A Text-Book of Horseshoeing, for Horseshoers and Veterinarians

by A. Lungwitz

PART I.

CHAPTER I.
THE GROSS ANATOMY OF THE HORSE.

The supporting structure of the horse’s body is the bony framework or skeleton ([Fig. 1], page 18). We distinguish in the skeleton the bones of the head, trunk, and limbs.

The bones of the head are numerous and, excepting the lower jaw, are solidly united with one another. In general, we distinguish in the head only the upper and lower jaws (1 and 1′). Both form various cavities; for example, the cranial cavity, in which the brain lies, the orbital cavities (eye-sockets), the nasal passages, and the mouth. Besides, the teeth are set in the jaws.

The trunk comprises the bones of the spinal column, thorax, and pelvis.

The spinal or vertebral column (2 to 6), which bears the head at its anterior end, is the chief support, of the entire skeleton. It consists of from fifty-two to fifty-four single and irregular bones called vertebræ, placed in the upper part of the median vertical plane of the body. Each vertebra, with the exception of those of the tail (coccygeal or caudal vertebræ), is traversed by a large opening called the vertebral foramen. The vertebræ are placed end to end in a row, and through them runs a continuous large canal called the vertebral or spinal canal, in which lies the spinal cord. The horse has seven cervical, eighteen dorsal, six lumbar, five sacral, and sixteen to eighteen caudal vertebræ. The sacral vertebræ are grown together to form one piece called the sacrum.

Fig. 1.

Skeleton of the Horse.—1, bones of the head; 1′, lower jaw; 2, cervical vertebræ; 3, dorsal vertebræ; 4, lumbar vertebræ; 5, sacral vertebræ (sacrum); 6, coccygeal vertebræ; 7, ribs; 8, sternum (breast-bone); 9, pelvis; 9′, ilium; 9″, ischium; 10, scapula (shoulder-blade); 11, humerus; 12, radius; 13, ulna; 14, carpus (knee); 15, large metacarpal bone (cannon); 16, rudimentary metacarpal bones (splint-bones); 17, os suffraginis (long pastern); 18, os coronæ (short pastern); 19, os pedis (hoof-bone); 20, sesamoid bones; 21, femur; 22, patella (knee-pan, stifle); 23, tibia; 24, fibula; 25, tarsus, or hock; 26, large metatarsal bone (cannon); 27, rudimentary metatarsals (splint-bones); 28, os suffraginis (long pastern); 29, os coronæ (short pastern); 30, os pedis (hoof-bone, “coffin bone”); 31, sesamoid bones.

The thorax is formed by the ribs and the breast-bone or sternum. The horse has eighteen ribs on each side (7), and all articulate with the dorsal vertebræ. The first eight pairs unite by their lower ends directly to the sternum or breast-bone, and are therefore called true ribs, while the last ten pairs are only indirectly attached to the sternum, and are consequently called false ribs. The sternum (8) lies between the forelegs, and helps to form the floor of the chest cavity. The space enclosed by the bones of the thorax is called the thoracic, pulmonary, or chest cavity, and contains the heart and lungs. The bones of the pelvis form a complete circle or girdle. The upper part, called the ilium (9′), articulates on its inner side with the sacrum (5), while its outer side is prolonged to form a prominent angle, which is the support of the hip, and is called the “point of the hip.” The posterior part of the pelvis is called the ischium (9″), and that part lying between the ilium and the ischium and forming part of the floor of the pelvis is called the pubis.

The space between the thorax and the pelvis, bounded above by the lumbar vertebræ and shut in below and on the sides by the skin and muscular walls of the belly (abdomen), is called the abdominal cavity. This cavity opens directly into the pelvic cavity, and contains the stomach, intestines, liver, spleen, pancreas, kidneys, and a part of the generative organs. The thoracic and abdominal cavities are separated by a muscular partition, the diaphragm.

The bones of the limbs may be likened to columns, upon which the body rests; they articulate with one another at various angles, are tubular in structure, and strong.

The bones of the fore-limbs do not articulate directly with the bones of the trunk, but are attached to the body by means of the skin and muscles. From above to below we distinguish the following bones:

1. The scapula, or shoulder-blade (10), a flat, triangular bone, prolonged at its upper border by a flat, very elastic cartilage, called the scapular cartilage. At its lower end the scapula articulates with—

2. The upper end of the humerus (11), forming the shoulder-joint (scapulo-humeral articulation). The humerus articulates at its lower end with—

3. The radius (12) and the ulna (13), to form the elbow joint. These two bones are the basis of the forearm. The ulna, smaller and weaker than the radius, lies behind and projects above it to form the point of the elbow. The lower end of the radius articulates with—

4. The carpus, or knee (14), which comprises seven small, cubical bones disposed in two horizontal rows, one above the other. The upper row comprises four bones and the lower row three. The lower row rests upon—

5. The large metacarpal or cannon bone, and the two rudimentary metacarpal or splint-bones. The lower end of the radius, the upper ends of the metacarpal bones, and the small carpal bones together form the carpal or knee-joint (wrist of man). Of the metacarpals, the middle one is the largest, longest, strongest, and most important, and is called the large metacarpal, cannon, or shin-bone (15). It articulates at its lower end with the os suffraginis, or long pastern (17), and with the two small sesamoid bones (20). On each side of the upper part of its posterior surface lie the two long, slender splint-bones (16). The inner splint-bone is sometimes affected with bony thickenings (exostoses) called “splints.”

6. The bones of the phalanges (all bones below the cannon) will be fully described in another place.

The bones of the hind limbs articulate directly with the pelvis at the hip-joint. They are stronger than the bones of the anterior limbs. We distinguish the following bones in the hind legs:

1. The highest bone in the hind limb is the femur (21). It is the strongest bone in the entire body. It lies in an oblique direction downward and forward, and at its lower end articulates with—

2. The patella (22), the tibia (23), and the fibula (24), to form the stifle-joint (knee of man). The patella plays over the anterior surface of the lower end of the femur. The fibula is small, and lies against the upper and outer side of the tibia. The latter at its lower end articulates with—

3. The bones of the tarsus, or hock (25), which are six small, irregular bones disposed in three rows, one above another. The os calcis, or heel-bone, and the astragalus are in the uppermost row, and are the most important. The former projects above the true hock-joint from behind, to form a long lever, the upper end of which is called the “point of the hock,” and the latter articulates with the tibia. The tarsal (hock) bones articulate below with—

4. The metatarsal bones (26 and 27), which are longer, and the cannon narrower from side to side, than the corresponding metacarpal bones, but are otherwise similar.

5. The phalanges of a hind limb (28 to 31) are also narrower than those of a fore-limb, but are nearly alike in other respects.

All the horse’s bones present small, but more or less distinct openings (nutrient foramina) for the passage of blood-vessels and nerves. Many bones possess roughened elevations and depressions, to which ligaments, tendons, or muscles are attached. With the exception of the os pedis, all bones are enveloped in a sort of “bone-skin” called periosteum. The bones unite among themselves to form either movable or immovable unions. A movable union between two or more bones is termed a “joint,” or articulation. The articulating ends of the bones, presenting on one side a convex surface (head or condyle) and on the other a corresponding concave surface (glenoid or cotyloid cavity), are covered with elastic articular cartilage. The bones are bound together by means of ligaments, which are tough, fibrous, cord-like, or sheet-like structures. Ligaments are either (1) capsular or (2) funicular (cord-like). Every articulation in the limbs possesses a capsular ligament, and all, except the shoulder-joint, have several funicular (cord-like) ligaments. The capsular ligaments are lined upon their inner face with a delicate membrane (synovial membrane) which secretes the synovia, or “joint-water,” whose function is to lubricate the joint and prevent friction; they enclose the joint in a sort of air-tight cuff or sack. The funicular ligaments are very strong and often large, and are the chief means of union of the bones. The immovable articulations are termed sutures; they are found principally in the head. The mixed joints are found between the bodies of the vertebræ, each two of which are united by an elastic fibro-cartilage which, in the form of a pad, lies between them, and by its elasticity allows of very slight movement, though the spinal column as a whole can execute manifold and wide movements, as shown by the neck and tail.

Joints which permit motion in all directions are known as free joints; such are the shoulder- and hip-joints (ball-and-socket joints). Those which admit of motion in but two directions (flexion and extension), and often to a very limited extent, are called hinge-joints,—e.g., the elbow, hock, and fetlock. The joints between the long and short pasterns and between the latter and the pedal bone are imperfect hinge-joints, because they allow of some other movements besides flexion and extension. The articulation between the first and second cervical vertebræ (atlas and axis) is called a pivot-joint.

The skeleton represents a framework which closely approaches the external form of the body, and by reason of its hardness and stiffness furnishes a firm foundation for all other parts of the body. By reason of the great variety of position and direction of the bones, and of the fact that changes of position of each single part of this complicated system of levers may result in the greatest variety of bodily movements, we can readily understand how the horse is enabled to move from place to place. Of course, the bones have no power of themselves to move, but this power is possessed by other organs that are attached to the bones. These organs are the muscles, and, owing to their ability to contract and shorten themselves, and afterwards to relax and allow themselves to be stretched out, they furnish the motive power that is communicated to and moves the bones.

Fig. 2.

Outer Muscles of the Horse.—1, cervical trapezius; 2, dorsal trapezius; 3, mastoido-humeralis; 4, great dorsal muscle; 5, long abductor of the arm; 6, long extensor of the forearm; 7, large extensor of the forearm; 8, short extensor of the forearm; 9, sterno-trochinus (deep pectoral); 10, sterno-aponeuroticus; 11, great serratus; 12, common extensor of the metacarpus; 13, common extensor of the toe (anterior extensor); 14, common extensor of the long pastern (lateral extensor); 15, oblique extensor of the metacarpus; 16, external flexor of the metacarpus; 17, internal flexor of the metacarpus; 18, oblique flexor of the metacarpus; 19, fascia lata; 20, superficial gluteus (anterior portion); 21, middle gluteus; 22, superficial gluteus (posterior portion); 23 and 24, femoral biceps; 25, semitendinosus; 26, semimembranosus; 27, anterior extensor of the toe; 28, lateral extensor of the toe; 29, perforans muscle (deep flexor of toe); 30, oblique flexor of the phalanges; 31, perforatus tendon (superficial flexor of phalanges); 32, Achilles tendon (ham-string).

The muscles of the body massed together are the red flesh which we observe in every slaughtered animal. They are not, however, so shapeless as they appear while in this condition; on the contrary, they present well-arranged muscular layers of variable size, thickness, length, and position. ([See Fig. 2].) The muscles clothe the skeleton externally, give the body its peculiar form, and, by their special power of contraction, change the relative positions of the bones and thus make it possible for the animal to move. For this reason, the muscles are called the active, and the bones the passive, organs of motion. By carefully examining a muscle it will be found to consist of actual, minute, reddish, muscular fibres. As a rule, muscles terminate in more or less strong, glistening, fibrous cords called tendons, or fibrous sheets termed aponeuroses, by which they are attached to the bones. In the limbs are muscles terminating in very long tendons, which act as draw-lines upon the distant bones of the foot (long and short pasterns and pedal bone) and set them in motion. Such long tendons are enclosed in sheaths of thin, membranous tissue, known as tendon sheaths. The inner surface of such a sheath is in direct contact with the surface of the tendon, and secretes a thin slippery fluid (synovia) which lubricates the tendon and facilitates its gliding within the sheath.

