THE HEAD AND NECK. 



133 



four-sided prisms with a nearly square section. The 

 foremost, pm% and the hindermost, m% differ from 

 the others, being more triangular in section, the apex 

 of the triangle pointing forwards in the first aud 

 backwards in the last. 



To understand properly the arrangement of the 

 enamel folds and of the dentine within, and the 



Fig. 21. — a, grinding-surface of unworn molar tooth 

 of Anehitherium ; 6, corresponding surface of un 

 worn molar of young horse ; c, the same tooth after 

 it has been some time in use. In the latter, the 

 uncolored portions are the dentine or ivory, the 

 shaded parts the cement filling the cavities and sur- 

 rounding the exterior. The black line separating 

 tliese two structures is the enamel, or hardest con- 

 stituent of the tooth. 



cement on the outside of them, as seen in a section 

 or in the naturally worn surface of one of the mid- 

 dle teeth, it is necessary to examine it in its unworn 

 and even unfinished state, before the thick coat of 

 cement has been deposited around the ridges and 

 projections of the surface. Such teeth can always 

 be found within the sockets of the prepared skulls 



