68 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



cusp, as may be seen in many of the lower primates, is really 

 double. 



In the upper molar teeth, to the three primary cusps which 

 form a cup, a fourth has been added (see Fig. 50^). The two 

 outer or buccal cusps are distinguished as the A.E. cusp (antero- 

 external), the P.E. cusp (postero-external) ; the two inner as the 



a.e. 



7 2 



1+2 



B. 



Fig. 50. — A. The tritubercular Type of Tooth. The corresponding cusps are shown 

 in the crowns of an Incisor (.B), Canine (C), Bicuspid (Z>), Upper Molar (£), and 

 a Lower Molar (F). 



A.I. (antero-internal) and P.I. (postero-internal). In the upper 

 molars the cusps are situated alternately and the P.E. and A.I. 

 cusps are united by an oblique enamel ridge, which represents the 

 posterior margin of the crown of the primitive tritubercular tooth 

 (Fig. 5 E). In the molar teeth of civilized races, especially in 

 their wisdom teeth, the 4th or posterior internal cusp is often 

 absent, the primitive tritubercular tooth thus reappearing. In 

 the lower molars two cusps have been added to the three primary 

 ones, making five in all. The fifth cusp is situated at the 

 posterior border of the crown ; the others are arranged in opposite 

 pairs. The fifth cusp has become lost in the 2nd and 3rd lower 

 molars of civilized races. 



The Roots. — The upper molar teeth have three roots, two outer 

 and one inner, but in the wisdom teeth, especially of civilized 

 races, the roots are usually fused. The lower molars have two 

 roots, but each root appears to be essentially double in nature. 

 In lower primates the upper biscusps have three roots, but in man 

 these are usually fused so as to form one or sometimes two roots. 

 The lower bicuspids have usually one root, but as in lower apes, 

 they may have two. 



