AGE, AS INDICATKD BY THE TEETH, 321 



a positively regular rule of wear. Perfect regularity iu wear and in the 

 effect of wear is seldom found in nature. In some instances it will be 

 observed that the enamel is higher and more prominent than in others. 

 This difference does not indicate or in any degree depend on age, but 

 simply on the comparative hardness or softness of the enamel and den- 

 tine. 



The Tusks. — In horses, as distinguished from mares, great assistance 

 in determining the age is derived from the presence of the Tusks, which 

 are generally wanting in the latter. The tusks usually begin to appear 

 in a very slight degree about three and a half or four years old. Their 

 sharp points then just pierce the gums, and they continue to grow until 

 fully developed about five or five and a half years old. They do not meet 

 like other teeth, and therefere do not suffer from wear from that cause. 

 They suffer, however, from wear in the course of mastication, and in fact 

 undergo greater changes than any other teeth, and so form a valuable 

 guide as to age. 



The tusk is a very peculiar shaped elongated tooth. Internally it 

 consists of dentine, and is protected on the outside only by enamel. The 

 enamel, however, overlaps the dentine, and hence arises the sharp edge or 

 hook of the newly developed tusk, which may be felt if the finger be 

 brought round it from behind. 



This sharpness gradually wears off. After seven it has disappeared, 

 and in each succeeding year the tusk becomes not only rounder and 

 blunter, but its upper portion wears off. It also appears yellow, on ac- 

 count of the dentine becoming exposed by reason of the enamel wearing 

 off its exterior surface. The tusks, unlike other teeth, do not apparently 

 increase in length with years, but become shorter and shorter. In fact 

 the effect of wear is greater on them than on other teeth, and it is also 

 greater than the process of the receding of the gum. In very old horses 

 the tusk is very little above the level of the gum. Mares sometimes have 

 four small rudimentary tusks. 



The alterations, which gradually take place in the foi-m of the tusks, 

 are shown in Figure 16*8. 



Collateral circumstances be taken into consideration.— In 

 judging of the age of the horse by the teeth, every collateral circumstance 

 requires to be taken into consideration, such as the form of the mouth, 

 the way in which the teeth meet and close on each other, the food on 

 which the animal has been kept, any irregularity in the upper teeth which 

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