160 AGE OF THE DOMESTIC ANIMALS. 



with which it merges, while its sides are bordered by small 

 gutters. The internal border is convex and the external border 

 is concave, which gi\'es the tooth a carved shape and which in- 

 dicates the side to which it belongs. The root is rounded, 

 conical, and yellow in color ; its extremity, in a young tooth, 

 sliows the opening of the dental canal. 



Structure. — The incisors have approximately the same 

 structure as the tush tooth of a horse ; they are composed of a 

 dentine with the free portion covered by a continuous layer of 

 enamel. The enamel is thickest on the external surface and 

 gradually disappears on the root. In the young tooth there is 

 a large, simple, dental cavity filled with the dental pulp; but as 

 the animal gets older, a dark-yellow dentine is deposited until 

 the cavity is filled up and the tooth ceases to grow. It is not 

 pushed from its alveolar cavity, as the incisors of the horse are, 

 as the free portion is worn away. 



Like the horse, the ox has two sets of incisors, — the tempo- 

 rary or milk-teeth and the permanent ones. The milk-teeth 

 are distinguished from the permanent ones from their behig 

 smaller and narrower ; their enamel is thinner and more trans- 

 parent and they are more curved to the side. Their roots are 

 very sliort, and are pushed out by the replacing permanent 

 incisors. 



The incisor scarcely reaches its full development when it 

 commences to be worn by its contact and constant friction 

 with food and with the cushion of the upper jaw. The wearing 

 commences at the anterior border and removes tlie enamel 

 toward the posterior part of the upper face. When it has 

 completely removed the conical eminence and the lateral gutters, 

 the tooth is said to be leveled and the table is formed. From 

 the almost horizontal direction of the incisor teeth, the wearing 

 of their tables takes place in an oblique direction to their long 

 axis. 



The table at first is large, consisting of a plate of dentine 

 surrounded by a border of enamel, and having in its centre a 

 transverse line of dark yellow, made by the uncovered, later de- 



