Part 74

The pastern joint is made by the union of the two pastern bones.

The coffin joint is made by the union of the small pastern, coffin, and small sesamoid, or navicular bones, the latter being set behind and beneath the joint surface of the coffin bone in such way as to receive largely the weight of the small pastern.

Three tendons serve to move the bones of the foot one on another. Two of these flex, or bend, the joints, while the other extends, or straightens, the column of bones. (Plate XXXIII, fig. 5.)

The flexor pedis perforans, or deep flexor of the foot, passes down behind the cannon bone, lying against the suspensory ligament in front, crosses the fetlock joint in the groove made by the union of the two sesamoids, and is attached to the bottom on the coffin bone, after covering the navicular, by a wide expansion of its fibers. It is the function of this tendon to flex the coffin bone and, with it, the horny box.

The flexor pedis perforatus, or superficial flexor of the foot, follows the course of the preceding tendon and is attached to the middle of the ankle. The function of this tendon is to flex the foot at the fetlock.

The extensor pedis runs down in front of the leg, is attached on the most prominent point of the coffin bone, and has for function the straightening of the bones of the ankle and foot.

The bones, ligaments, and tendons are covered by a loose connective tissue, which gives a symmetry to the parts by filling up and rounding off, and all are protected by the skin and hoof.

The skin of the fetlock and ankle is generally characterized by its thickness and the length of its hairs, especially around the hind parts of the fetlock joint in certain breeds of horses. The most important part of this envelope is that known as the coronary band.

The coronary band is that portion of the skin which secretes the horn of which the wall of the hoof is made. This horn much resembles the nail which grows on the fingers and toes of man. It is composed of cylindrical tubes, which are held together by a tenacious, opaque matter. The horn extends from the coronary band to the lower border of the hoof. (Plate XXXII, fig. 1.)

The hoof is a box of horn, consisting of a wall, sole, and frog, and contains, besides the coffin, navicular, and part of the small pastern bones, the sensitive laminae, plantar cushion, and the lateral cartilages. (Plate XXXIII, fig. 4.)

The sole of the foot incloses the box on the ground surface, is shaped like the circumference of the foot, except that a V-shaped opening is left behind for the reception of the frog, and is concave on the lower surface. The sole is produced by the velvety tissue, a thin membrane covering the plantar cushion and other soft tissues beneath the coffin bone. The horn of the sole differs from the horn of the wall in that its tubes are not straight and from the fact that it scales off in pieces over the whole surface.

The frog is a triangular-shaped body, divided into two equal parts by a deep fissure, extending from its apex in front to the base. It fills the triangular space in the sole, to which it is intimately attached by its borders. The horn of the frog is produced in the same manner as the sole; but it differs from both the wall and sole in that the horn is soft, moist, and elastic to a remarkable degree. It is the function of the frog to destroy shock and to prevent slipping.

The sensitive laminae are thin plates of soft tissue covering the entire anterior surface of the coffin bone. They are present in great numbers, and by fitting into corresponding grooves on the inner surface of the horn of the wall the union of the soft and horny tissues is made complete. (Plate XXXII, fig. 1.)

The plantar cushion is a thick pad of fibrous tissue placed behind and under the navicular and coffin bones and resting on the sole and frog, for the purpose of receiving the downward pressure of the column of bones and to destroy shock. (Plate XXXII, fig. 4.)

The lateral cartilages are attached, one on each side, to the wings of the coffin bone by their inferior borders. They are thin plates of fibro-cartilage, and their function is to assist the frog and adjacent structures to regain their proper position after having been displaced by the weight of the body while the foot rested on the ground. (Plate XXXII, fig. 2.)

FAULTS OF CONFORMATION.

A large percentage of horses have feet which are not perfect in conformation, and as a consequence they are especially predisposed to certain injuries and diseases.