As long as the bones, articulations, muscles, and tendons of the limbs remain healthy, just so long will the legs maintain their natural direction and position. Frequently, however, this normal condition of the limbs is gradually altered by disease of the bones, joints, and tendons, and defects in the form and action of the lower parts of the limbs arise that often require attention in shoeing.

THE FOOT.

A. The Bones of the Foot.

Since the horse is useful to man only by reason of his movements, his foot deserves the most careful attention. The horseshoer should be familiar with all its parts. [Fig. 3] shows the osseous framework of the foot, consisting of the lower end of the cannon bone (A), the long pastern (B), the two sesamoid bones (C), the short pastern (D), and the pedal bone (E). The lower end of the cannon, or large metacarpal bone (A) exhibits two convex articular surfaces (condyles) separated by a median ridge running from before to behind, and all covered by articular cartilage. On both the external and the internal aspects of the lower end of the cannon are small uneven depressions in which ligaments take their attachment.

Fig. 3.

The condyles of the cannon articulate with the os suffraginis (long pastern) and the two sesamoids ([Figs. 3, C], and [4, B]) in such a manner that in the forefeet the cannon makes an angle with the long pastern of from one hundred and thirty-five to one hundred and forty degrees, and in the hind feet of from one hundred and forty to one hundred and forty-five degrees.

The long pastern (first phalanx) ([Fig. 4, A]) is about one-third the length of the cannon; its upper and thicker end presents two condyloid cavities (a) (glenoid cavities), separated by a median groove, which exactly fit the condyles and ridge at the lower end of the cannon. The lower end of the long pastern is smaller than the upper, and is provided with two condyles, between which is a shallow groove (e). The anterior face of the bone is smooth, rounded from side to side, and blends into the lateral borders. The posterior face is flatter, and shows a clearly marked triangle to which ligaments attach.

The two sesamoid bones ([Fig. 4, B]) are small, and somewhat pyramidal in shape, and, lying against the posterior part of the condyles of the cannon bone, increase the articular surfaces at the upper end of the long pastern.

Fig. 4.

Os suffraginis with both sesamoid bones in position, as in [Fig. 3. A], os suffraginis; B, sesamoid bones; a, upper joint-surface of long pastern; b, joint-surface of sesamoid bones; c, roughened surface at upper end; d, roughened surface at lower end, both for attachment of ligaments; e, lower joint-surface.

The short pastern (second phalanx) ([Figs. 5] and [6]) lies under the first phalanx and above the os pedis; it is somewhat cubical in shape. Its upper articular surface ([Fig. 5, a]) presents two glenoid cavities to correspond with the condyles of the first phalanx. The lower articular surface ([Fig. 5, d]) resembles the lower end of the first phalanx. The upper posterior border of this bone is prominent and prolonged transversely ([Fig. 6, a]), to serve as a supporting ledge for the first phalanx, as a point of attachment for the perforatus tendon, and as a gliding surface for the perforans tendon.

Fig. 5.

Short pastern (os coronæ)
viewed in front and in profile:
a, upper joint-surface;
b, anterior surface;
c, lateral surface;
d, lower joint-surface.

Fig. 6.

Short pastern seen from behind:
a, smooth surface over
which the perforans tendon glides;
b, lower joint-surface.

The lowest bone of the limb is the third phalanx or os pedis ([Fig. 7]). In form it is similar to the hoof. The anterior or wall-surface (a) is rough, like pumice stone. Above and in front is the pyramidal eminence to which the tendon of the anterior extensor of the phalanges attaches. Behind, the bone extends backward to form the inner and outer branches (c, c) or wings of the os pedis. The upper, articular surface (b) slopes backward and downward. The lower, solar or plantar surface ([Fig. 8, a]) is slightly concave, and presents posteriorly a half-moon-shaped excavation, with a roughened border called the semilunar crest (c), to which the perforans tendon attaches; just above this crest are two small holes (e) known as the plantar foramina, through which the plantar arteries pass into the bone. The surfaces of wall and sole come together in a sharp edge, which is circular in its course. It is easy to tell whether a pedal bone is from a fore or a hind limb; the os pedis of a hind leg has a steeper and more pointed toe, and a more strongly concaved solar surface than the same bone of a fore-leg. Not only is the outline of the sharp inferior border of the os pedis of a front foot more rounded at the toe, but when placed on a flat surface the toe does not touch by reason of being turned slightly upward, much as a shoe designed to give a “rolling motion.” The os pedis of a hind foot is narrower from side to side (pointed), and does not turn up at the toe.

Fig. 7.

Os pedis seen in profile and in front; a, anterior face with pyramidal eminence above; b, joint-surface; c, wings or branches of hoof-bone; d, notch which, by the attachment of the lateral cartilage, is converted into a foramen and leads to e, the preplantar fissure.

Fig. 8.

Lower surface of hoof-bone; a, anterior portion covered by the velvety tissue of the sole; b, wing of the os pedis; c, semilunar crest, to which the perforans tendon attaches; d, plantar fissure leading to e, plantar foramen.

The right and left hoof-bones are also, as a rule, easily distinguished by variations in the surfaces of wall and sole. The shape of the os pedis corresponds to the form of the horny box or hoof, and therefore a knowledge of this bone is absolutely necessary.

The navicular bone (os naviculare, nut-bone, [Figs. 9] and [10]) is an accessory or sesamoid bone to the os pedis. It is a small bone, transversely elongated and situated behind and below the os pedis and between the wings of the latter. It adds to the articular surface of the pedal joint. Its under surface is smooth, and acts as a gliding surface for the perforans tendon, which is quite wide at this point.

Fig. 9.

Fig. 10.

Fig. 9 represents the upper surface of the navicular bone; Fig. 10 the lower surface of the same: a, anterior border; b, slight elevation in middle of under surface.

The long axes of the three phalanges (os suffraginis, os coronæ, and os pedis) should unite to form a straight line, when viewed either from in front or from one side; that is, the direction of each of these three bones should be the same as the common direction of the three considered as a whole.

In young colts both the long and short pasterns are in three parts and the pedal bone in two parts, all of which unite later in life to form their respective single bones.

In mules and asses the os pedis is comparatively small and narrow. In cattle all three phalanges are double, and split hoofs cover the divided os pedis.

B. The Articulations of the Foot.

There are three articulations in the foot—namely, the fetlock, coronary, and pedal joints. All are hinge-joints, the fetlock being a perfect hinge-joint, and the other two imperfect hinge-joints. Each has a capsular ligament, and also several funicular or cord-like ligaments which are placed at the sides of (lateral ligaments), or behind (on the side of flexion) the joints.

I. The fetlock or metacarpo-phalangeal articulation is formed by the condyles at the lower end of the cannon bone and the glenoid cavities formed by the union of the articular surfaces of the sesamoids and the upper end of the first phalanx. The following ligaments are about this joint:

Fig. 11.

1. Two lateral ligaments, an external and an internal ([Fig. 11, a]).

2. Two lateral sesamoid ligaments (f).

3. An intersesamoid ligament ([Fig. 12, b]), a thick, fibrous mass, binding the sesamoid bones almost immovably together, extending above them and presenting on its posterior face a smooth groove, in which glide the flexor tendons of the phalanges (perforans and perforatus).

4. The suspensory ligament of the fetlock ([Figs. 11, c], [12, c], and [13, c], pages 29 and 30). This may also be called the superior sesamoid ligament. It is a long and very powerful brace, originating on the lower row of carpal bones (bones of the hock in the hind leg) and on the upper end of the cannon between the heads of the two splint-bones, and dividing at the lower third of the cannon into two branches (c), which are attached one to each sesamoid bone. Below these bones these two branches are prolonged obliquely downward and forward on opposite sides of the long pastern to pass into the borders of the anterior extensor tendon of the toe at about the middle of the long pastern ([Fig. 14, b′], page 32).

Fig. 12.

Fig. 13.

[Fig. 11] shows a side view, and Figs. 12 and 13 a posterior view of the phalangeal bones, with their articular ligaments. The lettering is the same in all three figures: a, lateral ligament of fetlock-joint; b, intersesamoid ligament; c, suspensory ligament of the fetlock; d, median branch of inferior sesamoid ligament; d′, lateral branches of inferior sesamoid ligament; e, deep inferior sesamoid ligament; f, lateral sesamoid ligaments; g, inferior coronary ligaments; h, superior coronary ligaments; h′, median coronary ligaments; i, lateral pedal ligament; k, lateral coronary ligament and suspensory ligament of the navicular bone; l, interosseous ligament.

5. The inferior sesamoid ligament ([Figs. 11, d′], [12, d, d′], and [13, d′, E]). This originates at the lowest part of the sesamoid bones and intersesamoid ligament, and consists of three parts or branches. The median branch (d) is the longest and strongest, and takes its lower attachment in the middle of the fibro-cartilaginous lip found on the upper border of the posterior face of the second phalanx. The two lateral branches (d′) approach each other as they descend, and terminate on the sides of the roughened triangle on the posterior face of the first phalanx.

6. The deep inferior sesamoid ligament ([Fig. 13, e]) is quite short, and consists of a number of distinct, thin fibrous bands lying directly against the bone and entirely covered by the median and lateral inferior sesamoid ligaments. These fibrous bands cross one another in passing from the sesamoids to the first phalanx.

II. The coronary joint is the simplest of the three articulations of the foot. The long pastern furnishes two condyles and the short pastern two glenoid cavities. Besides a capsular ligament there are—

1. Two lateral coronary ligaments (k) and,

2. Six posterior coronary ligaments,—namely, two superior coronary ligaments (h), two median coronary ligaments (h′), and two inferior coronary ligaments (g).

III. The pedal articulation (“coffin” joint) is an imperfect hinge-joint, and is formed by the condyles at the lower end of the short pastern and the two glenoid cavities in the united upper surfaces of the pedal and navicular bones. Besides the capsular ligament ([Figs. 12] and [13, l]), which binds all three bones together, there are the following accessory ligaments:

1. Two strong lateral ligaments, an external and an internal ([Fig. 11, i]), whose posterior borders are lost in the lateral cartilages which cover them.

2. Two lateral suspensory ligaments of the navicular bone (k). They begin on the posterior border and ends of the navicular bone, and terminate on the lower part of the anterior surface of the os suffraginis, where they are lost in the lateral ligaments of the coronary articulation.

3. The lateral ligaments of the lateral cartilages, navicular bone, and os pedis. They are short, and unite the navicular bone with the os pedis and lateral cartilages.

Of the three phalangeal articulations, the pedal is the only one that permits of any lateral movement; hence it is an imperfect hinge-joint.

C. The Locomotory Organs of the Foot.

Though the muscles are the organs which produce motion, the horseshoer need concern himself only with the tendons of those muscles which extend and flex the phalanges. These tendons are either extensors or flexors. The extensors lie on the anterior face and the flexors on the posterior face of the phalanges.

