100 COMPOSITION OF THE TEETH. 



the gums, is covered with enamel, a substance very different from 

 common bone. 



Every tooth in its natural condition has a cavity in it, which 

 commences at the extremity of each root, and extends from it to 

 the body of the tooth, where it enlarges considerably. This cavity 

 is lined by a membrane, and contains a nerve, with an artery 

 and vein, which originally entered the tooth, by a foramen near 

 the point of the root, as is evidenf during the growth of the teeth. 

 These vessels, and the nerve, have been traced into the teeth ; 

 although in many subjects the foramina appear to be closed up. 

 A third substance has lately been discovered by Prof. Ret- 

 zius of Stockholm as entering into the..co'nGfyoit]6n*of the teeth 

 of man, called the cortical substance or cementum. It com- 

 mences at the lower edge of the enamel and surrounds com- 

 pletely the fang. In many of the lower animals it is found also? 

 on the faces of the compound teeth, filling up the spaces between 

 the vertical ridges of enamel. 



Composition of the Teeth. 



The bone or ivory of the teeth, now called Dentine, (see 

 Fig. 24,) constitutes the whole of the root, and a greater part 

 of the body and neck. The cavity in the centre, for the lodg- 

 ment of the pulp, (cavitas pulpi) in whichever of the teeth it 

 is examined, presents an exact similarity of shape to the bodies 

 and fangs of the teeth, as though the latter had been moulded 

 upon the pulp. The ivory is of a polished pearly whiteness, 

 like that of a piece of white satin. It is composed chemically 

 both of animal and earthy matter, but in different propor- 

 tions from ordinary bone. If exposed for a considerable 

 Fig. 24. time to the action of a weak acid solu- 



tion, the earthy matter is dissolved, 

 and there is left a flexible, tenacious, 

 dense, and homogeneous mass, much 

 resembling cartilage, but more dense. 

 If, on the contrary, it is exposed to the 

 action of fire, the animal matter is first 

 blackened, then consumed, and there 

 is left a white, hard, friable residue of calcareous matter. 



