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 the bone.  It presents the appearance of fine longitudinal streaks, which, when closely examined with a needle, are found to consist of numerous fine leaves.  These extend downwards from the lower border of the coronary cushion to the inferior margin of the os pedis.  At this point each terminates in several large villous prolongations, which extend into the horny tubes at the circumference of the sole.  At the point of the toe this membrane sometimes shows a V-shaped depression, into which fits a inverted V-shaped prominence on the inner surface of the wall at this point.

The sensitive laminae increase in width from above to below.  Their free margin is finely denticulated, while their sides are traversed from top to bottom by several folds (about sixty), which, examined microscopically, are seen to consist of secondary leaves, or laminellae.