THE PERMANENT TEETH ; 397 
Tn the mare the canines usually are very small and do not erupt, reducing the num- 
ber to 36 or 38.1 
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 in the young 
horse. They have the peculiarity (not found in existing mammals other than the 
equidz) of presenting, instead of the simple cap of enamel on the crown, a deep 
invagination, the infundibulum, which becomes partly filled up with cement. 
Hence as the tooth wears, the masticatory surface (or ‘“‘table’’) 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 
Fie. 336.—Uprer Inctsor TeeTH oF Horse. Fic. 337.—Lower Inctsor TEETH oF Horse. 
The labial surfaces of the teeth have been exposed by removal of the bone. Subject was five years old. 
and the embedded parts converge. The average total length of an incisor at five 
or six years of age is about two and a half to three inches (ca. 7 cem.). They taper 
regularly from exposed crown to apex, without any constriction, and in such a 
manner that in young horses the masticatory surface is broad transversely; toward 
the middle the two diameters of a cross-section are about equal; near the apex 
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 mas- 
ticatory surfaces at different ages represent a series of cross-sections. As the ex- 
posed crown wears down, the embedded part (reserve crown) emerges from the 
1 Bllenberger found, as the result of extensive observations (S000 subjects), that about 
2 to 3 per cent. of mares have erupted canines in both jaws; that 6 to 7 per cent. have them in 
the upper jaw; while 20 to 30 per cent. have them in the lower jaw. The numerical variation in 
the above formula results from the fact that the first premolar (“‘wolf-tooth”’) is often absent and 
further that it is doubtful to what set it belongs when present. It is commonly not included in 
the enumeration. 
