THE TEETH AND THEIR DEVELOPMENT 185 



stretching their head, which also prevents them 

 from being well reined in. 



The changes which successively take place in 

 the incisor teeth throughout the whole period of 

 the life, form the most certain test of the age of 

 the horse. 



The teeth of a horse are forty in number, and 

 in the mare thirty-six, being destitute of the four 

 canines or tushes. The incisor, or cutting teeth, 

 are temporary, and are replaced by others from 

 two and a half years to four or five. The marks 

 on them which distinguish the age become obliter- 

 ated at eight years. 



Teeth are composed of three distinct structures, 

 viz., dentine, enamel, and crusta petrosa. The 

 dentine surrounds the " pulp cavity " in all teeth. 

 The enamel forms the white shell, and the crusta 

 petrosa or cement joins the three structures to- 

 gether. The incisor tooth of the horse is known 

 from other incisor teeth, not only by its shape, 

 but by having a cavity in the crown, termed the 

 " infundibulum " or pit, which descends about a third 

 way down the body of the tooth and ends in a 

 blind pouch. 



In the early stages of their formation, the 

 incisor teeth of the horse resemble a cellular body 

 whose sides are soft and membranous, and quickly 

 become hard and thickened, and are then reflected 

 at the side of the table. From this primary dental 

 production emanate two cavities that have no com- 

 munication with each other, and which are essen- 

 tially different, the largest being situated next the 

 root. (See Plate v, fig. n.) This contains the 

 pulpy substance and the blood-vessels and nerves 

 of the tooth. The other cavity, or so - called 



