424 CHAPTER 58. 



they can recede no further. Hence all further wear shows its effects Ly 

 diminishing the length of the teeth. 



Loss of circularity. — In the very young horse the teeth are arranged 

 almost in the form of a semicircle. Year by year this form decreases, 

 until in old horses the teeth are arranged in something like a straight 

 line. Compare figs. 0, 1, 2, 3, and 4 with figs, li, 15, 16, and 17. 



If the reader should hajDjDen to be in the neighbourhood of a cavalry 

 barrack, he will have the best possible opportunity of studying the age 

 of living horses, because in every regiment an accurate register is kept of 

 the age of every horse. As all the horses are bought at four or five 

 years old, it is almost impossible that any mistake can occur. 



Memorandum. — The drawings of the teeth have all been made from 

 nature ; and hence, although pretty normal specimens have been selected, 

 yet in various ways they present in some instances irregularities and 

 deviations from a positively regular rule of wear. Perfect regularity in 

 wear and in the effect of Avear is seldom found in nature. In some in- 

 stances 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 dentine. 



875. The Tuslis. 



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 therefore do not sufl'er 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 veiy peculiar-shaped elongated tooth. Internally it con- 

 sists 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 

 account of the dentine becoming exposed by reason of the' enamel wear- 

 ing off its exterior surface. The tusks, unlike other teeth, do not appa- 

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

 fact the eft'ect 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 some- 

 times have four small rudimentary tusks. 



The alterations, which gi-adually take place in the form of the tusks, 

 are shown in a series in fio;. 28. 



