Fig. 28.
Foot from which the near half of the horny wall and a greater part of the so-called fleshy wall have been removed, in order to show the relation of the lateral cartilage to adjacent structures: a, vertical section of the skin prolonged downward through the pododerm (foot-skin) to show clearly that the latter is but a continuation of the former; a′, hairless place on the skin; b, perioplic band; b′, line indicating the upper border of the same; b″, surface of section of the periople, or perioplic horn-band; c, coronary cushion; c′, (left) line which marks the upper border of the coronary cushion; c″, section of wall at the toe; d, podophyllous tissue (sensitive laminæ); e, horny sole; f, white line; g, horny frog; h, fleshy frog; i, lateral cartilage.
In order to study the pododerm we should not wrench the hoof off with violence, but should allow the foot to partially decompose by leaving it for six to eight days at ordinary room temperature; it can then be removed without injuring the pododerm. After the hoof has been removed the entire pododerm presents a more or less dark-red color (flesh-color), which is due to the great number of blood-vessels that it contains. For this reason different parts of the pododerm have received the prefix “fleshy,” as for example, fleshy wall, fleshy sole, fleshy frog, etc. The pododerm is what the uninformed horseshoer calls the “quick.” I will here remark that the three layers of the external or hair-skin are represented in the foot; however, the epidermis is in an entirely different form,—namely, the horny box or hoof. The internal layer or subcutaneous tissue of the hair-skin is absent in those parts of the foot where the pododerm covers the os pedis. There remains, therefore, only the middle layer, derm, or pododerm, which secretes the hoof, and which is the prolongation and representative of the middle layer of the hair-skin. The pododerm is distinguished from the derm of the hair-skin chiefly by the absence of hairs, oil- and sweat-glands, and the presence on its outer surface of fleshy, sensitive laminæ and small thread-like projections called villi.
The pododerm consists of five different parts: the perioplic band, the coronary band, the sensitive laminæ (podophyllous tissue), the velvety tissue of the sole, and the velvety tissue of the fleshy frog.
1. The perioplic band ([Fig. 28, b]) is a narrow ridge, about one-fifth to one-fourth of an inch wide, lying between the hair-skin and the coronary band. Somewhat broader at the toe than on the sides, it broadens out near the bulbs of the heels, over which it passes to end in the velvety tissue of the fleshy frog. It is separated from the coronary band by a narrow depression called the coronary furrow (Moeller). The surface of the perioplic band glistens faintly, and is thickly studded with numerous thread-like projections called villi, which are from one-twenty-fourth to one-twelfth of an inch in length. The perioplic band secretes the soft horn of the perioplic ring and the perioplic or varnish-like outer layer of the wall.
2. The coronary band ([Fig. 27, c]) lies between the perioplic band and the sensitive laminæ or fleshy leaves. It presents a prominent convex band or cushion about three-fourths of an inch wide, which extends entirely around the foot from one bulb of the heel to the other. In front it directly covers the anterior extensor tendon of the toe, and at the sides the lateral surfaces of the os coronæ and the upper part of the lateral cartilages, while farther back towards the heels the lateral cartilages project considerably above both coronary and perioplic bands. The coronary band is more convex (rounded) in front than on the sides of the foot, and is flattened in the region of the bulbs of the heels. Its surface is thickly covered with villi, which are longer and stronger than those of the perioplic band. At the bulbs of the heels the coronary band turns forward and inward along the fleshy frog nearly to its summit. This portion of the coronary band is from one-third to one-half an inch wide, and is called the bar portion of the coronary band. It is also covered with villi, which are directly continuous with those of the fleshy frog. The coronary band secretes the principal part (middle layer) of the horny wall of the hoof, including the bar portion (bars) of the wall.
Fig. 29.
Plantar surface of a foot deprived of its horny capsule by prolonged maceration: a, laminæ of the bars; b, velvety tissue of the sole; c, velvety tissue of the frog; d, median cleft of the fleshy frog, into which the velvety tissue dips; e, bulbar portion of the perioplic band, which passes insensibly into the velvety tissue of the fleshy frog.