The inferior border is bounded by the sensitive laminæ.
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.
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.