The Keratogenous, or Horn-Producing Membrane, is in reality an extension of the dermis of the digit. It covers the extremity of the digit as a sock covers the foot, spreading over the insertion of the extensor pedis, the lower half of the external face of the lateral cartilages, the bulbs of the plantar cushion, the pyramidal body, the anterior portion of the plantar surface of the os pedis, and over the anterior face of the same bone. In turn, as the human foot with its sock is covered by the boot, this is encased by the hoof, the formation of which we shall study later.

To expose the membrane for study the hoof must be removed. This may be done in two ways. By roasting in a fire, and afterwards dragging off the horny structures with a pair of pincers, a knife having first been passed round the superior edge of the horny box. Or by maceration in water for several days, when the hoof will become loosened by the process of decomposition, and may be easily removed by the hands. The latter method is less likely to injure the sensitive structures, and will expose them with a fresh appearance for observation.

For purposes of description the keratogenous membrane is divided into three regions:

1. The Coronary Cushion.
2. The Velvety Tissue.
3. The Podophyllous Tissue, or the Sensitive Laminae.

1. The Coronary Cushion. In the foot stripped of the hoof the coronary cushion is seen as a rounded structure overhanging the sensitive laminae after the manner of a cornice. It extends from the inner to the outer bulbs of the plantar cushion, and is bounded above by the perioplic ring, and below by the laminae.

When in situ it is accommodated by the Cutigeral Groove, a cavity produced by the bevelling out of the superior portion of the inner face of the wall of the hoof. Its superior surface is covered by numerous elongated papillae, 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 laminae.

Fig. 17.   The Keratogenous Membrane (Viewed From The Side)

Fig. 17. - The Keratogenous Membrane (Viewed From The Side). (The Hoof Removed By Maceration.) 1. The Sensitive Laminae, 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 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.

Fig. 18.   The Keratogenous Membrane (Viewed From Below)

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.

Examined on the foot, deprived of its horny covering, the sensitive laminae are, the majority of them, in close contact with each other. In the normal state this is not so. The interstices between the leaves are then occupied by the horny leaves, to be afterwards described as existing on the inner surface of the wall.

Reaching and rounding the heels, the sensitive laminae extend forward for a short distance, where they interleave with the horny laminae of the bars.

Much discussion has centred round the point as to whether or no the cells of the sensitive laminae take any share in the formation of the horn of the wall. This will be alluded to in a future chapter.