14 



PHYSIOLOGY AT THE FARM. 



Fig. 1. 



horse vertical folds of enamel and cement penetrate the crown 

 of the tooth. Even the incisors or nippers of the horse have 

 a single fold, composed of cement and 

 enamel, which dips from the horizontal 

 flat surface of the crown into an excava- 

 tion of the dentine, as represented in fig. 

 1. This fold is peculiar to the incisors 

 or nippers of the horse, being found in 

 the incisor teeth of no other animal. This 

 fold, or process as it is called in anatomy, 

 consists, as just said, of cement and en- 

 amel, the enamel being interposed between 

 the cement and the dentine, in which ex- 

 ists the excavation wherein the fold is 

 lodged. When the tooth begins to wear, 

 the fold becomes an island of enamel 

 enclosing a cavity partly filled by cement, 

 partly by tartar and substances derived 

 from the food, which cavity constitutes the 

 "mark." In aged horses the incisors are 

 worn down beyond the limits of the fold, 

 whence the " mark " disappears. In the 

 mid-incisors the cavity disappears at the 

 sixth year, in the incisor on each side of these at the seventh 

 year, and in each outer incisor at the eighth year, in the lower 

 jaw. In the incisors of the upper jaw the mark remains some- 

 what longer. 



The folds of cement and enamel, which in like manner pene- 

 trate the dentine from the summits of the crowns in the 

 molar teeth, are not single, as in the incisors, but several the 

 effect of which is that, as the tooth wears, the hard enamel, 

 being most resistant, continues to present a sharp edge, of 

 varied pattern, of signal power in the trituration of the food. 



LONGITUDINAL SECTION 

 OF THE INCISOR TOOTH 

 IN THE HORSE. 



d, The dentine ; e, the en- 

 amel ; c, c, the cement ; 

 s, mass of tartar and 

 particles of food. 



