papillæ, set so closely as to give the appearance of the 'pile' of velvet.
This is observed to the best advantage with the foot immersed in water.

_The Superior Border_ of the cushion is bounded by the _Perioplic Ring_,
the cells of which have as their function the secreting of the _Periople_,
a layer of thin horn to be noted afterwards as covering the external face
of the wall. From the perioplic ring the cushion is separated by a narrow
and shallow, though well-marked, groove.

The inferior border is bounded by the sensitive laminæ.

[Illustration: FIG. 17.--THE KERATOGENOUS MEMBRANE (VIEWED FROM THE SIDE).
(THE HOOF REMOVED BY MACERATION.) 1. The sensitive laminæ, or podophyllous
tissue; 2, the coronary cushion; 3, the perioplic ring; 4, portion of
plantar cushion; 5, groove separating perioplic ring from coronary cushion;
6. the sensitive sole.]

The upper portions of the laminæ, those in contact with the cushion, are
pale in contrast with the portions immediately below, and thus there is
given the appearance of a white zone adjoining the inferior border of the
cushion.

Widest at its centre, the cushion narrows towards its extremities, which,
arriving at the bulbs of the plantar cushion, bend downwards into the
lateral lacunæ of the pyramidal body, where they merge into the velvety
tissue of the sole and frog.

The papillæ of the coronary cushion secrete the horn tubules forming the
wall, and the papillæ of the perioplic ring secrete the varnish-like veneer
of thin horn covering the outside surface of the hoof.