THE AGE. 433 



deposited in cavities in the jaw of the embryo, the germs of 

 the former lying above those of the hatter. These germs are 

 well supplied with vessels for their nutriment by means of a 

 delicate membraneous envelope. The growth of the milk-teeth 

 is much more rapid in their earlier development than that of 

 the permanent teeth. The tooth is a mere pulp mitil it has 

 attained considerable size. It then begins to have deposited 

 the proper bony substance of the body of the tooth, and its 

 hard covering for that part which is afterward to be exposed 

 aoove the gums. This is called the enamel of the tooth, and 

 IS so hard as to resist iron, and even a file with difficulty will 

 make an impression on it. The enamel is deposited with re- 

 markable adaptation to the objects for which it is intended, be- 

 ing quite heavy over those parts of the tooth to which the food 

 is exposed in nipping and grinding, but comparatively thin 

 over the remainder of the tooth above the gum, and extending 

 in a thin scale but a little way beneath it. It is also arranged 

 so as to greatly strengthen the tooth. But, notwithstanding 

 all this, the teeth of the horse must wear away. 



The enamel, as it passes over the nipper, or incisor tooth,, 

 dips down into the bony substance, leaving an oval depression 

 in the crown of the tooth, called the mark, and rises to rather 

 a sharp edge on the front margin of the tooth. The mark, as 

 it is called, is occasioned by the food blackening the bottom of 

 the depression before described, as formed by the dipping of 

 the enamel as it passes over the crown of the tooth. Some 

 persons have an idea that the mark is formed by a circle of 

 ;he enamel being sunk in the middle of the tooth, with a hole 

 :n the center, and talk of the filling up of the tooth. But this 

 IS a very erroneous idea. The depression does not fill up, but 

 the wearing down of the enamel by u-se gradually changes the 

 shape and appearance of the mark, and eventually entirely 

 obliterates it, or rubs it out. 



The structure of the grinder is quite different from that of 

 the nipper, or incisor. Instead of one sack, or bag, of pulp 

 28 



