70 THE PERMAXEin: DENTITION. 



is the deeper, both being continued on the incased 

 portion. This is not the case with the lower grinders, 

 which have but one narrow and frequently indistinct 

 groove. The internal surface, in both jaws, presents 

 only one groove, and that but little marked. It is 

 placed backward in the upper teeth, and is most ap- 

 parent toward the root. The anterior and posterior 

 faces of the respective teeth, which are in contact with 

 each other, are nearly level, but at the extremities of 

 the arcades the isolated faces are converted into a nar- 

 row border. 



" The grinders are separated from each other by their 

 imbedded portion, particularly at the extremities of 

 the arcades, an arrangement which strengthens them 

 by throwing the strain put upon the terminal teeth 

 toward the middle of the line. They exhibit a variety 

 of roots. In the first and last, either above or below, 

 there are three, while the intermediate teeth have four 

 in the upper jaw, and only two in the lower. The 

 root, if examined a short time after the eruption of 

 the free portion, looks only like the shaft of the latter, 

 without fangs,* but a wide internal cavity. The roots 

 form when the teeth begin to be pushed from their 

 sockets ; they cease to grow as soon as their cavities 

 are filled with new dentine, but the tooth, constantly 

 growing, causes the walls inclosing it to contract ; so 

 that in extreme age the shaft, completely worn away, 

 leaves several stumps formed by the roots. 



" The replacement of the twelve molars is not at all 

 like what happens with the incisors. They grow im- 



* Fang for root is obsolete. Fang signifies croim especially 

 the pointed teeth of animals of prey and the poison-fang of ser- 

 pents. Fang for both root and crown causes confusion. 



