distance towards its centre, after which it becomes gradually more thin.

_The Internal Border_ has the shape of an elongated V with the apex
pointing forwards. It is much thinner than the external border, and,
like it, is dovetailed into the horny laminæ of the inflections of the
wall--namely, the bars. In front of the termination of the bars it is
dovetailed into the sides and point of the frog. Where unworn by contact
with the ground, the horn of the sole is shed by a process of exfoliation.

3. THE FROG.--Triangular or pyramidal in shape, the frog bears a close
resemblance to the form of the plantar cushion, upon the lower surface of
which body it is moulded. It offers for consideration two faces, two sides,
a base, and a point or summit.

[Illustration: FIG. 22.--HOOF WITH THE SENSITIVE STRUCTURES REMOVED. 1,
Superior face of horny frog; 2, the frog-stay; 3, the lateral ridges of
the frog's superior surface; 4, the horny laminæ at the inflections of the
wall.]

_The Superior Face_ is an exact cast of the lower surface of the plantar
cushion. It shows in the centre, therefore, a triangular depression, with
the base of the triangle directed backwards. Posteriorly, the depression
is continued as two lateral channels divided by a median ridge. The median
ridge widens out as it passes backwards, forming the larger part of the
posterior portion of the frog. This median ridge fits into the cleft of the
plantar cushion. It serves to prevent displacement of the sensitive from
the horny frog, and has been rather aptly termed the '_Frog-stay_.'

_The Inferior Surface_ is an exact reverse of the superior. The triangular
depression of the superior surface is represented in the inferior surface