_I^1_, _I^2_, First and second permanent incisors; _Di^3_, third
deciduous incisor. The cheek teeth are numbered according to popular
usage.
]
In the mare the canines usually are very small and do not erupt,
reducing the number to 36 or 38.[81]
=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 equidæ) 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 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.
[Illustration:
FIG. 246.—LOWER INCISOR AND CANINE TEETH OF HORSE, FIVE YEARS OLD.
]
[Illustration: