ive sole.]

The upper portions of the laminae, 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 lacunae of the pyramidal body, where they merge into the velvety
tissue of the sole and frog.

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

[Illustration: FIG. 18.--THE KERATOGENOUS MEMBRANE (VIEWED FROM BELOW).
(THE HOOF REMOVED BY MACERATION.) 1, The sensitive sole; 2, the sensitive
frog[A]--(a) its median lacuna, (6) its lateral lacuna; 3. V-shaped
depression accommodating the toe-stay; 4, the sensitive laminae which
interleave with the horny laminae of the bar.]

[Footnote A: The sensitive frog thinly invests the plantar cushion or
fibre-fatty frog, the outline of which is here indicated.]

2. THE VELVETY TISSUE.--This is the portion of the keratogenous membrane
covering the plantar surface of the os pedis and the plantar cushion. To
the irregularities of the latter body--its bulbs, pyramidal body, and its
lacunae--it is closely adapted. Its surface may, therefore, be divided into
_(a) The Sensitive Frog_, and _(b) The Sensitive Sole_.

_(a) The Sensitive Frog_ is that part of the velvety tissue moulded on the
lower surface of the plantar cushion. The shape of the plantar cushion has
already been described as identical with that of the horny frog. It only
remains to state that, like the coronary cushion, the surface of the
sensitive frog is closely studded with papillae. The cells clothing the
papillae are instrumental in forming the horny frog.

_(b) The Sensitive Sole_.--As its name indicates, this is the portion of
the keratogenous membrane that covers the plantar surface of the os pedis.
It also is clothed with papillae, which again give rise to the formation of
that part of the horny box to which they are adapted--namely, the sole.

3. THE PODOPHYLLOUS TISSUE, OR SENSITIVE LAMINAE.--This portion of the
keratogenous membrane is spread over the anterior face and sides of the os
pedis, limited above by the coronary cushion, and below by the inferior
edge of th