Chapter I: The Gross Anatomy of the Horse (1)

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.

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.

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.

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.

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.

Short pastern (os coronæ) viewed in front and in profile: _a_, upper joint-surface; _b_, anterior surface; _c_, lateral surface; _d_, lower joint-surface.]

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_.

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.]

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 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:

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. 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.

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).

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.

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‴_).

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.)]

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).

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.

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.

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.]

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_.

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).

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_.

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.

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.

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.

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.=

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.]