82 AGE OF HORSES. 



depression that is more or less filled with cement, which 

 soon becomes discoloured by the food the animal eats. 

 The hole thus made in the tooth is called the " marlcr 

 As this layer of cement varies from one-tenth to 

 one-half of an inch in thickness, the " mark " wears 

 nut in the teeth of some horses, much quicker than 

 it does in tliose of others. The outer enamel which 



Fig. 12. — A transverse section of a lower right front incisor, showing 

 the different layers of the tooth, with their relative thickness 

 (enlarged). A, front face. B, rear face. C, cement. E, outer 

 enamel. E', central enamel. I, external dentine. I', internal and 

 darker coloured dentine. 



surrounds the crowns of the teeth is, in the first instance, 

 covered with a very thin layer of cement, which is soon 

 rubbed off. After an incisor has been a short time 

 in use, its table (cutting surface) presents two, more 

 or less, irregular rings of enamel (see Fig. 12) : the 



