THE TEETH OF THE HORSE 



341 



Incisor Teeth. — These are twelve in number. The six in each jaw are placed 

 close together, so that their labial edges form almost a semicircle. They have the 

 peculiarity (not found in existing mammals other than the equidae) of presenting, 

 instead of the simple cap of enamel on the crown, a deep invagination, the infundi- 

 bulum, which becomes partly filled up with cement. Hence as the tooth wears 

 the table surface has a central ring of enamel surrounding this cavity in addition 

 to the peripheral enamel. The cavity becomes darkened by deposits from the 

 food, and is commonly termed the "cup" or "mark." Each tooth is curved so 

 that the labial surface is convex and the roots converge. The average length of 

 the incisors at five or six years of age is about two and a half to three inches (ca. 7 

 cm.). They taper regularly from crown to root, without any constriction, and 

 in such a manner that in young horses the exposed crown is broad transversely; 

 toward the middle, the two diameters of a cross-section are about equal; near the 

 root the antero-posterior diameter is considerably greater than the transverse. 



This fact is of value in the determination of age by the teeth, since the table 

 surface at different ages represents a series of such cross-sections. As the exposed 



Fig. 246. — Lower Incisor and Canine Teeth op Fig. 247. — Upper Incisor and Canine Teeth of 



Horse, Five Years Old. Horse, Five Years Old. 



/', /-, /3, Incisors; C , canine. 



crown wears down the embedded part (reserve crown) pushes up out of the alveolus, 

 so that the tables of the first and second lower incisors are at first oval, with the 

 long diameter transverse ; later — at about fourteen years usually for the first lower 

 incisors — the tables are triangular, wnth the base at the labial edge. At the same 

 time the infundibulum or cup becomes smaller, approaches the lingual border, and 

 finally disappears; it remains longer on the upper incisors, as it is deeper in them. 

 Anotlier marked feature in old age is the progressive approach to a horizontal direc- 

 tion as seen in profile; at the same time the teeth become parallel and finally con- 

 vergent. 



Canine Teeth. — These are four in number in the male; in the mare they are 

 usually absent or rudimentary.' They interrupt the interdental space, dividing 

 it into two unequal parts. The upper canine is situated at the junction of the pre- 

 maxilla and the maxilla; the lower canine is placed nearer the corner incisor. 

 The canines are simple teeth, smaller than the incisors, and are curved with the 

 concavity directed backward. The crown is compressed, convex, and smooth 



^ It is interesting to notice that vestigial canines are not at all uncommon in mares, espe- 

 cially in the lower jaw. They are very small, and do not usually erupt; their presence is indi- 

 cated in the latter case by a prominence of the gum. This is in conformity with the fact that 

 they were present in both sexes in Eocene and Miocene ancestors of existing equidae. 



