422 CHAPTER 58. 



somewhat later in the other teeth. It is indicated by a slight discolora- 

 tion of the tooth at the above point. There is, however, no actual hole, 

 because with advancing years the upper part of the original cavity has 

 become filled up with a sort of spurious dentine, which is more yellow 

 than the true material, of which the body of the tooth consists. As age 

 increases, this indication of the fang-hole, which is sometimes called the 

 " Secondary mark," becomes rather more plain. It, however, affords no 

 reliable data by which to judge of the age, and is only mentioned in this 

 place lest the reader should mistake it for the remains of the infundibu- 

 lum. The enamel, it will be remembered, is pearly white, whilst the 

 mark of the fang-hole is brownish yellow. The position of the fang-hole 

 is shown, fig. 18 B. 



874. Further changes indicating the age. 



It will be seen that about nine the " marks," never very reliable, 

 entirely fail us, and indeed after seven or eight they can hardly be said 

 to afford any very reliable data. Veterinary surgeons and horsemen 

 prefer to judge the age mainly by the changes which gradually take 

 place in the shape of the teeth. But the public always rely a good deal 

 on the marks. 



From eight years old and upwards the best indications of the age are 

 given by the gradual alterations, which take place in the shape of the 

 teeth from wear and in the closing of the mouth. 



Lateral breadth, &c. The teeth originally are broad laterally at their 

 upper surfaces, otherwise called their crowns or " tables," and thin from 

 front to rear. (Figures 4, 5, and G.) They narrow gradually towards 

 their necks and fangs. Hence as their upper surfaces wear off, the teeth 

 become narrower year by year. In very old horses there is often a posi- 

 tive interval between the teeth (figures 16 and 17), and they appear like 

 sticks in the jaw. 



The gradual effect of wear in producing this alteration is shown in 

 figure 20, where successive portions of the upper surface of the tooth are 

 represented as having been removed by the saw. The original form of 

 the tooth is shown in ig. 21. 



The amount of wear on the upper surface of the teeth is greater in 

 the young mouth than it is afterwards, because in youth the teeth meet 

 more fairly than they do in after years. (Compare Figs, ia and 27.) 

 . The rate of wear gradually decreases as years increase, because the teeth 

 do not meet so directly, but on the contrary project more and more for- 

 ward in something like two parallel lines. For example, a quarter of an 

 inch will usually be worn "off the surface between five and six years old, 

 whilst probably not more than that quantity will be worn off between 

 twenty-and twenty-five years old. 



Triangularity. A further very well-marked indication of increasing 

 age is given by the increasing depth from front to rear in the upper sur- 

 faces or crowns of the teeth. This increase of depth will be noticed if 

 figs. 7 and 8 are carefully compared with figs. 4, 5, and 6. Further wear 

 causes the crowns of the teeth to assume a triangular form. The cause of 



