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4. The Kind of Movement Admitted in Joints
The movements admissible in joints may be divided into four kinds: gliding and angular movements, circumduction, and rotation. These movements are often, however, more or less combined in the various joints, so as to produce an infinite variety, and it is seldom that only one kind of motion is found in any particular joint. Gliding Movement.—Gliding movement is the simplest kind of motion that can take place in a joint, one surface gliding or moving over another without any angular or rotatory movement. It is common to all movable joints; but in some, as in most of the articulations of the carpus and tarsus, it is the only motion permitted. This movement is not confined to plane surfaces, but may exist between any two contiguous surfaces, of whatever form. Angular Movement.—Angular movement occurs only between the long bones, and by it the angle between the two bones is increased or diminished. It may take place: (1) forward and backward, constituting flexion and extension; or (2) toward and from the median plane of the body, or, in the case of the fingers or toes, from the middle line of the hand or foot, constituting adduction and abduction. The strictly ginglymoid or hinge-joints admit of flexion and extension only. Abduction and adduction, combined with flexion and extension, are met with in the more movable joints; as in the hip, the shoulder, the wrist, and the carpometacarpal joint of the thumb. Circumduction.—Circumduction is that form of motion which takes place between the head of a bone and its articular cavity, when the bone is made to circumscribe a conical space; the base of the cone is described by the distal end of the bone, the apex is in the articular cavity; this kind of motion is best seen in the shoulder and hip-joints. Rotation.—Rotation is a form of movement in which a bone moves around a central axis without undergoing any displacement from this axis; the axis of rotation may lie in a separate bone, as in the case of the pivot formed by the odontoid process of the axis vertebræ around which the atlas turns; or a bone may rotate around its own longitudinal axis, as in the rotation of the humerus at the shoulder-joint; or the axis of rotation may not be quite parallel to the long axis of the bone, as in the movement of the radius on the ulna during pronation and supination of the hand, where it is represented by a line connecting the center of the head of the radius above with the center of the head of the ulna below. Ligamentous Action of Muscles.—The movements of the different joints of a limb are combined by means of the long muscles passing over more than one joint. These, when relaxed and stretched to their greatest extent, act as elastic ligaments in restraining certain movements of one joint, except when combined with corresponding movements of the other—the latter movements being usually in the opposite direction. Thus the shortness of the hamstring muscles prevents complete flexion of the hip, unless the knee-joint is also flexed so as to bring their attachments nearer together. The uses of this arrangement are threefold: (1) It coördinates the kinds of movements which are the most habitual and necessary, and enables them to be performed with the least expenditure of power. (2) It enables the short muscles which pass over only one joint to act upon more than one. (3) It provides the joints with ligaments which, while they are of very great power in resisting movements to an extent incompatible with the mechanism of the joint, at the same time spontaneously yield when necessary. The articulations may be grouped into those of the trunk, and those of the upper and lower extremities.
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