[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 laminæ which
interleave with the horny laminæ 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
lacunæ--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 papillæ. The cells clothing the
papillæ 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 papillæ, 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 LAMINÆ.--This portion of the
keratogenous membrane is spread over the anterior face and sides of the os