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 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. 5. _Gliding_ joints are formed by the articulation of plain (almost flat) surfaces. Examples of gliding joints are found in the articulations between the bones of the wrist and those of the ankle. They are the simplest of the movable joints and are formed by one bone gliding, or slipping, upon the surface of another. *The Machinery of the Body.*—A machine is a contrivance for directing energy in doing work. A sewing machine, for example, so directs the energy of the foot that it is made to sew. Through its construction the machine is able to produce just that form of motion needed for its work, and no other forms, so that energy is not wasted in the production of useless motion. The places in machines where parts rub or turn upon each other are called _bearings_, and extra precautions are taken in the construction and care of the bearings to prevent friction.