Fig. 14.

Right forefoot viewed from in front and from the external side: a, anterior extensor tendon of the toe; b, suspensory ligament of the fetlock; b′, branch of the same passing forward and uniting with the extensor tendon of the toe; c, extensor tendon of the os suffraginis (absent in the hind leg), called the lateral extensor.

The anterior extensor of the phalanges ([Fig. 14, a]) extends the long and short pasterns and the hoof-bone; it is broad, and made somewhat broader by receiving the branches of the suspensory ligament (b′) that come from the sesamoid bones. It takes a firm attachment on the pyramidal eminence of the os pedis. In the forefeet the long pastern has a special extensor tendon (c), which is known as the lateral extensor. When the muscles to which these tendons are attached act,—that is, when they draw themselves together, or contract, as we term this action,—the foot is carried forward (extended).

There are two flexor tendons of the phalanges,—namely, the superficial (perforatus tendon) and the deep (perforans tendon).

Fig. 15.

Right forefoot seen from behind: a, lower end of the perforans tendon, cut through and hanging down, so that its anterior surface is visible; a′, lower expanded end (plantar aponeurosis) of this tendon, which attaches itself to the semilunar crest of the os pedis; a″, shallow groove which receives the slight elevation on the under surface of the navicular bone; a‴, piece of the perforans tendon enclosed by the ring formed by the perforatus tendon; b, perforatus tendon bent over backward so that its anterior surface is visible; b′, ring of the perforatus tendon; b″, terminal branches of the same; the perforans tendon passes through the space between these two branches; c, navicular bone; d, suspensory ligament of the same; e, smooth surface on the os coronæ over which the perforans tendon glides; f, the smooth groove (sesamoid groove) on the posterior surface of the intersesamoid ligament for the gliding of the perforans tendon; g, body of the suspensory ligament of the fetlock; g′, terminal branches of the same, attaching to the sesamoid bones.

1. The superficial flexor or perforatus tendon ([Figs. 15, b], and [16, a, b]) lies behind, immediately under the skin, and covers the deep flexor or perforans tendon. At the gliding surface between the sesamoid bones ([Fig. 15, f]) it broadens, and forms a ring or tube ([Fig. 15, b′]) through which the perforans tendon (a‴) passes, while a short distance farther down it bifurcates, or divides into two branches ([Figs. 15, b″], and [16, b]), which terminate, one on either side, partly on the inferior lateral borders of the first phalanx and partly on the fibro-cartilage of the second phalanx. It acts simultaneously on the long and short pasterns.

2. The deep flexor or perforans tendon ([Figs. 15, a], and [16, c]) is cylindrical and stronger than the perforatus tendon; above the fetlock-joint it lies between the perforatus and the suspensory ligament of the fetlock. At the sesamoid bones it passes through the ring formed by the perforatus tendon ([Fig. 15, b′]), then becomes broad and double-edged, passes between the two terminal branches of the perforatus, glides over the fibro-cartilage of the second phalanx and over the inferior surface of the navicular bone, and finally ends on the semilunar crest of the third phalanx. In common with the perforatus tendon it flexes the foot.

Fig. 16.

Right forefoot seen from behind and a little from the external side: a, perforatus tendon; b, terminal branches of the same; c, perforans tendon; d, annular ligament which attaches to the sesamoid bones: d′, the “x” ligament, which attaches by four branches to the os suffraginis; d″, an upper branch of the same (the lower branches are not shown in the figure); e, reinforcing sheath of the perforans tendon, covering the under surface of the latter and attached by its branches at e′ to the lower end of the os suffraginis; f, suspensory ligament of the fetlock.

If at a point a few inches above the fetlock a limb be cut through from behind, the knife will pass successively through the following structures: skin, perforatus tendon, perforans tendon, suspensory ligament, cannon bone, lateral extensor tendon, anterior extensor tendon, and, lastly, the skin on the anterior surface of the limb. The flexor tendons are frequently thickened and shortened by inflammation due to injury, and as a result the foot is pulled backward and the hoof gradually becomes more nearly upright,—i.e., stubby, steep-toed. A knowledge of the normal condition of the tendons is, therefore, absolutely necessary to the horseshoer. Both flexor tendons are embraced and held in place by ligaments and fascia passing out from the phalanges ([Figs. 16, d′], and [24, e, f]). The extensor and flexor tendons essentially contribute to the strong union of the phalangeal bones, and especially to the support and stability of the fetlock-joint. The gliding of the tendons is made easy by the secretion of a lubricating fluid, called synovia, from the inner surface of the sheaths which surround them. In thin-skinned well-bred horses with sound limbs one can not only distinctly feel the tendons through the skin, but can see their outline. When the tendons and bones are free from all inflammatory thickenings, and the tendon sheaths are not visibly distended, we say that the leg is “clean.”

Mucous Bursæ and Tendon Sheaths.

Accessory to the tendons, there are in the foot roundish, membranous sacs (mucous bursæ) and membranous tubes (tendon sheaths). Both contain a liquid resembling synovia (“joint-water”), which facilitates the gliding of the tendons. These bursæ and sheaths are often distended to form soft tumors, known as hygromata (“wind-puffs,” “wind-galls”).

(a) Mucous Bursæ.—They lie beneath tendons at those places where the tendons pass over bony prominences.

1. The mucous bursa of the anterior extensor tendon of the toe is about the size of a walnut, and lies between the tendon and the capsular ligament of the fetlock-joint ([Figs. 17, g], and [18, e]).

2. The mucous bursa of the extensor tendon of the long pastern (lateral extensor) is somewhat smaller, and lies, likewise, beneath the tendon, between it and the capsular ligament of the fetlock-joint ([Fig. 17, h]).

3. The mucous bursa of the navicular region lies between the under surface (gliding surface) of the navicular bone and the flexor pedis perforans tendon (deep flexor). Its width equals the length of the navicular bone, and it extends upward and downward beyond the bone. Above, it is separated from the sheath of the perforans tendon (“great sesamoid sheath”) by a membranous partition; below, it passes to the attachment of the perforans tendon to the semilunar crest of the os pedis.

(b) There is but one tendon sheath in the foot—the sheath common to the two flexor tendons (great sesamoid sheath). It encloses the flexor tendons from the middle third of the cannon down to the middle of the short pastern, and is intimately united with the flexor pedis perforans tendon ([Fig. 17, f, f′, f″, f‴]. [Fig. 18, d, d′, d″, d‴]).

Fig. 17.

Right forefoot seen from the external side; f, f′, f″, f‴, great sesamoid sheath (tendon sheath); g, mucous bursa beneath anterior extensor tendon of the toe; h, mucous bursa beneath extensor tendon of long pastern; i, synovial distension of the fetlock-joint; 7, suspensory ligament; 9, cannon bone; 10, outer sesamoid bone; 12, fetlock-joint; 13, lateral cartilage; 14, suspensory ligament of the lateral cartilage. (Ellenberger in Leisering’s Atlas and Veterinary Anatomy, Sisson, Saunders.)

Fig. 18.

Right forefoot seen from the inner side; d, d′, d″, d‴, great sesamoid sheath; e, mucous bursa beneath anterior extensor tendon of the toe; f, synovial distension of fetlock-joint; 10, inner sesamoid bone; 11, “x” ligament; 14, fetlock-joint; 15, lateral cartilage; 16, suspensory ligament of lateral cartilage (Ellenberger in Leisering’s Atlas and Veterinary Anatomy, Sisson, Saunders.)

Altering the Relative Tension of the Flexor Tendons
and Suspensory Ligament of the Fetlock-Joint.

The body-weight imposed at the fetlock-joint is supported, in large part, by the suspensory ligament; somewhat less weight is borne by the perforans tendon, and a still smaller amount by the perforatus. The coronary joint is supported chiefly by the perforatus, assisted by the perforans. The pedal joint is pressed forward and upward by the perforans tendon passing in a curve beneath the navicular bone. Each of these three structures bears its normal proportion of the body-weight when the three phalanges, as viewed from the side, form a continuous straight line from the fetlock-joint to the ground. In such a case the obliquity of the long pastern will be the same as that of the toe ([see Foot-Axis, p. 70]).

Fig. 19.

Right forefoot viewed from the external side: A, os coronæ; B, os pedis; C, external lateral cartilage; a, lateral pedal ligament; b, ligament uniting the lateral cartilage with the os coronæ; c, aponeurosis joining lateral cartilage and os pedis.

Raising the toe by means of a tip, a full shoe with thinned branches or a toe-calk, or paring away the quarters will tilt the os pedis backward, break the foot-axis backward in the pedal joint and to a less extent in the coronary joint, and increase the tension of the perforans tendon considerably and of the perforatus slightly. These tendons tightening behind the fetlock-joint force it forward, causing the long pastern to stand steeper, and taking some strain from the suspensory ligament. Hence, the perforans tendon is under greatest tension, and the suspensory ligament under least tension, when the foot-axis is broken strongly backward.

Shortening the toe, or raising the quarters by heel-calks or thickened branches, will tilt the os pedis forward, break the foot-axis forward in the pedal joint, and will greatly lessen the tension of the perforans tendon. The aggregate tension of perforans and perforatus tendons being diminished, the fetlock sinks downward and backward, the long pastern assumes a more nearly horizontal direction, and the tension of the suspensory ligament is increased. Thus, the perforans tendon is under least tension, and the suspensory ligament under greatest strain, when the foot-axis is broken strongly forward.

D. The Elastic Parts of the Foot.

Fig. 20.

Os pedis and inner face of one lateral cartilage; a, toe of os pedis; a′, pyramidal eminence to which the extensor tendon attaches; a″, wing of pedal bone; b, lateral cartilage; C, points of attachment of suspensory ligament of lateral cartilage; d, point of insertion of ligament to the short pastern; e, point of insertion of ligaments from navicular bone.

All bodies which under pressure or traction change their form, but return again to their original shape as soon as the pressure or traction ceases, are called elastic or springy. Nearly all parts of the horse’s foot, except the bones, possess more or less elasticity. The lateral cartilages and the plantar cushion are elastic to a high degree, but the coronary band, the laminæ, the articular cartilage, and the horny box or hoof are less elastic. This property or characteristic is possessed by the respective parts of the foot in accordance with their function, location, and structure.

Fig. 21.

Plantar cushion seen from below: a, base or bulb of the plantar cushion; b, summit; c, median lacuna or cleft in which lies the “frog-stay” of the horny frog.

Fig. 22.

Plantar cushion seen from above: a, base (bulbs) of same; b, summit; c, suspensory ligament of plantar cushion; d, place at which the elastic ligament connecting the os suffraginis and the lateral cartilage unites with the plantar cushion.

The two lateral cartilages ([Figs. 19, C] and [20, b]) are irregular, quadrangular plates, attached to the wings of the os pedis, and extending so far upward and backward that one can feel them yield to pressure on the skin above the coronet, and can thus test their elasticity. The perforans tendon and the plantar cushion lie between the lateral cartilages, and on the sides and behind are partially enclosed by them. The internal concave surface of the lateral cartilage ([Fig. 20]) is attached to the plantar cushion, the os pedis, and the navicular bone, and, like the external, slightly convex surface, is covered with many blood-vessels (veins) [Fig. 25, B].

