_The two-year-old colt_ shows a well-worn set of incisor teeth, and the
ruminant at this age has replaced the nippers or centrals.

_The third_, _fourth_ and _fifth years_ are indicated by the replacement of
the temporary nippers, dividers and corners in the horse, and the first and
second dividers and corner teeth in ruminants.

[Illustration: FIG. 52.--Longitudinal section of incisor tooth: cup;
cement; enamel; ivory; and pulp cavity.]

In the horse the permanent nippers are full grown and in wear at _three
years_ of age; the permanent dividers are full grown and in wear at _four
years_ of age; and the permanent corners are full grown and in wear at
_five years_ of age. The table surfaces of the incisor teeth of a
five-year-old horse show different degrees of wear. At this period in the
animal's age, the nippers have been in wear two years, the dividers one
year, and the corners are beginning to show wear. In ruminants, all of the
chisel-shaped table surfaces of the incisors show considerable wear when
the animal is five years old.

After the animal has a full set of permanent teeth, we judge the age by the
degree of wear or the appearance of the table surfaces of the incisors,
their shape, the angle with which they meet and the general appearance of
the head.

[Illustration: FIG. 53.--Cross-section of head of young horse showing
replacement of molar tooth.]

There are several different factors that may cause the wear on the teeth,
and the appearance of their table surfaces to vary in the different