maintained by a pushing out of the tooth, and a corresponding shortening of
the portion that is fixed or imbedded in the jaw.

_The table surface of the unworn incisor_ tooth is covered with enamel, and
in the middle portion the enamel forms a deep cup. After the tooth has
become worn the margin of the table portion is then limited by a ring of
enamel. This is termed the encircling enamel ring. The central portion of
the table shows a second ring, the central enamel ring, that limits the cup
margin (Fig. 54).

[Illustration: FIG. 54.--Transverse section of incisor tooth: peripheral
cement; peripheral enamel; ivory; central enamel; and central cement.]

_As the table surface_ represents a cross section of the tooth, its
appearance and shape will then depend on the portion of the tooth that it
represents. From year to year, there is a gradual shortening in the lateral
diameter, and an apparent increase in the diameter from before to behind.
These changes in shape are from a long, narrow table surface to an oval,
from oval to circular and from circular to triangular (Fig. 55). As the
original free portion of the tooth wears off, the cup becomes shallow and
smaller until the remnant is represented by a mere dot of enamel that
finally disappears from the posterior portion of the table. After the cup
has moved from the central portion of the crown and occupies a more
posterior position, the dental star, which represents a cross section of
the pulp cavity, puts in its appearance. It first takes the form of a brown
or dark streak, and later a circular dark spot which gradually increases in
size with the wear on the tooth and the age of the animal.

[Illustration: FIG. 55.--Table surfaces of nippers at different ages: four
years; six years; nine years, and fifteen years of age.]