The teeth of a horse are not less peculiar than its limbs.  The living engine, like all others, must be well stoked if it is to do its work; and the horse, 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 apposed 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 apparatus of great efficiency, and one which is repaired as fast as it wears, owing to the long-continued growth of the teeth.