Section lengthwise through middle of the plantar cushion: a, glome (bulb) of heels; b, apex or point of fleshy frog; c, fibro fatty tissue of plantar cushion; d, median cleft which receives the frog-stay of the horny frog.
The plantar cushion ([Figs. 21], [22], [23]) is composed almost entirely of yellow elastic and white fibrous tissues, with adipose (fat) cells distributed throughout their substance. It is similar in form to the horny frog, and lies between it and the perforans tendon ([Fig. 24, a]). The bulbs are formed by the posterior thicker portion which lies between the lateral cartilages and is divided into two parts by the cleft or median lacuna ([Figs. 21, a], and [23, d]). The summit is attached to the plantar face of the os pedis in front of the semilunar crest, and the bulbs are attached to the lateral cartilages. It is covered inferiorly by the velvety tissue of the frog (pododerm).
Fig. 24.
Right forefoot viewed from below, behind, and the external side. This figure shows clearly the position of the plantar cushion. The external lateral cartilage and the tissues covering the plantar cushion and under surface of the os pedis (velvety tissue of the sole and fleshy frog) have been removed: a, fleshy frog or plantar cushion; a′, bulbs of plantar cushion; the remaining visible parts belong to the so-called “fleshy frog;” a″, groove (median lacuna) in the lower surface of the fleshy frog, in which lies the frog-stay of the horny frog; b, suspensory ligament of the plantar cushion passing out of the bulbs; b′, small elastic cords passing to the lateral cartilage; c, elastic ligament coming from the lateral cartilage and uniting with the suspensory ligament of the plantar cushion; d, small tendinous cord beginning in the skin behind the fetlock-joint and ending on the os suffraginis in common with b and c; e, tendinous reinforcing sheath of the perforans tendon; f, reinforcing stay of the perforatus tendon; g, perforatus tendon; h, perforans tendon; i, suspensory ligament of the fetlock; k, plantar surface of the os pedis, to which the plantar cushion is joined by fibrous bands.
E. The Blood-Vessels and Nerves.
Vessels which carry blood from the heart to the tissues are called arteries, while those which return the blood to the heart from the tissues are called veins. Arteries and veins are connected by very small, thread-like vessels called capillaries, which originate in the smallest arteries and are so minute that they can not be seen without the aid of a microscope. The capillaries penetrate the soft tissues in every direction, and finally unite to form small veins. For our purpose we need consider only the arteries and veins.
The arteries carrying blood from the heart ramify and subdivide in all parts of the body, and thus reach the foot. They are thick-walled, very elastic tubes, without valves, and carry bright-red blood, which flows in spurts, as can be seen when an artery is cut. If a finger be pressed lightly over an artery lying near the surface, the blood-wave can be felt as a light stroke (pulse). The character of the pulse is important, because in inflammations of the pododerm or horn-producing membrane of the foot we can ascertain by feeling that the pulse is stronger than usual in the large arteries carrying blood to the inflamed foot.
On either side of the phalanges below the fetlock-joint there lies an artery called the digital artery ([Fig. 25, a]). The pulse can be felt in it as it passes over the fetlock at [A, Fig. 25]. It gives off the following collateral (side) branches: 1. The artery of the first phalanx (perpendicular artery), with anterior and posterior branches. 2. The artery of the plantar cushion, which supplies with blood the plantar cushion, the velvety tissue of the sole and frog, the bar portion of the coronary band, and the sensitive laminæ of the bars. 3. The coronary artery, which carries blood to the coronary band, os coronæ, ligaments of the coronary and pedal joints, flexor tendons, and skin.
The terminal branches of the digital arteries are the preplantar and plantar ungual arteries. The preplantar artery passes through the notch in the wing of the os pedis, then along the preplantar fissure, splitting up into many branches, which spread over and penetrate the porous surface of the os pedis. The plantar artery courses along the plantar fissure, enters the plantar foramen, and passes into the semilunar sinus of the os pedis, where it unites with the terminal branch of the opposite digital artery, forming the semilunar arch.