134 



THE VITAL FUNCTIONS. 



tirely of phosphate of lime ; the proportion of the 

 carbonate of that earth entering into its composition 

 being very small ; and the animal portion is albu- 

 men, with a small quantity of gelatin. 



A layer of a still harder substance, termed the 

 enamel, usually covers the ivory, and, in teeth of 

 the simplest structure, forms the whole of their 

 outer surface : this is the case with the teeth of man 

 and of carnivorous quadrupeds. These two sub- 

 stances, and the apparent direction of their layers, 

 are seen in Fig. 277, which is the section of a 



simple tooth, e is the outer case of enamel, o the 

 osseous portion, and p the cavity where the vascular 

 pulp which formed it was lodged. The enamel is 

 composed almost wholly of phosphate of lime, con- 

 taining no albumen, and scarcely a trace of gelatin : 

 it is the hardest of all animal substances, and is 

 capable of striking fire with steel. It exhibits a 

 fibrous stnicture, approaching to a crystalline 

 arrangement ; and the direction of its fibres, as 

 shown by the form of its fragments when broken, 

 is every where perpendicular to the surface of the 

 ivory to which it is applied. The ends of the fibres 

 are thus alone exposed to the friction of the sub- 

 stances upon which the teeth are made to act; and 