Fig. 23.

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.

Fig. 25.

Side view of forefoot, showing blood-vessels and nerves: a, digital artery; b, anterior artery of the os suffraginis; d, anterior coronary artery, or circumflex artery of the coronet; e′, preplantar ungual artery; f′, inferior communicating arteries passing out from the semilunar artery of the os pedis, through minute holes just above the lower border of the bone; they unite to form (f″) the circumflex artery of the toe; A, digital vein; B, superficial venous plexus of coronary band and lateral cartilage; C, podophyllous venous plexus; G, circumflex vein of the toe; 1, plantar nerve; 2, anterior digital branch of same; 3, posterior digital branch of same; 4, small cutaneous branches of same.

After the arterial or pure blood passes through the capillaries it is collected by the veins, to be returned to the heart; then it is driven to the lungs for purification, and is again returned to the heart, from whence it is pumped through the arteries to all parts of the body.

Fig. 26.

Foot viewed from below and behind: a, digital arteries; c, arteries of the plantar cushion; f‴, small branches of the semilunar artery of the os pedis, which ramify in the velvety tissue of the sole; A, digital vein; B, venous plexus of the heels or bulbs; D, solar venous plexus; G, circumflex vein of the toe; 3, posterior digital branch of the plantar nerve; 4, cutaneous branches of the same.

The veins are more numerous than the arteries; they have thinner walls, and the larger ones are provided with valves that prevent the impure blood from flowing backward. The veins carry impure or dark-red blood towards the heart, and if one is opened the dark blood flows in a steady stream; it does not spurt. The great number of veinlets in the lower parts of the foot form a complex net-work (plexus) of vessels which are in such manifold and close union with one another that checking the flow of blood in one part does not seriously interfere with the flowing of the blood towards the larger veins. The following are the most important of these net-works of veins or venous plexuses: (1) the solar venous plexus ([Fig. 26, D]); (2) the podophyllous venous plexus ([Fig. 25, C]); (3) superficial coronary venous plexus ([Fig. 25, B]); (4) bulbar venous plexus ([Fig. 26, B]). All these plexuses of small veins contribute to form the digital veins ([Figs. 25] and [26, A]).

Nerves are roundish white cords which come from the brain and spinal cord; they generally accompany arteries. They divide and subdivide into smaller and smaller branches till they become invisible to the naked eye and are lost in the tissues. The nerves that are found in the foot come from the spinal cord, and because the largest nerves of the foot accompany the digital arteries they are called digital nerves ([Fig. 25, 1]). The branches ramify throughout all parts of the foot except the horny box and the hair. Nerves, according to their use or function, are classed as motor and sensory. The motor nerves end in muscles which they stimulate to action and control. The sensory nerves terminate in the skin and in the soft tissues just under the horny box or hoof (pododerm), and render these parts sensitive; that is, they convey certain feelings, as, for example, the pain caused by bruising, pricking, or close-nailing, to the brain and consciousness.

F. The Protective Organs of the Foot.

The protective organs are the skin and the horny box or hoof.

The external skin, or hide, covers the entire body; in the feet it covers the bones, tendons, and ligaments, even passing in under the hoof and directly covering the os pedis. This portion of the skin, enclosed by the hoof and therefore invisible, is called the pododerm or foot-skin. In Germany it is called the hoof-skin (huflederhaut), because it is a continuation of the outer visible skin, and because it secretes the hoof,—that is, the hoof is produced by it. That part of the skin which is covered with hair is known as the external or hair-skin.

(a) The hair-skin ([Fig. 27, a]) consists of three superposed layers,—(1) the external superficial layer, or epidermis; (2) the middle layer, derm or leather-skin (so-called because leather is made from it); (3) the internal layer, or subcutaneous connective tissue.

1. The external layer, or epidermis, is composed merely of single flattened, horn-like cells (scales) lying side by side and over one another, and uniting to form one entire structure,—a thin, horn-like layer, without blood-vessels or nerves. It extends over the entire surface of the body, and protects the underlying, very sensitive middle layer from external influences. The oldest cell-layers lie on the outer surface, and are being continuously brushed off in patches or scales, while new ones are constantly being formed on the outer surface of the middle layer.

2. The middle layer, leather-skin or dermis, is composed of solid, fibrous, and elastic tissues, and contains many blood-vessels, small nerves, sweat-and oil-glands, and hair follicles from which the hair grows. The hair upon the posterior surface of the fetlock-joint is usually long and coarse, forming a tuft known as the “foot-lock,” which encloses a horny spur, called the ergot. Common-bred horses have, as a rule, larger and coarser footlocks than thoroughbreds. The derm or leather-skin, which produces the hair and epiderm, is the thickest and most important layer of the skin.

3. The inner layer, or subcutaneous tissue, unites the middle layer with the muscles, tendons, ligaments, bones, or other structures. It is that loose fibrous mesh or net-work through which the butcher cuts in removing the hide from the carcass.

Fig. 27.

Foot from which the horny capsule or hoof, has been removed by prolonged soaking: a, skin; on the left the hair has been rubbed away; b, perioplic band; c, coronary cushion; d, podophyllous tissue (fleshy leaves); at the lower border of the figure can be seen the minute thread-like processes or villi which grow down from the lower end of each fleshy leaf.

(b) The hoof-skin ([Figs. 27] and [28, b, c, d]), or pododerm, is completely enclosed by the hoof. Although it is only an extension of the derm or middle layer of the hair-skin, it differs from the latter in structure and relations.

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.

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.

Plantar surface of right fore-hoof: a, a, bearing-surface of the toe; a, b, bearing-surface of the side walls or mammæ; b, c, bearing-surface of the quarters; d, buttress, or angle formed by wall and bar; e, bar; f, sole; f′, branches of the sole; g, white line; it passes between the sole and bars and ends at g′; h, horny frog; i, branches of the frog; k, heels, bulbs, or glomes of the hoof; l, median lacuna of horny frog. Between the bars and the horny frog lie the lateral lacunæ of the frog.

The hoof is divided into three principal parts, which are solidly united in the healthy foot,—namely, the wall, the sole, and the frog. That part of the hoof which is almost wholly visible when the foot is on the ground ([Fig. 30, b, c]), and which protects the foot in front and upon the sides, is known as the wall. In position, course, direction, and arrangement of its parts it simulates the different parts of the pododerm from which it is developed. It extends from the edge of the hair just above the coronary band to the ground; backward it gradually decreases in height (length), passes around the bulbs of the heels, and turns forward and inward ([Fig. 32, d, e], and [34, a, b]) to form the bars, which are finally lost in the edge of the sole near the summit of the frog. It thus forms at each heel an angle ([Fig. 31, d], and [32, d]) known as a buttress, which encloses a branch of the horny sole. Externally the wall is smooth, covered with the varnish-like periople, and presents indistinct ring-like markings ([Fig. 30]). Its inner surface, on the contrary, presents a great number of horn-leaves which are spoken of collectively as the keraphyllous tissue ([Figs. 32, g], and [35, f]). The upper or coronary border of the wall is thin and flexible, and on its inner aspect is the coronary groove, into which fits the coronary band ([Fig. 30, f]). The lower border of the wall, called the “bearing-edge” or plantar border ([Fig. 31, a]), is the one to which the horseshoe is fastened. By dividing a hoof from before to behind along its median line, outer and inner halves or walls are produced, and by dividing the entire lower circumference of the wall into five equal parts or sections, a toe, two side walls or mammæ, and two quarters will be exhibited ([Figs. 32] and [33]). In order to designate these regions of the hoof still more accurately, they are spoken of as outer and inner toes, quarters, and heels.

Fig. 32.

Wall and bars seen from below: a, toe; b, side wall, or mamma; c, quarter; d, buttress; e, bar; g, horn-leaves; h, space occupied by the frog.

The direction (slant) and length of the wall vary in one and the same hoof, as well as between fore and hind hoofs. The portion of the wall of fore-hoofs is the most slanting,—that is, forms the most acute angle with the surface of the ground,—and is also the longest. Towards the quarters the wall gradually becomes very nearly vertical; in almost all hoofs the posterior part of the quarters slants downward and inward towards the median vertical antero-posterior plane of the foot. At the same time the wall, in passing back from the toe to the heel, becomes gradually shorter in such a manner that the heights of the toe, side walls, and quarters are related to one another about as 3: 2: 1 in front hoofs and as 4: 3: 2 in hind hoofs. The outer wall is, as a rule, somewhat more slanting than the inner. Viewing a foot in profile, the toe and heel should be parallel; that is, the line from the hair to the ground at the toe should be parallel to the line from the hair to the ground at the buttress. All deviations of the wall from a straight line (outward or inward bendings) are to be regarded as faults or defects.

Fig. 33.

A hoof in profile; a, toe (one half); b, side wall; c, quarter.

Fig. 34.

Vertical section through the middle of a hoof, with horny frog removed, to show the position of the bar: a, b, marks the line at which the wall bends forward and inward towards the median line of the foot to become the bar. Bar runs forward and passes imperceptibly into the sole c; a, a′, the light shading shows the part of the bar that was in contact with the horny frog.

The thickness of the wall is also variable. In front hoofs the wall is thickest at the toe, and becomes gradually thinner towards the quarters, while in hind hoofs, there is very little difference in the thickness of the wall of the toe, sides, and quarters. The more slanting half of the hoof is always the thicker; thus, for example, the outer wall of a base-wide foot is always longer and more oblique than the inner wall, and is also thicker. According to Mayer, the thickness of the wall at the toe varies from three- to five-eighths of an inch, and at the quarters from two to three-eighths of an inch. These measurements are dependent upon the size and breeding of the horse.

Fig. 35.

The outer wall of the hoof has been removed by cutting vertically through the middle of the toe, down to the upper surface of the sole, then horizontally backward into the quarter, and, finally, upward through the quarter: a, perioplic horn-band; b, coronary groove; it turns inward and forward at c to form the upper border of the bar; d, surface of section of the wall at the toe; d′, at the quarter; e, surface of horizontal section of the wall near its lower border; f, keraphyllous layer of the wall; at f′ it turns forward and inward to cover the bar; f″, horny leaves standing free and passing insensibly into the white horn of the middle layer or true wall; g, horny sole; h, white line; i, small horn-spur in middle of toe; k, part of horny frog which is in intimate union with the upper edge of the bar; l, frog-stay of horny frog; it divides the trough-like depression of the upper surface of the frog into m, the two upper channels of the frog.

