JOINTS, in anatomy. A joint or articulation may be defined to be the union of any two segments of the skeleton of an animal body through the intervention of a structure or structures of a different nature. The textures which enter into the formation of the more complex joints are bone, cartilage, fibro-cartilage, ligaments, and synovial mem brane. Bone forms the fundamental part of all joints; ligament, in various modifica tions, is employed as the bond of union between the bony segments; while the three remaining textures chiefly occur in those joints in which there is free motion. The joints vary in the degree of motion from almost perfect immobility to the greatest amount and extent of motion that are compatible with the maintenance of the bony seg ments in their proper relation with each other.
Joints have been divided by anatomists into two great classes, to which the terms synar throsis and diarthrosis are applied. In synarthrosis the parts are continuous—that is to say, there is no synovial sac intervening between the bones; and the joints belonging to this class are so very limited in their motion as to be considered by some as immovable; in diarthrosis the articular surface of each of the bones is covered with cartilage, and between these cartilaginous plates is a synovial sac; and mobility is the distinguish ing feature of this class of joints. In briefly describing the leading varieties oethese two classes of joints we shall, as far avoid the barbarous terms which have been introduced into this department of anatomy.
In synarthrosis the articulation is said to be by suture when the bones seem to grow somewhat into one another, and to become interlocked and dovetailed together, each bone having a jagged or serrated margin, or when there is a degree of beveling of one bone, so that it is overlapped by the other. Both these kinds of suture are at once seen in the human skull, the serrated suture being well seen in the union of the two parietal bones, the beveled suture being shown in the overlapping of the temporal bone above the side of the parietal, and a combination of the two being exhibited by the coronal suture between the frontal bone and the anterior edges of the parietal bones. In
all these there is a thin ligamentous membrane interposed between the bones, which disappears as the growth of the cranium becomes completed.
When a slight amount of motion of one bone upon another is required to be com bined with great strength, the contiguous surfaces of the bones are united by a thick and .strong layer of fibro-cartilage, with which a little elastic tissue is intermixed. This -is an intermediate variety between the two classes of joints, but approximates most nearly to synarthrosis. As examples of this kind of joint may be mentioned the articulation between the bodies of the vertebrae and that between the two pubic bones at what is termed the symphysis. See PELVIS.
In diarthrosis the degree and nature of the motion are very various. There may be merely a little gliding motion between the ends of the bones, as, for example, in the articulations between the various bones of the carpus and tarsus. See HAND and FOOT. In these cases the surfaces are plane, or one is slightly concave and the other slightly convex; and the motion is limited in extent and direction by the ligaments of the joint, or by some projecting point of one of the bones. In some cases instead of a slight con cavity and convexity, one bone presents a cup-like depression, while the termination of the other assumes a hemispherical, or more or less globiTrar shape. Hence the name of ball and socket that is applied to such joints. The best example of this variety is the hip joint (q.v.), and the next best is the shoulder. In these joints the ball is kept in apposi tion with the socket by means of what is termed a capsular ligament, which may be described as a barrel-shaped expansion of ligamentous structure, attached by its extremities around the margin of the articular surfaces composing the joint, and forming a com plete investment of it, but not so tight as materially to restrict its movements. This species of joint is capable of motion of all kinds, as any one may readily test for himself, especially in the shoulder-joint.