GC8 MAMMALIA. 



socket, its limits on each side being distinctly marked by a defined 

 line. This membrane secretes enamel, and coats the convex 

 surface of the tooth with a thin layer (b) of that dense substance. 

 From this beautiful arrangement it results, that while the ante- 

 rior end of the tooth is perpetually worn away by attrition against 

 hard substances, the ivory is abraded more rapidly than the 

 enamel that coats it in front ; thus, therefore, the tooth constantly 

 preserves its chisel-like shape, and presents the sharp cutting 

 edge formed by the layer of enamel. 



(746.) The second kind of teeth, composed of bone and enamel, 

 are limited in their growth ; and the entire crown or projecting 

 portion is invested with enamel covering its surface. The teeth 

 of all the CARNIVORA, of the QUADRUMAXA, and also of MAN, 

 are of this description. From marked differences in their form 

 in different regions of the mouth, such teeth are conveniently 

 divisible into different groups, called respectively incisores, lani- 

 ares or canine teeth, pseudo-molar es or false grinders, and 

 molar es or grinding teeth. 



Whatever may be the shape of teeth of this class, their mode 

 of growth is similar to that observed in those of our own species. 

 We have chosen, in order to illustrate this, the growing perma- 



Fi^.313. 



nent teeth of a young Lion, wherein the different organs employed 

 in their formation are easily distinguishable. The ivory that forms 

 the bulk of the tooth (Jig. 318, b) is formed by the surface of an 

 internal pulp (a) ; and as it slowly accumulates, encroaching upon 

 the central cavity, and penetrating more deeply into the socket, 

 the fang is gradually formed, and the central pulp shrinks until, in 

 the fully formed tooth, it becomes reduced to a thin membrane, 

 richly supplied with vessels and nerves, which lines the small cen- 

 tral cavitv that remains. 



