CO THE PERMAXEKT DENTITION. 



tecting varnish, but is not everywhere of the same 

 thickness. On the salient portions it is extremely 

 thin, and the friction caused by the food, the lips, and 

 the tongue soon wears it away altogether. It is more 

 abundant in depressed situations, as in the longitudi- 

 nal groove on the anterior face of the tooth, and par- 

 ticularly at the bottom of the infundibulum. The 

 quantity accumulated in this cul-de-sac is not, how- 

 ever, always the same. We have seen it almost null, 

 and on the other hand, we possess an incisor unworn, 

 or nearly so, in which the cavity is almost entirely 

 obstructed by it. We are not aware that, up to the 

 present time, any account has been taken of these dif- 

 ferences in calculating the progress of wear; but it is 

 manifest that they shorten or prolong the time re- 

 quired for the effacement of the infundibulum." 



The grinder teeth, the horse's millstones, present 

 various and interesting contrasts. They are sepa- 

 rated from the incisors by a space that will average 

 about four inches in extent, the sharp-pointed tushes 

 (in males) only intervening. The space between the 

 grinders and tushes is called the diastema (place for 

 the bit). The upper grinders, except the first and 

 last, are nearly quadrangular in form. The first and 

 last, which exceed the others about a third of an inch 

 in antero-posterior (front to rear) diameter, terminate 

 in obtuse angles, which are far more pronounced on 

 the inner than on the outer surface, thus affording the 

 tongue fuller and freer play, without the danger of its 

 being lacerated, as would be the case were the angles 

 sharp. The form of the lower grinders, with the same 

 exceptions in the case of the first and last, is nearly 

 rectangular; their antero-posterior diameter is the 



