if it is to make good its wear and tear, and to exert the
enormous amount of force required for its propulsion, must be well and
rapidly fed. To this end good cutting instruments and powerful and
lasting crushers are needful. Accordingly, the twelve cutting teeth of a
horse are close-set and concentrated in the fore-part of its mouth, like
so many adzes or chisels. The grinders or molars are large, and have an
extremely complicated structure, being composed of a number of different
substances of unequal hardness. The consequence of this is that they
wear away at different rates; and, hence, the surface of each grinder
is always as uneven as that of a good millstone.

I have said that the structure of the grinding teeth is very
complicated, the harder and the softer parts being, as it were,
interlaced with one another. The result of this is that, as the tooth
wears, the crown presents a peculiar pattern, the nature of which is not
very easily deciphered at first, but which it is important we should
understand clearly. Each grinding tooth of the upper jaw has an _outer
wall_ so shaped that, on the worn crown, it exhibits the form of two
crescents, one in front and one behind, with their concave sides turned
outwards. From the inner side of the front crescent, a crescentic _front
ridge_ passes inwards and backwards, and its inner face enlarges into a
strong longitudinal fold or _pillar_. From the front part of the hinder
crescent, a _back ridge_ takes a like direction, and also has its
_pillar_.

The deep interspaces or _valleys_ between these ridges and the outer
wall are filled by bony substance, which is called _cement_, and coats
the whole tooth.

The pattern of the worn face of each grinding tooth of the lower jaw is
quite different. It appears to be formed of two crescent-shaped ridges,
the convexities of which are turned outwards. The free extremity of each
crescent has a _pillar_, and there is a large double _pillar_ where the
two crescents meet. The whole structure is, as it were, imbedded in
cement, which fills up the valleys, as in the upper grinders.

If the grinding faces of an upper and of a lower molar of the same side
are applied together, it will be seen that the opposed ridges are
nowhere parallel, but that they frequently cross; and that thus, in the
act of mastication, a hard surface in the one is constantly applied to a
soft surface in the other, and _vice versa_. They thus constitute a
grinding ap