A Text-Book of Horseshoeing, for Horseshoers and Veterinarians

by A. Lungwitz

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.