476 ORGANS OF DIGESTION. 



The bony part of a tooth has very nearly the same form with 

 the entire body; hence, upon the grinding surface, we have the 

 same modifications of shape as when the enamel is left on. The 

 application of a heated iron to it, turns it to a deep black from 

 the abundance of animal matter in it, which is one way to mark 

 out decidedly the distinction betwen it and enamel. The ani- 

 mal substance, when separated from the calcareous by muriatic 

 acid, is more compact than the corresponding substance of bone, 

 but, like it, is soft and flexible. 



The bony part is not vascular; Mr. Hunter, after repeated 

 trials in old and young subjects upon this point, never succeed- 

 ed in making an injection of it; neither could he trace vessels 

 from the pulp to a growing tooth. In growing animals, fed 

 upon madder, he found that the portion which was formed pre- 

 viously to the commencement of this diet, retained its primitive 

 colour, while the part formed during the administration of the 

 diet was affected by it and turned red: again, if the animal were 

 permitted to live some weeks after the madder was suspended, 

 to the preceding condition was superadded a new layer of white. 

 In this experiment, a conclusive difference from common bone 

 is established; for besides, in all cases, the facility of injecting 

 the latter with size, it is susceptible of being dyed throughout 

 by the administration of madder; though the formed parts do 

 not take the latter so readily as the forming. These experi- 

 ments, which are confirmed by my own observations, prove sa- 

 tisfactory the total absence of blood vessels in the texture of 

 the teeth; and that the colouring matter, when fixed in them, 

 does not depend upon a circulation, but upon its being deposited 

 as the tooth grows, and left there permanently. The teeth are 

 consequently not subjected to a mutation of particles, and to 

 being continually remodelled as the bones are; but when once 

 formed, they remain in the same state, without change. 



Every tooth has within its body a cavity, which varies in 

 form and size according to the class to which the tooth belongs: 

 this cavity is continued as a conoidal canal, through the whole 

 length of each root, and terminates, by a small opening, at its 

 point. The cavity is smooth on its internal surface, and is 

 filled with a soft pulpy matter which has no adhesion to the 



