 

 we can distinguish whether they are touched on the outside or in- 

 side, above or below, on the right or the left. The tactile sensibility 



