IN THE LOWER ANIMALS, ETC. 243 



jaw is greatly developed, to give space on its in- 

 ternal surface for the insertion of the internal 

 pterygoid muscle, while the external pterygoid is 

 inserted nearer to the condyle ; their united forces 

 supplying the powerful lateral and oblicpie motions 

 referred to. 



The human teeth present us with a simple 

 arrangement of the three substances of which 

 mammalian teeth are composed, the greater por- 

 tion of the tooth consists of dense nonvascular 

 dentine, the crown being covered by a simple cap 

 of enamel, which is conformed in every part to 

 the contour of the crown; and the enamel fibres 

 are arranged at nearly a right angle to the surface 

 on which they rest, the enamel possessing greater 

 density and hardness, and a lower degree of vita- 

 lity than any other animal tissue. 



The roots of the human teeth, and, in a very 

 minute degree, the crowns also, are invested with 

 a layer of cementum ; a structure showing a close 

 affinity to true bone, and which, endowed with a 

 far higher amount of vitality than either enamel 

 or dentine, has also the least density. 



Perhaps the structure of the teeth of gramini- 

 vorous mammals could not be better illustrated, 

 than by our conceiving the idea of a number of 

 such simple teeth, modified in outline and soldered 

 together by the greatly increased proportion of 

 cementum, not only to the surface of the roots, 

 but of the crowns also ; thus forming a compound 



n 2 



