278 HISTORY OF THE HUMAN BODY 



Hemisphere (Platyrrhini) there are three premolars and either 

 two or three molars, but in those of the Eastern Hemisphere 

 (Catarrhini) there are always two of the former and three of 

 the latter, giving the constant formula of: 



2-I-2- 1 * 



= V2 



2-1-2-3 6 



Of these the five medial teeth of each jaw are replaced by a 

 second set, the three molars not, the formula for the milk 

 dentition being: 



2-1-2-0 



2-1-2-0 



20 



In these points man corresponds completely with the other 

 Catarrhini. 



Regarding the evolution of the various shapes of mammalian 

 teeth from the primitive conical type, the canines and incisors 

 present a simple problem, since the first retain .almost their 

 typical form, and the second show merely a labio-lingual 

 flattening. The derivation of the complex cheek teeth, how- 

 ever, presents a serious problem, for the solution of which two 

 main theories have been offered. According to the first, or 

 tritubercular, theory, the fundamental postulate must be laid 

 down that every tooth., no matter how complex, represents a 

 single primary element, and that the modifications are due 

 to the development of additional cusps for the purpose of in- 

 suring a better articulation with the opposing surfaces of the 

 teeth of the other jaw. The number of cusps which may thus 

 develop on the contact surface of a tooth is si.v f the first of 

 which to appear is the protocone, or primary cusp. Associated 

 with the protocone are two secondary ones, the paracone f 

 which is anterior to it, and the metacone, which is posterior 

 (Fig. 76, I and II). These may become connected by crests 

 or ridges and the tooth may become still more complicated 

 by the bending of the crests and cusps into a V-shaped figure, 

 the trigon (Fig. 76, III). A tooth, or a dentition, in which 

 the three cusps, para-, proto-, and meta-cone, still lie in a 



