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.
Some other peculiarities of the dentition of the horse must be noticed,
as they bear upon what I shall have to say by and by.
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