The horn wall is composed of three superposed layers. These from without to within are: (1) the periople, secreted by the perioplic band. It is very thin, glistening, and varnish-like in appearance, and covers the entire outer surface of the wall, except where it has been removed by the rasp, and prevents rapid evaporation of moisture from the horn. (2) The middle or protective layer ([Fig. 35, d]) is the thickest, strongest, and most important of the three layers; it forms the principal mass of the wall, and is developed or secreted by the coronary band, which fits into the coronary groove. There are in the coronary groove a great number of small, funnel-shaped openings into which project the horn-producing villi or papillæ of the coronary band. (3) The inner layer or keraphyllous layer ([Fig. 35, f]) consists of prominent, parallel horn-leaves lying side by side over the entire inner surface of the middle layer of the wall, and continuing beyond the buttresses to the ends of the bars ([Fig. 35, f′]). This layer of horn-leaves (keraphyllous layer) has in a general way about the same shape and arrangement as the layer of fleshy leaves (podophyllous layer) which secretes it; for the horn-leaves fit in with the fleshy leaves in such a way that every fleshy leaf is embraced by two horn-leaves, and every horn-leaf by two fleshy leaves ([Fig. 36]). The keraphyllous layer and the horn of the inmost part of the middle or protective layer are always white, even in pigmented (colored) hoofs.

Fig. 36.

Cross-section of keraphyllous and podophyllous laminæ (horny and fleshy leaves): a, inmost part of the solid wall; the horn-tubes approach very close to the horny leaves; b, body of the podophyllous membrane; c, horny portion of a horn-leaf directly continuous with the middle or principal layer of the wall; c′, a rudimentary horn-leaf that does not reach the body of the podophyllous membrane; c″, cross-section of horny leaves from the sides of which branch many secondary leaves (leaflets) composed of soft (young) horn-cells. These soft cellular horn-leaflets dovetail with the podophyllous or fleshy leaflets; d, podophyllous laminæ extending from the body of the podophyllous membrane; d′, podophyllous laminæ which have branched in their course to the wall, and thus given rise to c′, rudimentary horn-leaves; d″, cross-section of podophyllous leaflets extending from the sides of the podophyllous leaves; each two such leaflets secrete a keraphyllous leaflet between them; e, injected arterial vessels.

Fig. 37.

Vertical section of the horny sole magnified: a, funnel-shaped openings which contain the horn-producing villi of the fleshy sole; they are of various sizes; b, horn-tubes; c, intertubular horn.

Fig. 38.

Horny frog, with the posterior portion of the perioplic horn-band and the periople which covers the quarters removed from the hoof as one piece by maceration: a, trough-shaped depression of upper surface, which is divided posteriorly into the two upper channels of the frog by b, the frog-stay; c, part of the frog that is joined to the bar and forms the lateral wall of the depression (channels) on upper surface of frog; d, lateral surface of horny frog which, in its upper part, adheres to the bar, but below, at d′, lies free; e, point or summit of the frog; f, perioplic horn-band; f′, periople of the quarters.

The horn sole ([Fig. 31, f], and [Fig. 35, g]) is secreted by the velvety tissue of the sole. A sole from which the loose flakes of old horn have been removed is about as thick as the wall. It covers the under surface of the foot, and presents upon its upper surface a convexity which exactly fits into the concavity on the under surface of the os pedis. This upper surface is thickly covered by a multitude of minute funnel-shaped openings for the reception of the villi of the velvety tissue of the sole ([Fig. 37]). The lower surface of the sole is more or less concave, rough, uneven, and often covered by loose scales of dead horn. Behind, the sole presents a triangular opening whose borders lie partly in contact with the horny frog and partly with the bars. This opening or re-entering angle divides the sole into a body ([Fig. 31, f]) and two wings or branches ([Fig. 31, f′]). The outer border of the sole unites through the medium of the white line with the lower part of the inner surface of the wall,—that is, with the keraphyllous layer of the wall. This white line ([Figs. 31, g], and [35, h]), of so much importance to the horseshoer, is formed by the horn-leaves, and by those short plugs of tubular horn which are secreted by the villi that are always found at the lower ends of the fleshy leaves. The white line may be said to exist wherever the horn-leaves can be discerned upon the plantar surface of the hoof. It not only passes around the circumference of the sole from heel to heel, but may be followed forward from the buttresses along the bars almost to the summit of the frog. The horn of the white line is soft, unpigmented (white), and possesses so very little resistance (strength) that it is often found crumbling or even absent in places. The visible part of the white line is usually of a grayish-black color, owing to the working in from below of dirt and liquid manure, and to staining by rust from the nails. The white line is very important, since it serves as the point from which we judge of the thickness of the wall, and because the horseshoe nail should penetrate it.

Fig. 39.

A horny frog cut vertically and lengthwise through its middle: a, upper surface; b, frog-stay; c, median lacuna of frog, which at c′, is overlaid with superposed layers of horn.

Fig. 40.

Longitudinal section of the wall magnified. The dark stripes parallel and close together are horn-tubes; the lighter surface between the tubes represents the intertubular horn. Notice that the horn-tubes are of various diameters. The space between a and b represents the small tubes of the outer, darker horn of the principal (middle) layer of the wall; the space between b and c the lighter, inner horn of the wall; c, d, the horn separating the wall proper from the horny leaves; d, e, the horny leaves (keraphyllous tissue), on which can be seen fine, parallel, vertical stripes; in the horn-leaf at f, f′, are seen fissures passing obliquely upward and outward towards the wall.

The Frog ([Figs. 31, h], [35, k, l], [38] and [39]), secreted by the velvety tissue covering the plantar cushion and presenting almost the same form as the latter, lies as a wedge between the bars and between the edges of the sole just in front of the bars, with both of which structures it is intimately united. Its horn is quite soft and very elastic. The median lacuna or cleft of the frog ([Fig. 31, l]) divides it into two branches ([Fig. 31, i]), which pass backward and outward into the horny bulbs ([Fig. 31, k]). In front of the median lacuna the two branches unite to form the body of the frog ([Fig. 31, h]), which ends in a point, designated the point, apex, or summit of the frog. On the upper surface of the frog, directly over the median cleft of the lower surface, there is a small projection called the frog-stay ([Figs. 35, l], [38] and [39, b]), which fits into the median cleft of the plantar cushion. Besides, the upper surface of the frog shows many minute openings, similar to but smaller than those of the sole and coronary groove, for the reception of villi. In unshod hoofs the frog, sole, bars, and bearing-edge of the wall are on a level; that is, the plantar surface of such hoofs is perfectly flat.

Fig. 41.

Cross-section of the wall, magnified:
a, horn-tubes;
b, intertubular horn.

The minute structure of the horn can scarcely be considered in detail in an elementary treatise such as this is. However, a few of the most important facts are as follows:

If we carefully examine a transverse section of the horn of the wall ([Fig. 41]), sole, or frog, we will see with the naked eye, though much better with a magnifying glass, many minute points quite close to one another, and greatly resembling the small openings which we have seen in the coronary groove of the wall and on the upper surface of the horny sole and frog. If, now, we examine a longitudinal section of the wall ([Fig. 40]) or sole, we will see a number of fine, dark stripes which are straight, parallel, quite close to one another, of different widths, and which are separated by bands of lighter horn also of different widths. A thin section or slice of the wall taken at right angles to the direction of these dark lines ([Fig. 41]) shows us that the minute points that are visible to the naked eye, when held up to the light or moderately magnified, prove to be small openings ([Fig. 41, a]). Since these openings, shown in [Fig. 41], represent the dark lines shown in [Fig. 40], because an opening is found wherever there is a dark line, we must regard all dark lines seen in longitudinal sections of wall, sole, and frog as hollow cylinders or tubes, though they are not always hollow, but are often filled with loosely adjusted, crumbling, broken down horn-cells. The dark edges of the openings (a) consist of thick layers of horn-cells (tube-walls). The entire structure is called a horn-tube, and the lighter-colored masses of horn ([Fig. 41, b]) between the tubes are known as intertubular horn.

With the exception of the horny leaves of the wall and bars, all the horn of the hoof is composed of horn-tubes and intertubular horn.

The horn-tubes of the wall, sole, and frog always run downward and forward parallel to the direction of the wall at the toe,—that is, in a direction parallel with the inclination of the hoof as a whole. Although the wall, sole, and frog differ from one another considerably with respect to the size and number of the horn-tubes, the quality of the intertubular horn, and the thickness and strength of the horn-cells, these differences are only of subordinate interest or importance to the horseshoer; but he who desires to learn more of this matter is referred to the work of Leisering & Hartmann, “Der Fuss des Pferdes in Rücksicht auf Bau, Verrichtungen und Hufbeschlag,” eighth edition, Dresden, 1893. This book also treats of the variations in the quality of hoofs, which is very important for the practical horseshoer to know. It, furthermore, considers the solidity and strength of the horn of the different parts of the hoof.

With respect to solidity, two kinds of horn are distinguished,—namely, hard and soft horn. The periople, the white line, and the frog are soft horn structures; the middle layer of the wall and the sole are hard or solid horn. The wall, however, is somewhat harder and more tenacious than the sole, for the latter passes off in more or less large flakes (exfoliates) or crumbles away on its lower surface, at least in shod feet, while no such spontaneous shortening occurs in the wall.

Fig. 42.

Vertical section through middle of a forefoot, the skin and pododerm being in red. (In the figure the direction of both long and short pasterns, B and D, is too nearly vertical—too steep). A, metacarpal bone (cannon); B, os suffraginis (long pastern); C, inner sesamoid bone (to render it visible a portion of the intersesamoid ligament was removed); D, os coronæ (short pastern); E, os pedis (foot-bone); F, navicular bone; a, extensor tendon; b, suspensory ligament of the fetlock; b′, superficial inferior sesamoid ligament; c, perforatus tendon or flexor of the os coronæ; c′, ring passing forward from this tendon and encircling the perforans tendon; d, perforans tendon; e, capsular ligament of fetlock-joint; f, capsular ligament of coronary joint; g, g′, capsular ligament of pedal joint; h, synovial sheath of the perforans tendon; i, plantar cushion and fleshy frog; i′, bulbs or glomes of plantar cushion; i″, indicates the lowest point reached by the plantar cushion, which in the figure is hidden below by the frog-stay of the horny frog; k, coronary band (red); l, podophyllous tissue (red); m, velvety tissue of the sole (red); n, velvety tissue of fleshy frog (red); o, wall; p, sole; q, frog; q′, the inner half of the frog-stay which reposes in the median lacuna of the fleshy frog; s, hair-skin (red).

Soft horn differs from hard horn in that its horn-cells never become hard and horn-like. It is very elastic, absorbs water quickly, and as readily dries out and becomes very hard and brittle and easily fissured and chapped. With respect to quality, we distinguish good and bad horn; the former is fine and tenacious (tough), the latter coarse and either soft and crumbling or hard and brittle. If not dried out, all horn is elastic, though soft horn is more elastic than hard. All horn is a poor conductor of heat.

The relative positions of the various parts of the foot are shown in [Fig. 42].

[Fig. 43] represents the exterior of a well-formed foot.

Fig. 43.

Right forefoot viewed from the side: A, lower end of the cannon; B, fetlock-joint; C, long pastern; D, coronet; E, hoof; F, heel; F′ inner heel; G, foot-lock covering the ergot.

CHAPTER II.
THE FOOT IN ITS RELATION
TO THE ENTIRE LIMB.

Fig. 44.

