3. The fleshy wall, or podophyllous tissue ([Figs. 27], [28, d], and [29, a]), is all that portion of the pododerm on which there are fleshy leaves. This leafy tissue covers the anterior surface of the os pedis and the lower portion of the external surface of the lateral cartilages. At the bulbs of the heels it turns inward at a sharp angle and extends forward and inward, between the bar portion of the coronary band and the posterior part of the velvety tissue of the sole, nearly to the middle of the solar surface of the foot, to form the laminæ of the bars ([Fig. 29, a]). The fleshy wall and fleshy bars are not covered with villi, but with numerous prominent, parallel, fleshy leaves placed close together, each of which runs in a straight line downward and forward from the coronary band to the lower border of the os pedis. Between the fleshy leaves are deep furrows in which, in a foot which has not been deprived of its horny capsule, lie the horny or insensitive leaves of the wall. The fleshy leaves (podophyllous laminæ) are related to one another somewhat as the leaves of a book; their posterior borders are attached to the body or basement membrane of the fleshy wall, while their anterior borders and sides are free. At their upper ends immediately below the coronary band the leaves are quite narrow, but they gradually increase in width down to the middle, and thereafter maintain that breadth to the lower border of the os pedis, where they terminate in free, fleshy villi, which differ in no respect from those of the fleshy sole. The number and length of the fleshy leaves vary; in a medium-sized foot there are about five hundred, while in a large foot there may be as many as six hundred. On the anterior surface of the os pedis the leaves are thickest and longest; on the sides and quarters they gradually decrease in length, while in the bar region they are the shortest and gradually disappear near the anterior ends of the bars. The width of the leaves decreases as they become shorter. Viewed with the naked eye the leaves appear flat and smooth, but under the microscope one can see on both sides of a fleshy leaf numerous small, fleshy leaflets parallel to one another and extending lengthwise with the larger leaf. The large ones are called principal leaves, and the small ones are known as collateral leaves, or simply as leaflets.
The fleshy leaves (podophyllous tissue) secrete the horny leaves (keraphyllous tissue) and serve to bind the horny wall to the pododerm. The strength of this union is due largely to the dovetailing of the horny leaves and their leaflets with the fleshy leaves and their leaflets.
4. The fleshy sole or velvety tissue of the sole ([Fig. 29, b]) is that part of the pododerm which covers all the under surface of the foot except the plantar cushion, the bar laminæ, and the bar portion of the coronary band. It is sometimes slate-colored or studded with black spots, but is usually dark-red. It is thickly set with villi, which are especially long and strong[1] near its periphery. The fleshy sole covers the solar plexus, or net-work of veins, and secretes the horny sole.
5. The velvety tissue of the frog ([Fig. 29, c]) covers the lower surface of the plantar cushion, and in the region of the bulbs (e) passes insensibly into the perioplic band. In comparison with the fleshy sole, it has much finer and shorter villi and contains fewer blood-vessels. It secretes the soft, horny frog.
Fig. 30.
Side view of hoof recently removed: a, the perioplic horn-band; it is swollen from prolonged maceration in water; the upper border shows adhering hairs; the inner surface (perioplic groove) presents many minute openings; a′, the perioplic horn-band broadens in passing over the bulb or glome of the heel, and is finally lost in the horny frog; a″, section of wall removed. That part of hoof on the right of b is called the toe; between b and c is the side wall or “mamma,” and between c and d the “quarter;” e, projecting horny frog; f, coronary groove with numerous minute openings; g, keraphyllous layer of the wall (horny leaves).
(c) The horn capsule or hoof ([Fig. 30]) is the entire mass made up of the horn-cells secreted from the whole surface of the pododerm, and next to the shoe is the organ with which the horseshoer has most to do. The horn capsule or hoof is nothing more than a very thick epidermis that protects the horse’s foot, just as a well-fitting shoe protects the human foot. The hoof of a sound foot is so firmly united with the underlying pododerm that only an extraordinary force can separate them. In its normal condition the hoof exactly fits the soft structures within it; hence it is evident that local or general contraction of the hoof must produce pressure on the blood-vessels and nerve-endings of the pododerm, disturb the circulation of the blood and the nutrition of the foot, and cause pain.
Fig. 31.
