THE PERMANENT TEETH. 1 13 



Of the three membranes of the germ or follicle, one only 

 may be considered as permanent, that of the pulp or internal, 

 which secretes the bone of the tooth. 



The two outer, or those of the sac, cover the crown -of the 

 tooth ; and as this is pushed forwards by successive depositions 

 of bony matter from within, they are pressed upon and wasted 

 away by absorption, like the gum, in direct proportion with 

 the advancement of the tooth, so that in perfectly natural den- 

 tition, there is little tension or pressure felt. This is called 

 cutting the teeth, a name which expresses the fact, sufficiently 

 well, but literally conveys a wrong idea. 



In cases of difficult dentition, the membranes of the sac re- 

 tain their density and vascularity, and are probably thickened 

 by inflammation, and the bony layers formed from the pulp, 

 resisted in their advancement by these membranes, make 

 compression upon the pulp and dental nerves ; this, like 

 continued pressure made in other parts of the body, becomes 

 exquisitely painful, and gives rise to distressing sympathetic 

 disturbances. The relief procured by cutting the gums and 

 sac, will be more or less immediate, according to the degree 

 of compression and inflammation of the pulp. 

 The periosteum covering the fangs of the tooth, is a reflected 

 continuation of the periosteum lining the socket, and this again 

 is continuous with that lining the jaw. 



Of the Permanent Teeth. 



The adult or permanent teeth, are developed in a manner 

 almost exactly analogous to the deciduous or infantile. The 

 germs of many of them are distinctly perceptible in the gums 

 of the infant at birth. They are placed at first deep in the jaw 



after the inner membrane of the sac has been removed by absorption, the outer 

 one, according to Bell, deposits the pars petrosa, and fills up the intervening 

 space. This is a substance harder than the bone, but softer than the enamel ; 

 and the advantage derived from it is, that it is worn oft' by trituration more 

 readily than the enamel, so that the latter is constantly maintained in sharp 

 prominent lines upon the surface of the teeth. The same object is here insen- 

 sibly attained, as a natural consequence of the difference in density of these 

 parts, which the miller effects with much labor with his pick-hammer, on the 

 burr-stones of his mill. P. 

 10* 



