ARTHROLOGY.

What is known regarding the articulations of bones of joints. The several bones which form the skeleton are united by means of certain soft structures, forming a number of articulations or joints, the study of which is termed arthrology.

Before considering the different forms of joints, it will be advisable to describe briefly the various tissue, other than bone, which enter into and contributes towards their formation. These are chiefly cartilage, connective and elastic substance or tissues, and fat.

In health, one bone never comes directly in contact with another, cartilage or fibrous tissue being always interposed; an exception to this exists in the adult skull, most of the bones of which become firmly united by ossification of the interposed soft material.

Cartilage.—Cartilage, known also by the familiar name of gristle, is a firm, bluish-white elastic animal substance, somewhat transparent, resilient, and flexible, possessing great cohesive power. That which forms the original basis of the bony framework is termed temporary, and that which persists in the adult, permanent cartilage; the former disappears as it is replaced by bone, but the latter, of which alone we have to treat here, never under normal circumstances become ossified. Cartilage consists of corpuscles or cells, usually embedded in an intercellular substance.

The articular cartilage is important as it encrusts the articular surfaces of bones, helping to form joints by supplying smooth, elastic cushions, which diminishes both concussion and friction.

Connective Tissue (White fibrous tissue.)—In one form or another this tissue is found in all parts of the body. The chief varieties are the areolar and the fibrous; the former serving as a connecting medium, and support to the various organs, and to the structures of which they are formed. It appears as a loose, transparent mesh, its interwoven bundles forming spaces termed the areolar or cells.

Connective tissue contains nerves and blood-vessels, for the supply of neighboring structures as well as for its own nourishment. When healthy it is little sensitive to pain.

Yellow Elastic Tissue.—This differs from the white or connective tissue in being yellow, elastic, and not so tough or strong. Its fibers are usually large but when mixed with the white tissue in tendons the size diminishes.

Yellow elastic tissue is found nearly pure in the ligamentum nuchae, and tunica abdominis, the coats of the largest arteries and elsewhere. The lungs contain a large quantity of this tissue.

When white fibrous tissue is boiled gelatine is obtained, which is not the result of boiling the yellow elastic tissue.

Adipose Tissue.—Fat or adipose tissue consists of cells containing an oily material, and arranged in isolated groups, or slightly separated by meshes of areolar tissue. It is found in many parts of the body, and varies greatly in quantity; in joints it occurs between the ligaments, and serves the purpose of a packing material, while in the form of medulla or marrow it occupies the cavities of bones. In fat cell a nucleus is very rarely visible.

Ligaments.—Ligaments are dense, fibrous, connecting structures. They exist in most articulations, and are made up principally of white fibrous tissue. There are two kinds—capsular or bursal, and funicular or binding ligaments.

Capsular ligaments are membranous structures enclosing true joints. They consist of a dense interlacement of fibers attached to bones, round the edges of the articular cartilages; some regard them as a continuation of the periosteum or the covering of bone. They are never closely applied, their use being to form cavities round the joints, enclosing and protecting the synovial or lubricating apparatus inside.

Funicular or binding ligaments consist of rounded or flattened cords, or bands of fibrous tissue, passing from one bone to another, firmly attached to roughened portions of their surfaces. They hold the bones in their places, at the same time allowing the requisite amount of motion in the joints. Ligaments which are situated between bones are often termed interosseous. Annular ligaments are those which bind down and protect the tendons of muscles in certain joints, converting grooves in the bones into channels or tubes which are lined with synovial membrane, and through which the tendons play.

Some ligaments are composed almost entirely of yellow elastic tissue, such as the ligamentum nuchae (the elastic ligament of the neck) and the ligaments connecting the vertebral arches (arches between the bones of the spinal column).

Synovial Membranes.—These are thin membranes lining the capsular ligaments of joints, or they are interposed elsewhere between structures which move one upon another, and which would otherwise be injured by the friction. Near the borders of articular cartilages the membrane is generally found as a projecting fold, the projection being due to a small pad of fat, interposed between the membrane and the capsular ligament. These projections were once erroneously termed synovial glands, but their use is probably to assist in forcing the synovia between the opposing surfaces of cartilage.

Synovia.—Synovia or joint oil, is a viscid, transparent fluid, colorless, or pale yellow, physically resembling oil, but it contains very little fatty material, consisting chiefly of albumen, salts and water; it is secreted by the cells of the inner surface of the synovial membrane. When an animal is in active exertion, there is a greater demand for joint oil than when at rest, consequently there is an increased secretion of it.

Classes of Joints.—Joints may be divided into three classes—Immovable, movable and mixed.

Motion in Joints.—The following terms express the various movements allowed by joints—Extension tends to bring two bones as nearly into a straight line as the structure of the joint will permit; flexion is the reverse of this, and diminishes the angle that extension increases; abduction expresses the outward movement of a limb or bone from the central line of the body; adduction is the reverse action; rotation signifies the partial revolution of a bone or number of bones, as it were, on their own axis; circumduction implies the movement of the lower end of a bone or limb, where it describes a curve, as the bow of a circle, ellipse, etc. The term gliding explains itself, and is peculiar to movable and other joints having no frictional surfaces.

