[Footnote A: Extracted from a paper by J.A. Gilruth, M.R.C.V.S., in the
_Veterinary Record_, vol. v., p. 358.]

We have just referred to contraction of the heels as taking the place of a
normal expansion in those cases where ground frog-pressure was absent. We
shall readily understand this when we bear in mind the anatomy of the
parts concerned, especially that of the plantar cushion. This wedge-shaped
structure we have already described as occupying the irregular space
between the two lateral cartilages, the extremity of the perforans tendon,
and the horny frog.

Now, when weight or pressure is exerted from above on to this organ, and
the _frog is in contact with the ground below_, it is clear from the
position the cushion occupies that, whatever change of form pressure from
above will cause it to take, it must certainly be limited in various
directions.

[Illustration: FIG. 37. _a_, The dotted lines in this diagram represent the
expansion of the solar edge of the hoof at the moment of over-extension of
the fetlock-joint; _b_, the dotted line represents the change in form of
the coronary edge under similar circumstances.]

Because of the shape of the cushion its change of form cannot be forwards
(simultaneous pressure from above and below on to this wedge with its apex
forwards must tend to give it a backward change of form). Because of the
pastern being horizontal, and aiding in the downward pressure, its
change of form cannot be upwards. And because of the ground it cannot be
downwards. It follows, therefore, that the movement must be backwards and
outwards, being especially directed outwards because of its shape and the
median lacuna in its posterior half--this latter, the lacuna, accommodating