Flatfoot is that condition in which the sole has little or no convexity. It is a peculiarity common to some breeds, especially heavy, lymphatic animals raised on low, marshy soils. It is confined to the fore feet, which are generally broad, low-heeled, and with a wall less upright than is seen in the perfect foot.

In flatfoot there can be little or no elasticity in the sole, for the reason that it has no arch, and the weight of the animal is received on he entire plantar surface, as it rests upon the ground instead of on the wall. For these reasons such feet are particularly liable to bruises of the sole, corns, pumiced sole, and excessive suppuration when the process is once established. Horses with flatfoot should be shod with a shoe having a wide web, pressing on the wall only, while the heels and frog are never to be pared. Flatfoot generally has weak walls, and as a consequence the nails of the shoe are readily loosened and the shoe cast.

Clubfoot is a term applied to such feet as have the walls set nearly perpendicular. When this condition is present the heels are high, the fetlock joint is thrown forward, or knuckles, and the weight of the animal is received on the toes. Many mules are clubfooted, especially behind, where it seems to cause little or no inconvenience. Severe cases of clubfoot may be cured by cutting the tendons, but as a rule special shoeing is the only measure of relief that can be adopted. The toe should not be pared, but the heels are to be lowered as much as possible and a shoe put on with a long, projecting toe piece, slightly turned up, while the heels of the shoe are to be made thin.

Crookedfoot is that condition in which one side of the wall is higher than the other. If the inside wall is the higher, the ankle is thrown outward, so that the fetlock joints are abnormally wide apart and the toes close together. Animals with this deformity are "pigeon-toed," and are prone to interfere, the inside toe striking the opposite fetlock. If but one foot is affected, the liability to interfere is still greater, for the reason that the fetlock of the perfect leg is nearer the center plane.

When the outside heel is the higher the ankle is thrown in and the toe turns out. Horses with such feet interfere with the heel. If but one foot is so affected, the liability to interfere is less than when both feet are affected, for the reason that the ankle of the perfect leg is not so near to the center plane. Such animals are especially liable to stumbling and to lameness from injury to the ligaments of the fetlock joints. This deformity is to be overcome by such shoeing as will equalize the disparity in length of walls, and by proper boots to protect the fetlocks from interfering.

INTERFERING.

An animal is said to interfere when one foot strikes the opposite leg, as it passes by, during locomotion. The inner surface of the fetlock joint is the part most subject to this injury, although, under certain conditions, it may happen to any part of the ankle. It is seen more often in the hind than in the fore legs. Interfering causes a bruise of the skin and deeper tissues, generally accompanied with an abrasion of the surface. It may cause lameness, dangerous tripping, and thickening of the injured parts. (See also p. 387.)

Causes.--Faulty conformation is the most prolific cause of interfering. When the bones of the leg are so united that the toe of the foot turns in (pigeon-toed), or when the fetlock joints are close together and the toe turns out, when the leg is so deformed that the whole foot and ankle turn either in or out, interfering is almost sure to follow. It may happen, also, when the feet grow too long, from defective shoeing, rough or slippery roads, from the exhaustion of labor or sickness, swelling of the leg, high knee action, fast work, and because the chest or hips are too narrow.

Symptoms.--Generally, the evidences of interfering are easily detected, for the parts are tender, swollen, and the skin broken. But very often, especially in trotters, the flat surface of the hoof strikes the fetlock without evident injury, and attention is directed to these parts only by the occasional tripping and unsteady gait. In such cases proof of the cause may be had by walking and trotting the animal, after first painting the inside toe and quarter of the suspected foot with a thin coating of chalk, charcoal, mud, or paint.

Treatment.--When the trouble is due to deformity or faulty conformation it may not be possible to overcome the defect.

In such cases, and as well in those due to exhaustion or fatigue, the fetlock or ankle boot must be used. In many instances interfering may be prevented by proper shoeing. The outside heel and quarter of the foot on the injured leg should be lowered sufficiently to change the relative position of the fetlock joint by bringing it farther away from the center plane of the body, thereby permitting the other foot to pass by without striking.