Important points of joints to be considered will be described on Plate I by Roman figures to correspond with those given below.

Joints of the Front Leg.

I. Shoulder Joint.—This joint is formed by the lower end of the scapula, or shoulder blade, and the upper end of humerus or shoulder bone. This is a ball-and-socket joint, and is held in its place by ligaments and muscles. This belongs to the class of rotation joints, its action is outward, inward, backward and forward.

II. Elbow Joint.—This joint is formed by the lower part of the humerus and the upper portion of the radius and ulna. It is covered with ligaments and muscles similar to the above mentioned joint. This is a hinge joint the action of which is only forward and backward, or flexion and extension. It has no lateral movement.

III. Carpus or Knee Joint.—This joint has been explained very thoroughly in the previous chapter. There are three separate articular surfaces in this joint. This joint has a large capsular ligament, and has binding ligaments one above the other, the seventh being at the back of the three in the upper row, and the eight, when present, in a similar position with respect to the lower row. Thus there are three separate articular surfaces in this joint. The upper surface of the carpus articulates with the radius; this forms the hinge joint of the knee and is where flexion and extension takes place. Another articulation between the upper and lower carpus bones give slight motion to the knee, but not so much as the upper articular surface. Between the lower surface of the lower row of carpus bones and the upper extremity of the metacarpal, or cannon bones, is another articular surface which gives motion to the knee. This joint has a large capsular ligament, and has binding ligaments inside and outside. In addition to the flexion and extension movement the knee joint possesses, it also serves as a cushion and relieves concussion of the fore limb.

IV. Fetlock Joint.—This joint is formed above by the lower portion of the metacarpal, or cannon bone, and below by the part of the os suffraginis or the large pastern bone. At the back of this joint there are two small bones called the sesamoid bones. This joint has a capsular and two lateral binding ligaments. The motion of this joint is a backward and forward or flexion and extension, same as that of the knee.

V. Pastern Joint.—This joint is situated just above the hoof, and is formed by the lower end of the os suffraginis or large pastern bone and the upper end of the os coronae or small pastern bone. It has a capsular and lateral binding ligaments, same as the fetlock joint. It also has the same motion as the fetlock joint, and is often the seat of what is termed a high ringbone.

VI. Coffin Joint.—This joint is situated within the hoof. It is formed above by the lower end of the os coronae or small pastern bone, and below by the upper surface of the os pedis or coffin bone. Immediately behind this joint, and articulating with the two mentioned bones, is the navicular, or shuttle bone—it gets its name from its likeness to the shuttle of a sewing machine. This bone when diseased is the seat of navicular disease, or coffin-joint lameness. The action of this joint is very slight forward and backward.

Joints of the Hind Leg.

VII. Hip Joint. The hip joint is formed by the pelvis and femur bone. This is a true ball-and-socket joint similar to the shoulder joint. It has a capsular, and is called the round ligament, in the joint, holding the head or ball on the femur in the socket of the pelvis. This can be seen plainly by examining the joint. This round ligament is important, as it often becomes strained, being the seat of hip joint lameness. It is also held together by the strong muscles of the hip. Its motion is rotation similar to that of the shoulder joint.

VIII. Stifle Joint.—This joint is formed above by the lower end of the femur, or hip bone, and the upper end of the tibia or thigh bone. These two bones in front form a pulley-like surface on which the patella, or stifle bone, is situated. This bone, when the joint is in motion, glides up and down over the pulley-like surface. It sometimes becomes displaced, and this is termed dislocation of the patella or stifle bone. This is an important point to notice about this joint. It has a capsular and lateral, or binding ligament, which hold the stifle bone to its place as it plays upon the pulley-like process of this joint. The action of this joint is only forward and backward or flexion and extension. It has no lateral or side motion.

IX. Tarsus or Hock Joint.—This joint contains six bones. The two upper bones, one of which is a pulley-like bone placed in front, and the other placed behind, forms that portion of the hock which is called the point of the hock to which the muscles of the gambe are attached. It can be easily seen or felt. The upper surface of these two bones articulate with the lower portion of the tibia or thigh bone, and forms a true articulation of the hock joint. This part is what gives most of the motion to the joint. Below these are three other small, irregular bones, placed one upon the other, having an articular surface between them. Immediately behind these three small bones is what is called the cuboid bone. This bone also articulates with the three small irregular bones, helping to form the articular surface of the hock. The lower articular surface helps to give a small amount of motion to the joint. This joint is the seat of bone or bog spavin due to severe sprains, or poorly conformed joints.

Fetlock, pastern and coffin joints of the hind leg are so closely allied to those of the fore leg that it is not worth while discriminating between them.