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.