STRUCTURE OF THE INCISORS. 25 



extremity of the tooth ; but in very old horses, when the teeth 

 have worn down to the roots, and the dentine, no longer pro- 

 tected by the enamel, offers a diminished resistance to the fric- 

 tion to which they are exposed, the irritation of the dormant 

 periosteum sets up a new secretion, and cement is thrown out 

 often in very great quantities to strengthen weak roots. As the 

 inferior incisors are the shortest, and the first to wear away, we 

 more frequently find the large masses of cement in the lower 

 jaw. We again find excessive deposits of cement around the 

 roots of the teeth when they have been loosened or irritated in 

 their cavities by the rough pressure of bits, or have been injured 

 by any accidental cause. Tins occurs most frequently in vicious 

 or playful horses which have the habit of biting roughly at 

 foreign bodies. We see in this a wise provision of nature to 

 succor these important organs after accident, and from the 

 effects of old age. 



2. The Enamel (Fig. 12, EE ; Fig. 11, outer surface of d). 

 — The enamel is the true protecting layer of the teeth. Under- 

 neath the cement it forms a sort of armor which covers the sur- 

 face of the dentine and forms the walls of the cup. It does not 

 reach the cavity of the pulp. It is thicker, and covers a greater 

 extent on tlie anterior face of the tooth than it does on the pos- 

 terior. (Fig. 12, E E.) This furnishes an important factor in 

 determining the age of very old horses, when tlie tooth either 

 becomes triangular or biangular. In the wall of the cup the 

 enamel has about the same thickness throughout, although the 

 difference of a transverse or an oblique section may give a 

 deceptive appearance of a greater thickness at one point than at 

 another 



The enamel has a wonderful hardness. While still inclosed 

 in the germinal sack it is readily cut by a knife and its elements 

 can be dissected into Z-shaped prisms. From the moment 

 that it is exposed to the air it becomes so hard that it will strike 

 fire from flint. As it is more resisting than the dentine, it 

 constantly stands in relief on the surface of the table of the 

 tooth. 