Normal (regular) position
of fore-limbs.

As there are well-formed and badly formed bodies, so there are well-formed and badly formed limbs and hoofs. The form of the hoof depends upon the position of the limb. A straight limb of normal direction possesses, as a rule, a regular hoof, while an oblique or crooked limb is accompanied by an irregular or oblique hoof. Hence, it is necessary, before discussing the various forms of the hoof, to consider briefly the various positions that may be assumed by the limbs. In this discussion we shall deal with the living horse.

A. Standing Positions of the Limbs.

The position of a limb depends upon the varying lengths of its component bones and the angles at which they meet one another. To judge the standing position of a fore-limb one must stand in front of the horse; to judge a hind limb, stand behind the horse; the backward or forward deviations of both front and hind limbs are judged by standing at the side. But a horse does not always move as his standing position would lead one to suspect; standing and moving are different. Therefore, in order to arrive at a proper judgment, one must observe the limbs both at rest and in motion.

(a) The position of a limb viewed from in front is normal or straight ([Fig. 44]) when it stands vertical or perpendicular. A plumb-line dropped from the point of the shoulder (middle of the scapulo-humeral articulation) should pass down the middle line of the limb, dividing it into inner and outer halves of equal width, and meeting the ground at the middle of the toe.

Fig. 45.

Base-wide

Fig. 46.

Toe-wide

Fig. 47.

Toe-narrow
(“pigeon-toed”)

In the base-wide standing position ([Fig. 45]) the plumb-line falls to the inner side of the limb; the limb extends obliquely downward and outward. To this class belong also the knee-narrow (knock-kneed) position, in which the knees are too close together, while the feet stand wide apart, and the toe-wide position (splay-footed, [Fig. 46]) in which the toes point obliquely forward and outward. In base-wide positions either the entire limb extends downward and outward or the foot alone is turned outward.

The base-narrow position is frequently observed in horses with very wide breasts. The limbs run downward and inward, a plumb-line dropped from the point of the shoulder falling to the outer side of the leg and foot. A special form of the base-narrow position is the toe-narrow or pigeon-toed position ([Fig. 47]). In some instances the legs are straight and perpendicular down to the fetlock, while from there to the ground the phalanges incline obliquely inward. Another form is the knee-wide or bandy-legged position, in which the knees are placed too far apart, while the cannons and phalanges incline downward and inward.

Fig. 48.

Normal (regular) fore-limb
in profile.

Fig. 49.

Normal (regular) hind-limb
in profile.

Fig. 50.

Camped in front.

Fig. 51.

Calf-kneed.

Fig. 52.

Acute-angled foot
(low-jointed).

The position of a fore-limb viewed in profile is regular or normal ([Fig. 48]) when a perpendicular line dropped from the tuberosity of the acromian spine (point of union of the upper and middle thirds of the scapula or shoulder-blade) divides the leg from the elbow to the fetlock into anterior and posterior halves of equal width, and touches the ground immediately back of the bulbs of the heel. A perpendicular line dropped from the point of union of the middle and lower thirds of the scapula (shoulder-blade) will cut the humerus into halves, and meet the ground between the toe and the heel.[2] The foot-axis (line of direction of the three phalanges) and the wall at the toe form an angle of from forty-five to fifty degrees with the horizontal ground-surface.

From this normal or regular standing position, there are deviations forward as well as backward.

Forward Deviations.—“Standing in front” or “camped in front” ([Fig. 50]) is that position in which the entire leg from the body to the ground is placed too far forward. Sheep-kneed ([Fig. 51]) is that position in which the forward deviation is from the knee downward, the knee being placed too far under the body. “Weak-jointed,” “low-jointed,” or “acute-angled” ([Fig. 52]) is that position in which the limbs are perpendicular and straight down as far as the fetlock-joint, but the feet are placed too far in front.

Fig. 53.

Standing under.

Fig. 54.

Knee-sprung.

Backward Deviations.—Standing under in front ([Fig. 53]) is that deviation in which the entire leg from the elbow down is placed back of the perpendicular line and, therefore, too far under the body. When this deviation affects only the cannon bone, the horse stands bent forward at the knees,—a condition known as “goat-kneed,” “buck-kneed,” “over in the knees,” or, more commonly, “knee-sprung” ([Fig. 54]). When the backward deviation is only from the fetlock down, the animal is said to stand upright or “straight in the fetlock.”

Fig. 55.

Normal (regular) position
viewed from behind.

Fig. 56.

Base-wide
(cow-hocked).

Fig. 57.

Base-narrow.

Fig. 58.

Base-narrow position
of hind limbs
(bandy-legged).

(b) A hind leg viewed from behind is said to be regular or straight ([Fig. 55]) when a perpendicular line dropped from the tuberosity of the ischium ([see Fig. 1, 9″]) divides the entire limb into inner and outer halves of equal width and touches the ground opposite the median lacuna of the frog. Seen from the side, this line just touches the point of the hock and, passing down at some distance from the flexor tendons, meets the ground considerably back of the heels. A perpendicular line dropped from the hip-joint should pass through the foot, meeting the ground half-way between the point of the toe and the heel ([Fig. 49]). There are base-wide, base-narrow, toe-wide, and toe-narrow deviations in the hind limbs as in the fore-limbs.

The hind limbs are base-wide when they, either as a whole or in part, deviate outward from the normal. The “cow-hocked“ position ([Fig. 56]) is an example of the base-wide; in this case the points of the hocks are too close and turn towards each other, while the feet are widely separated and the toes turned outward. Base-narrow is that position of the hind legs in which either the entire leg deviates to the inner side of the perpendicular ([Fig. 57]), or the leg is about perpendicular down as far as the hock, but below this joint runs downward and inward ([Fig. 58]). In this latter case the hocks may be too far apart, the leg is bent outward at the hock and the animal is termed “bandy-legged,” “bow-legged.”

Viewing a hind limb from the side, it may be observed to deviate either forward or backward from the normal. Among forward deviations is the so-called “sabre-leg“ or “sickle-hock“ ([Fig. 59]), in which the hock-joint is too much flexed, the foot placed too far forward under the body, and the fetlock too slanting. In the position known as “camped behind” ([Fig. 60]) the leg is behind the body and the pastern is too upright, too nearly vertical.

It is possible for each limb of the same horse to assume a different direction. It more often happens that if the fore-limbs are base-wide the hind limbs are base-narrow, or vice versa. While there are some other deviations that differ somewhat from those already described, they are of less importance to the horseshoer.

Fig. 59.

Sabre-legged or sickle-hocked.

Fig. 60.

Camped behind.

B. Forms of Feet. Viewed from in Front,
from Behind, and in Profile.

In all the various positions of the limbs we find the feet in one of the following three forms, or very closely approaching one of them. By means of a proper knowledge of these three forms, the judging of the form, flight of the foot in travelling, and preparation of the hoof for the shoe, as well as the choice of the length of the shoe, are regulated, facilitated, and simplified.

Whether a horse’s feet be observed from in front or from behind, their form corresponds to, or at least resembles, either that of the regular position ([Figs. 61] and [62]), the base-wide or toe-wide position ([Figs. 63] and [64]), or the base-narrow or toe-narrow position ([Figs. 65] and [66]).

By the direction of the foot-axis—that is, an imaginary line passing through the long axis of the three phalangeal bones ([Figs. 61], [65], [67], [68] and [69])—we determine whether or not the hoof and pastern stand in proper mutual relation.

Fig. 61.

Fig. 62.

A pair of front feet of regular position viewed from in front and from behind.

In the regular standing position ([Figs. 61] and [62]) the foot-axis runs straight downward and forward, in the base-wide position ([Figs. 63] and [64]) it runs obliquely downward and outward, and in the base-narrow position ([Figs. 65] and [66]) it runs obliquely downward and inward.

Fig. 63.

Fig. 64.

A pair of feet of the base-wide (toe-wide) position seen from in front and from behind.

Viewing the foot from the side, we distinguish the regular (normal) position ([Fig. 68]), and designate all forward deviations as acute-angled (long toe and low heel, [Fig. 67]), and all deviations backward from the regular position as upright (short toe and high heel, [Fig. 69]), steep-toed, or stumpy.

Fig. 65.

Fig. 66.

A pair of feet of the base-narrow (toe-narrow) position seen from in front and from behind.

When the body-weight is uniformly distributed over all four limbs, the foot-axis should be straight ([Figs. 67] and [69]), not “broken” (bent); the long pastern, wall at the toe, and foot-axis should have the same slant.

Fig. 67.

An acute-angled hoof.

Fig. 68.

A normal-angled hoof.

Fig. 69.

An upright
(“stumpy”) hoof.

Fig. 70.

The “bear-foot.”

A peculiar form of foot is the so-called bear-foot ([Fig. 70]), in which the foot-axis, viewed from the side, is broken strongly forward at the coronet. The wall at the toe stands much steeper than the long pastern and is more or less convex; in other words, a low-jointed, sloping pastern is attached to an upright hoof. Such a foot is sometimes improperly called a “club-foot.”

C. Lines of Flight of Hoofs in Motion.

If we observe horses moving unrestrained over level ground, we will notice differences in the carriage of the feet. Viewed from in front, or from behind, in the regular standing position of the limbs the hoofs are carried forward in a straight direction, that is, in a line parallel with the median line of the body ([Fig. 71]). The toes likewise point straight forward; the hoofs alight properly (flat) on the ground. If the horse stands base-wide, the hoof is carried in a circle; from its position, which is behind and well out from the median line, the hoof passes first forward and inward until it is close to the supporting leg, and then outward to the ground ([Fig. 72]), where the shock is received principally upon the outer toe. The toes point either directly forward, as in the regular standing position ([Fig. 72]), or forward and outward as in the toe-wide position ([Fig. 73]). In the toe-wide position the hoof in its flight may cross the median line.

Exactly the reverse is true of the horse that stands base-narrow; in this case the hoof is moved in a circle whose convexity is outward,—that is, the hoof from its position behind, and close to the median line, is carried forward and outward and then inward to the ground ([Figs. 74] and [75]).

Viewed from the side, the line of flight of a hoof is determined largely by the obliquity (slant) of the foot-axis.

Fig. 71.

Fig. 72.

Fig. 73.

Fig. 74.

Fig. 75.

1. With a straight foot-axis of normal slant (45°-50°, [Fig. 76, A]), the hoof follows the arc of a circle and reaches its highest point when directly above the supporting hoof, i.e., when half-way in the stride.

2. With a straight, but acute-angled foot-axis (less than 45°, [Fig. 76, B]), the hoof rises rapidly, reaches its highest point before it has completed the first half of the stride, i.e., before it has passed the supporting hoof, and descending gradually in a long curve alights easily on the ground.

3. With a straight, but upright foot-axis (55° or more, [Fig. 76, C]), the hoof rises slowly, reaches its highest point in front of the supporting hoof, from which point it descends rapidly. The gait is “choppy,” and in the saddle-horse unpleasant for the rider. The length and the height of the stride are greatest in acute-angled feet; least in upright feet. Furthermore, length and height of stride are in a measure dependent on breeding, training, condition of the legs (whether stiffened by use or disease), length of the hoof and the weight of the shoe.

