

TEXTURE AND ORGANIZATION OF THE TEETH. 475 



ficult to penetrate it. It forms a crust upon the body scarcely 

 half a line in thickness, is more abundant upon the grinding 

 surface, and is reduced to a thin edge where it terminates at the 

 neck. When broken, it is seen to be fibrous, and the fibres are 

 so placed as to pass in a direction from the surface towards the 

 centre" of the tooth : by which all the friction to which the fibres 

 are exposed is applied against their extremities : an arrange- 

 ment on the principle of the articular cartilages, and, like them, 

 precisely suited to resist their being rubbed down in mastica- 

 tion, and also to prevent their splitting. 



Enamel consists principally in a phosphate of lime, with a 

 very small proportion of gelatine. When immersed in a weak 

 acid, its form is retained, but the slightest disturbance afterwards 

 causes it to crumble down into a white pulp. When animals 

 are fed upon madder, the colour of the enamel is not affected;* 

 though it .may be changed by dyes applied externally, as ex- 

 hibited by the inhabitants of the Pebw Islands, who, by the 

 use of plants turn it black, and by persons who chew tobacco, 

 in whom it becomes yellow. It is entirely devoid of blood ves- 

 sels. When exposed to heat it becomes very brittle, cracks 

 off from the enclosed bony part of the body, and presents a 

 singed appearance, from the small quantity of gelatine in it. 



The enamel is not so thick on the deciduous as on the per- 

 manent teeth; it is thicker on the cuspidati than on the incisors, 

 and on the first molar than on the second and third. It is very 

 readily dissolved in strong nitric or muriatic acid. 



The Osseous portion of the tooth is by much the most abun- 

 dant, as it forms the root, the neck, and the body also, with the 

 exception of the crust of enamel upon it. In its texture it 

 strongly resembles the petrous bone, and is even harder than 

 it, but has no cellular arrangement within. It consists in a se- 

 ries of longitudinal laminae, one within the other, and when de- 

 composed presents about seventy parts of the phosphate of lime 

 and other calcareous combinations, with about twenty of gela- 

 tine and ten of water, f 



* J. Hunter, loc. cit. I have also verified the same opinion by the same expe- 

 riment, 

 f Pepys. 



