2. THE PLANTAR CUSHION on FIBRO-FATTY FROG.—Composed of a fibrous meshwork, in the interstices of which are lodged fine elastic and connective fibres and fat cells, this wedge-shaped body occupies the space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog. It offers for consideration an antero-superior and an infero-posterior face, a base, an apex, and two borders.
The antero-superior face is in contact with the terminal expansion of the perforans tendon. The infero-posterior face is covered by the keratogenous membrane, and follows closely the shape of the horny frog, on whose inner surface it is moulded. It presents, therefore, at its centre a single conical prolongation, the Pyramidal Body, which is continued behind, as is the horny frog, in the shape of two lateral ridges divided by a median cleft. The base of the cushion lies behind, and consists of two lateral masses, the Bulbs of the Plantar Cushion. In front these are continuous with the ridges of the pyramidal body, while behind they become confounded with the lateral cartilages and the coronary cushion. The apex is fixed into the plantar surface of the os pedis, in front of its semilunar ridge. The borders, right and left, are wider behind than before, and are in relation with the inner faces of the lateral cartilages.
H. THE KERATOGENOUS MEMBRANE.
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 Laminæ.
1. THE CORONARY CUSHION. In the foot stripped of the hoof the coronary cushion is seen as a rounded structure overhanging the sensitive laminæ 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 laminæ.
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 papillæ, 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.