Fig. 76.

Flight of the hoof as seen from the side: A, flight of a regular hoof; B, flight of an acute-angled hoof; C, flight of an upright hoof.

Many deviations in the line of flight of hoofs and in the manner in which they are set to the ground occur; for example, horses heavily burdened or pulling heavy loads, and, therefore, not having free use of their limbs, project their limbs irregularly and meet the ground first with the toe; however, careful observation will detect the presence of one or the other of these lines of flight of the foot. Irregular carriage of the feet renders a horse unsuitable for general purposes only when it is very pronounced, in which case certain troublesome conditions, such as interfering and disease of joints, are of frequent occurrence.

D. The Influence of Weight in the Shoe
or Otherwise Attached to the Hoof,
in Altering the Flight of the Hoof.

There is nothing mysterious in the effect of weight upon the flight of the feet. On the contrary, the lines of flight are determined (as shown in pages 72-74, [Figs. 71-76]), first, by the relation of the transverse axes of the hinge-joints of the leg and foot to the line of progression (median line); second, by the length and obliquity of the hoof and pastern; third, by the height and length of stride which is natural to each individual.

Weight induces higher action and a longer stride. Inertia increases with the weight. A heavy shoe cannot be snatched from the ground as quickly as a light one, but when moving forward at a given velocity its greater momentum (momentum = mass (wt) × velocity: m = wt × v) carries the foot farther forward then does the lighter shoe. Thus, the heavier shoe, or weight attached to the hoof, lengthens the stride at both ends. The farther from the centre of rotation of the scapula the weight is placed, i.e., the nearer to the toe it is placed, the greater the muscular effort required to start it and to stop it.

Height of action, though largely the result of breeding, temperament, and the exhilaration that accompanies perfect health and entire absence of muscular fatigue, is to a certain extent influenced by the inclination of the pastern and toe to the cannon. The acute-angled foot, in the folding of the leg during the first half of the stride, moves through a longer arc of a circle whose centre is the fetlock joint than does the normal or the upright foot; rises more rapidly and to a higher point. ([See Fig. 76, B].) When the momentum of a foot moving rapidly and abruptly upward is increased by weight the result is extreme and even exaggerated flexion of all joints of the leg, and by allowing the hoof to grow long the flexion is still further increased. In the show ring, harness horses with fair natural action may be made to “climb” by shoes weighing from thirty to sixty ounces upon hoofs an inch or more longer than normal. The leverage of a heavy shoe on a long hoof is excessive, fatiguing and most injurious to ligament, tendon and muscle. The action, while high, is labored, pounding and altogether inelegant.

Fig. 77.

A 40 oz. right front shoe
(hoof-surface)
to increase knee-action in
a high acting harness horse.
For show-purposes only.

Fig. 78.

The same seen from the
ground-surface in profile:
a, bevel from inner border
of the web to outer border;
b, ends of the branches of full
thickness from outer to inner border.

In the training of trotters weight is often used to increase the length of the stride, or to cause a higher folding of a front foot, in order to prevent “scalping“ or “speedy-cut.“ As soon as the new gait becomes a fixed habit the weight should be gradually lessened. Weight is carried with less fatigue at a trot then at a pace, or at a gallop. It therefore steadies a trotter that is inclined to pace, or “break” into a run. The increased momentum of the weighted hoof makes for rhythm of movement, and increases the difficulty of skipping, dwelling, or mixing gaits.

In the base-wide (toe-wide) and base-narrow (toe-narrow) standing positions, the flight of the hoofs, as seen from in front or behind, is not straight forward, i.e., parallel to the line of progression of the body, but in arcs of circles. ([See Figs. 72-75], p. 73.) In these cases, increasing the weight of the hoofs, by increasing the momentum, must of necessity increase the tendency of the hoofs to move off at a tangent to the curves which they describe. In other words, weight increases the centrifugal force of a body moving in a curve. The outward swing of the hoofs of a base-narrow horse (paddling), and the inward swing of a base-wide horse (interfering), are made more pronounced by adding weight to any part of the hoof. The centrifugal force is greatest in base-wide feet when the weight is on the medial, or inner side of the hoof; in base-narrow feet when it is on the lateral or outer side.

A side weight, or side weight shoe is often of service in a cross firing pacer. This animal usually stands base-narrow (toe-narrow) behind, and in motion his hind hoofs describe a curve at first forward and outward and then inward till contact is made with the diagonal hoof or leg. The added weight (placed on the outer side) by increasing the centrifugal force carries the hoof just enough farther from the centre around which the hoof swings to prevent contact. ([See cross-firing, p. 138].)

Finally, it must not be forgotten that weight is always weight; that it cuts speed and devours endurance.

E. Forms of Hoofs.

A front hoof of the regular standing position ([Fig. 79]). The inner and outer walls differ but little in direction and thickness. The outer wall is a little thicker and somewhat more slanting than the inner ([see Figs. 61] and [62]), and its outer circumference describes a larger arc of a circle,—that is, is more curved, as can be seen both at its plantar border and at the coronet. The length of the quarter in relation to the length or height of the side wall and toe is about as 1: 2: 3. The toe forms an angle with the ground of forty-five to fifty degrees ([see Fig. 68]). The direction of the wall at the toe, viewed from the side, should be parallel with the direction of the long pastern.

Fig. 79.

Right fore-hoof of the regular position: a, side wall; b, quarter; c, beginning of the bar; d, buttress; e, middle of the bar; f, body of the sole; f′, branches of sole; g, white line; g′, apparent end of the bar; h, body of the frog; i, branch of the frog; k, bulbs (glomes) of the heel; l, middle cleft of frog; m, lateral cleft of frog.

A hoof of the base-wide position ([Fig. 80]) is always awry, because the outer wall is naturally somewhat longer and decidedly more slanting then the inner ([see Figs. 63] and [64]). The plantar border of the outer wall describes a large arc, whose sharpest curvature is where the side wall passes into the quarter. The plantar border of the inner wall is straighter (less curved); the outer half of the ground-surface (sole) of the hoof is, therefore, wider than the inner. So long as the hoof is healthy, both branches of the frog are equally developed. The wryness of the hoof depends upon the direction of the limb; therefore, a base-wide hoof should be regarded as a normally wry hoof, to distinguish it from hoofs which are wry from disease.

A hoof of the toe-wide position ([Fig. 81]) is distinguished from the preceding by the bending or curvature of the plantar border of the outer toe and inner quarter being often decidedly less pronounced than on the inner toe and outer quarter; therefore, two short curves and two long curves lie opposite each other; in other words, the inner toe and outer quarter, lying opposite each other, are sharply curved, while the outer toe and inner quarter, lying opposite each other, are much less sharply bent or curved. The toes are turned out. The feet are not set down flat upon the ground, but meet it with the outer toe.

Fig. 80.

Right fore-hoof of the
base-wide position.

Fig. 81.

Right fore-hoof of the
toe-wide position.

A hoof of the base-narrow position is normally wry, but never so pronounced as a hoof of the base-wide position. The inner wall is but little, more oblique than the outer, the difference being most noticeable at the quarters ([Figs. 65] and [66]). The curve of the plantar border of the wall is similar to that of a regular hoof, except that the inner side wall and quarter are a little more sharply curved in a base-narrow hoof. Occasionally the outer quarter is somewhat drawn in under the foot.

This form of hoof is most distinctly marked in animals that stand toe-narrow or are bandy-legged.

As to the forms of the hind hoofs, what has been said concerning the influence of position of the limbs upon the shape of the front feet will apply equally well to them. The hind hoof ([Fig. 82]) is not round at the toe, but somewhat pointed or oval. It greatest width is between the middle and posterior thirds of the sole. It usually has a strongly concave sole and a somewhat steeper toe than the fore-hoof; viewed from the side, the angle of the toe with the ground in the regular standing position is from fifty to fifty-five degrees.

Fig. 82.

Right hind hoof of the regular position: a, side wall; b, beginning of the quarter; c, beginning of the bar; d, buttress; e, middle of bar; f, body of the sole; f′, branch of sole; g, white line of the toe; g′, white line of the bar; h, body of the frog; i, branch of the frog; k, bulbs of heel; l, middle cleft of frog; m, lateral cleft of frog.

Finally, we also distinguish wide and narrow hoofs; they are not dependent upon the position of the limbs, but upon the race and breeding of the animal.

The wide hoof ([Fig. 83]) is almost round upon its plantar surface. Its wall runs quite oblique to the ground. The sole is but moderately concave, and the frog is strong and well developed. The narrow hoof ([Fig. 84]) is rather elliptical, with steep side walls, strongly concaved sole, and small, undeveloped frog. The horn of the narrow hoof is fine and tough; of the wide hoof, usually coarse. The wide hoof may readily become flat. Narrow hoofs are either the result of breeding or premature shoeing.

In enumerating the preceding forms of the hoof we have by no means referred to all the forms in which the hoof may be found; on the contrary, hoofs vary in shape and quality to such an extent that among a hundred horses no two hoofs can be found which are exactly alike. In fact, the same variety exists as in the faces of people, and we know that we can recall in succession even many more faces without finding two that are exactly alike. This explains the manifold differences in horse’s shoes with respect to size, form and other qualities.

Fig. 83.

Wide fore-hoof.

Fig. 84.

Narrow fore-hoof.

Suppose now a hoof is before us; it is first necessary to know whether or not it is healthy. Unfortunately, a perfectly healthy hoof is not so easy to find as one may think. We recognize a sound hoof by the following marks: Seen from in front or from the side, the course of the wall from the coronet to the ground, in the direction of the horn-tubes, is straight,—that is, bent neither in nor out. A straight edge, placed upon the wall in the direction of the horn-tubes, touches at every point. The wall must show neither longitudinal nor transverse cracks or fissures. If there be rings, their position and course are important. Rings which pass around the entire circumference of the wall parallel to the coronet indicate nothing more than disturbances of nutrition of the hoof; but the hoof cannot pass for sound when the rings have any other position and direction than the one mentioned, or if the rings upon any part of the wall are more marked than elsewhere, even though they may be parallel to the coronary band. Marked ring-building upon the hoofs of horses which have regular feeding, grooming, and work indicates a weak hoof. Viewed from the ground-surface and from behind, the bulbs of the heels should be well rounded, strongly developed, and not displaced. The concave sole should show no separation along the white line. The frog should be strong, well developed, and have symmetrical branches and a broad, shallow, dry median lacuna. The lateral lacunæ of the frog should be clean and not too narrow. The bars should pass in a straight direction forward and inward towards the point of the frog. Any bending outward of the bars towards the branches of the sole indicates the beginning of a narrowing of the space occupied by the frog,—that is, contraction of the heels. The horn of the branches of the sole in the buttresses and in their proximity should show no red staining. The lateral cartilages should be elastic. No part of the foot should be weakened at the cost of other parts. By firm union of all strong parts the strength and vigor of the hoof is in no sense disturbed. If one desires to ascertain the exact form and state of health of the hoof, it must never be inspected and judged alone, but in connection with the entire limb.

