138 



CHORDATE ANATOMY 



Significantly, in man, although even the upper canines are hardly larger 

 than incisors, they have nevertheless the long roots of the animal tusk. 



That general tendency to shorten the mammalian face, which has 

 brought down the cats to three and four cheek teeth and the higher 

 primates to five, continues in man by a general reduction in size of all 

 the teeth and by closing the diastema, the open space next the canines. 

 Consequently, human teeth are a continuous series and no tooth is very 

 much larger than another, for the canines ceased to be tusks at the begin- 



fCANINE 

 INCISORS I PREMOLARS A^^^^^l— v 



.--" 'MOLARS 



\ "PREMOLARS 



INCISORS '^CANINE 

 Fig. 126. — Human teeth viewed from the left side. The human dental formula is: 

 is, c'l, pmS, mh. As a result of the shortening of the human jaws the third molars fre- 

 quently do not erupt. The elongated root of the canine tooth suggests that as in lower 

 primates the ancestors of man may have had fangs. (Redrawn after Braus.) 



ning of human evolution. Along with these, has gone a change in the 

 direction of the incisors, correlated with the appearance of a chin. In 

 the apes the incisors protrude, in man they stand upright. Incidentally, 

 the human "bite" becomes horseshoe-shaped, with the rows of cheek teeth 

 no longer parallel, as in all lower forms, even the apes. In addition, the 

 triangular upper molars of the apes, with three cusps, become in man 

 quadrangular with four, and, correlated with the reduced size of the single 

 teeth, their pulp cavities become relatively still farther reduced, not to 

 sacrifice unduly the thickness of the tooth wall. Some fossil human molar 

 teeth, however, are taurodont, having a relatively large pulp cavity as in 

 Neanderthal man. 



All these differences between apes and men are, however, bridged by 

 various fossil creatures, on the whole human, some of the genus Homo 



