The best examples of joints that are slightly, but not freely, movable are found in the front of the spinal column. The cartilaginous pads between the vertebræ permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebræ in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another.
Structure of the Movable Joints.—By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed[pg 231] projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articular surfaces are quite smooth and dense, having no Haversian canals, and they are covered with a layer of cartilage. Strong ligaments pass from one bone to the other to hold each in its place (A, Fig. 103). Some of these consist simply of bands, connecting the joint on its different sides, while others form continuous sheaths around the joint.
Fig. 103—Outside and inside view of knee joint. 1. Tendons. 2. Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is lined, except upon the articular surfaces, by the synovial membrane.
The interior of the joint, except where the bone surfaces rub upon each other, is covered with a serous lining, called the synovial membrane (B, Fig. 103). This secretes a thick, viscid liquid, the synovial fluid, which prevents friction. The synovial membrane does not cover the ends of the bones, but passes around the joint and connects with the bones at their edges so as to form a closed sac in which the fluid is retained.
Kinds of Movable Joints.—The different kinds of movable joints are the ball and socket joint, the hinge joint, the pivot joint, the condyloid joint, and the gliding joint. These are constructed and admit of motion, as follows:
1. In the ball and socket joint the ball-shaped end of one bone fits into a cup-shaped cavity in another bone, called the socket. The best examples of such joints are[pg 232] found at the hips and shoulders. The ball and socket joint admits of motion in all directions.
2. In the hinge joint the bones are grooved and fit together after the manner of a hinge. Hinge joints are found at the elbows and knees and also in the fingers. The hinge joint gives motion in but two directions—forward and backward.
3. A pivot joint is formed by the fitting of a pivot-like projection of one bone into a ring-like receptacle of a second bone, so that one, or the other, is free to turn. A good example of the pivot joint is found at the elbow, where the radius turns upon the humerus. Another example is the articulation of the atlas with the axis vertebra as already noted. The pivot joint admits of motion around an axis.
4. The condyloid joint is formed by the fitting of the ovoid (egg-shaped) end of one bone into an elliptical cavity of a second bone. Examples of condyloid joints are found at the knuckles and where the wrist bones articulate with the radius and ulna. They move easily in two directions, like hinge joints, and slightly in other directions.