746 



EXAMINATION FOR AGE. 



increase in length, on account of their wear not keeping pace 

 with the rate at which the teeth are pushed out of their sockets. 



5. The arch fomied by the incisors in each jaw becomes, gradu- 

 ally, flatter and flatter. At first, when a horse has a " full mouth," 

 the tables of the incisors are so broad from side to side, that the 

 resulting arch is as round .as practicable, in order to afford room 

 for the teeth, which, then, more or less radiate outwards. As the 

 teeth wear down, they obtain more and more room, on account 



Fig. 199. — Transverse section of a lower right central incisor, showing 

 the different layers of the tooth, with their relative thickness 

 (enlarged). A, front face. b, rear face. C, external cement, 

 c', central cement. e, external enamel. e', central enamel. 

 I, external dentine. 1', internal and darker coloured dentine. 

 {From Goitbaux and Barrier.) 



of their becoming narrower from side to side ; and the jaw, 

 consequently, " falls in." 



6. Owing to the irritation set up, by the movement of the incisor 

 teeth in their sockets, when they become short with age, the lining 

 membrane (periosteum) of the sockets sometimes secretes, around 

 the stumps, an excess of cement, which helps to keep these stumps 

 in their place, and to increase the area of their cutting surface. 