F. Growth of the Hoof and Wear
of the Hoof and Shoe.

All parts of the horn of the hoof grow downward and forward, the material for this growth being furnished by the remarkably large quantity of blood which flows to the pododerm. The growth of the hoof is regulated by the nerves.

As a rule, the hoof grows uniformly,—that is, one section of the wall grows just as rapidly as another. A visible indication of growth is the increase in height and width of the hoof from colthood to maturity.

The rapidity of growth of the wall varies, amounting in a month to from one-sixth to one-half of an inch. The average monthly growth in both shod and unshod horses of both sexes is, according to my own experiments, one-third of an inch. Hind hoofs grow faster than front hoofs, and unshod faster than shod. The hoofs of stallions grow more slowly than those of mares and geldings.

Abundant exercise, proper grooming (flexibility and moistness of the horn), regular dressing of the wall, and running barefoot from time to time favor growth; while little or no exercise, dryness, and excessive length of the hoof hinder growth.

The time required for the horn to grow from the coronet to the ground is, therefore, equally variable, and is, moreover, dependent upon the height (length of toe) of the hoof. At the toe the horn grows down in from eleven to thirteen months, at the mammæ or sides in from six to eight months, and at the quarters in from three to five months. The time required for the renewal of the entire hoof we term the period of hoof renewal. If, for example, we know exactly the rapidity of horn growth in a given case, we can estimate without difficulty the length of the “period of hoof renewal,” as well for the entire hoof as for each individual section of the wall. The duration of many diseases of the hoof (cracks, clefts, partial bendings of the wall, contractions, etc.) can be foretold with relative certainty only by knowing the period of hoof renewal.

Irregular growth sometimes takes place. The chief cause of this is usually an improper distribution of the body-weight over the hoof,—that is, an unbalanced foot. Wry hoofs of faulty positions of the limbs are often exposed to this evil; a faulty preparation of the hoof (dressing) for the shoe, as well as neglect of the colt’s hoofs, is in the majority of cases directly responsible for this condition.

If in the shortening of the wall a part is from ignorance left too long, or one-half of the hoof shortened too much in relation to the other half, the foot will be unbalanced. The horse will then touch the ground first with the section of wall which has been left too high, and will continue to do so until this long section has been reduced to its proper level (length) by the increased wear which will take place at this point. In unshod hoofs this levelling process takes place rapidly; such, however, is not the case in shod hoofs, for here the shoe prevents rapid wear, and, indeed this levelling process is often rendered impossible through the welding of high steel calks to the shoe. If this fault in trimming be repeated at the next and subsequent shoeings, and if the faulty relation of the ground-surface of the hoof to the direction of the foot-axis remain during several months, the portion of wall left too high will grow more rapidly, the walls will lose their natural straight direction and become bent. If, for example, the outer wall has been left too long during a considerable period of time, a crooked hoof results ([Fig. 85]) in which the rings are placed closer together upon the low (concave) side than upon the high (convex) side. If for a long time the toe is excessively long, it will become bent; or if this fault affects excessively high quarters they will contract either just under the coronary band or will curl forward and inward at their lower borders. These examples are sufficient to show both the importance of the manner in which a horse places his foot to the ground and its influence upon the loading, growth, and form of the hoof.

Fig. 85.

Crooked (right) fore-hoof.

Wear of the Shoe and of
the Hoof upon the Shoe.

The wear of the shoe is caused much less by the weight of the animal’s body than by the rubbing which takes place between the shoe and the earth whenever the foot is placed to the ground and lifted.

The wear of the shoe which occurs when the foot is placed on the ground is termed “grounding wear,” and that which occurs while the foot is being lifted from the ground is termed “swinging-off wear.“ When a horse travels normally, both kinds of wear are nearly alike, but are very distinct when the paces are abnormal, especially when there is faulty direction of the limbs. While in the majority of horses whose limbs have been stiffened by age and overwork both kinds of wear are most marked at the toe of the shoe, we see relatively fewer cases of “grounding wear” at the ends of the branches (as in laminitis); on the contrary, we always notice “swinging-off wear” at the toe of the shoe. It is worthy of notice that length of stride has much to do with the wear. We observe that with shortening of the stride both kinds of wear occur at the toe of the shoe, and this is rapidly worn away, as is the case with horses which are fretful and prance under the rider, draw heavy loads, or from any other cause, as disease or infirmity, are obliged to shorten their steps. With increase of length of stride the wear of the shoe becomes more uniform.

Fig. 86.

A normal-angled foot
with straight foot-axis.
The shoe shows uniform wear.

Fig. 87.

An upright foot with foot-axis
broken forward by reason of too
high quarters. The shoe shows
“grounding“ wear at ends of
branches, and “swinging-off“
wear at toe.

Fig. 88.

A hoof with foot-axis broken
backward by reason of surplus
horn at the toe. The shoe shows
excessive “grounding“ and
“swinging-off” wear at the toe.

The position and form of the shoe have a marked influence upon its wear; at the place where the shoe is too far under the hoof either as a result of shifting or of having been nailed on crooked, or where the outer branch has not the necessary width, or does not form a sufficiently large curve, the wear will be increased.

Also the relative length of side walls, or of toe and heels, influences rapidity of wear of the shoe. If through ignorance or carelessness one side wall be left too long, the branch beneath will meet the ground before other parts of the shoe and will wear faster ([see Figs. 87], [88] and [89]).

The wear of the hoof upon the shoe occurs as a result of the movements of the quarters. Visible indications of this are the brightly polished, often sunken places upon the bearing-surface of the ends of the branches, showing that scouring occurs between the horn and the iron. Shoes which show brightly polished places in their anterior halves have been loose. The wear of the quarters upon the shoe is not always uniform, but is usually greater on the inner than on the outer quarter, especially in base-wide feet. The degree of this wear of the hoof may be from nothing to one-fourth of an inch or more from one shoeing to the next. Finally, we should remember that this usually invisible scouring away of the hoof gradually causes the nails at the quarters to become loose, and that this is more clearly marked in the front than in the hind hoofs.

G. Physiological Movements of the Hoof.
(Mechanism of the Hoof.)

These movements comprise all those changes of position within and of the hoof which are brought about by alternately weighting and relieving the foot, and which are manifest as changes of form of the hoof. The following changes in form of the hoof are most marked at the time that the hoof bears greatest weight,—that is, simultaneous with the greatest descent of the fetlock-joint.

1. A lateral expansion over the entire region of the quarters, occurring simultaneously at the coronary and plantar borders. This expansion is small, and in general varies between one-fiftieth and one-twelfth of an inch.

2. A narrowing of the anterior half of the hoof measured at the coronary border.

3. A decrease in height of the hoof, with a slight sinking of the heels.

4. A flattening (sinking) of the sole, especially in its branches.

These changes of form are much more pronounced in the half of the hoof that bears the greater weight.

Fig. 89.

Transverse vertical section through the middle of a right fore shod hoof of base-wide form, viewed from behind. The outer wall having been insufficiently lowered has caused increased wear of the underlying branch of the shoe: a, wear of inner branch (beneath the relatively short wall); b, greater wear of outer branch beneath the relatively long wall; c, the horn between the dotted line and the shoe represents the surplus length of this outer wall.

A hoof while supporting the body-weight has a different form, and the tissues enclosed within it a different position, than when not bearing weight. Since loading and unloading of the foot are continually alternating, the relations of internal pressure even in the standing animal are continuously changing, so that, strictly speaking, the hoof is never at rest.

The changes in form take place in the following order: the body-weight falls from above upon the os coronæ, os pedis, and navicular bone, and at the moment that the foot is placed upon the ground is transmitted through the sensitive laminæ and horny laminæ to the wall. At the instant that the fetlock reaches its lowest point the os pedis bears the greatest weight. Under the body-weight the latter yields, and with the navicular bone sinks downward and backward. At the same time the upper posterior portion of the os coronæ ([Fig. 90, A]) passes backward and downward between the lateral cartilages (a), which project above the upper border of the wall, and presses the perforans tendon down upon the plantar cushion. The plantar cushion being compressed from above, and being unable to expand downward, is correspondingly squeezed out towards the sides and crowded against the lateral cartilages, and they, yielding, press against and push before them the wall at the quarters. The resistance of the earth acts upon the plantar surface of the hoof, and especially upon the frog, and it, widening, crowds the bars apart, and in this manner contributes to the expansion of the quarters, especially at their plantar border ([see Fig. 90]). The horny sole under the descent and pressure of the os pedis sinks a little—that is, the arch of the sole becomes somewhat flattened. All these changes are much more marked upon sound unshod hoofs, because in them the resistance of the earth upon the sole and frog is pronounced and complete. These changes in form are more marked in front feet than in hind. In defective and diseased hoofs it may happen that at the moment of greatest weight-bearing, instead of an expansion a contraction may occur at the plantar border of the quarters.

Fig. 90.

Vertical, transverse section of a foot seen from behind: A, os coronæ; B, os navicularis; C, os pedis; a, lateral cartilage; b, anterior portion of fleshy frog; c, section of perforans tendon; d, suspensory ligament of the navicular bone; l, wall; m, sole; n, white line; o, frog.

Three highly elastic organs there are which play the chief part in these movements,—namely, the lateral cartilages, the plantar cushion, and the horny frog. Besides these structures, indeed, all the remaining parts of the horn capsule, especially its coronary border, possess more or less elasticity, and contribute to the above-mentioned changes of form.

In order to maintain the elastic tissues of the foot in their proper activity, regular and abundant exercise, with protection against drying out of the hoof, are absolutely necessary, because the movements of the different structures within the foot and the changes of form that occur at each step are indispensable in preserving the health of the hoof. Long-continued rest in the stable, drying out of the hoof, and shoeing decrease or alter the physiological movements of the foot, and these lead under certain conditions to foot diseases, with which the majority of horse owners are entirely unacquainted.

As an outward, visible indication of the mobility of the quarters upon the shoe we may point to the conspicuous, brightly polished, and often sunken spots, or grooves, upon the ends of the branches. They are produced partly by an in-and-out motion of the walls at the quarters, and partly by a forward and backward gliding of the quarters upon the shoe.

The benefits of these physiological movements within the hoof are manifold:

1. Through them shock is dispersed and the body protected from the evil consequences of concussion or shock.

2. These movements increase the elasticity of the entire limb, and in this way contribute much to a light and elegant gait.

3. They maintain a lively circulation of blood in the vessels of the pododerm, and this insures a rapid growth of horn.

Since it is a generally accepted fact that shoeing interferes with the physiological movements of the hoof, alters them, indeed, almost suppresses them, and that all these movements are spontaneous and natural only in sound unshod hoofs, we are justified in regarding shoeing as a necessary evil. However, it is indispensable if we wish to render horses serviceable upon hard artificial roads. If, in shoeing, consideration be given to the structure and functions of the hoof, and particularly to the hoof-surface of the shoe, the ends of the branches being provided with a smooth, level bearing-surface, which allows free play to the elastic horn capsule, in so far as this is not hindered by the nails we need have no fear of subsequent disease of the hoofs, provided the horse is used with reason and receives proper care.