=Movement of Joints.=—The movements admissible in joints may be divided into four kinds: gliding, angular movement, circumduction, and rotation. These movements are often, however, more or less combined in the various joints. It is seldom that there occurs only one kind of motion in any particular joint. _Gliding movement_ is the most simple kind of motion that can take place in a joint, one surface gliding or moving over another without any angular or rotary movement. _Angular movement_ occurs only between the long bones. By it the angle between the two bones is increased or diminished. It may take place in four directions: forward and backward, consisting of flexion and extension; or inward and outward from the medial line of the body, consisting of adduction and abduction. Abduction of a limb is movement away from the medial line of the body. Adduction of a limb is movement toward the medial line of the body. _Circumduction_ is that limited degree of motion which takes place between the head of a bone and its articular cavity, whilst the extremity and sides of the limb are made to circumscribe a conical space, the base of which corresponds with the inferior extremity of the limb, the apex with the articular cavity; this kind of motion is best seen in the shoulder- and hip-joint. _Rotation_ is the movement of a bone upon an axis, which is the axis of the pivot on which the bone turns, as in the articulation between the atlas and axis, when the odontoid process serves as a pivot around which the atlas turns; or else is the axis of a pivotlike process which turns within a ring, as in the rotation of the radius upon the humerus. _Pronation_ is a form of rotation in which the inferior extremity of the radius passes before the ulna, and thus causes the hand to execute a kind of rotation from without inward. _Supination_ is a form of rotation in which the movement of the forearm and hand are carried outward so that the anterior surface of the latter becomes superior. =The Ligamentous Structure.=—Ligaments are dense, fibrous, connecting structures. They are made up principally of white fibrous tissue and exist in all true joints. There are four kinds of ligaments associated with true joints: The first kind, the _capsular ligament_ which encloses all true joints, is thin and consists of interlaced fibers attached to the bone at the edges of the articular cartilages. It either partly or wholly surrounds the joint, enclosing and protecting a synovial apparatus, which, by secreting a liquid resembling serum, lubricates the joint to prevent friction. The second kind, the binding or _lateral ligaments_, consist of flattened or rounded cords or bands of fibrous tissue. Such a ligament extends from one bone to the other, and firmly attached to their roughened surfaces, holds the bones in place and at the same time allows the required amount of motion. The third kind, located between the joints, is called _interosseous ligament_.