232 MOTION AND COORDINATION 



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 for- 

 ward 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 ex- 

 ample 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 slip- 

 ping, upon the surface of another. 



The Machinery of the Body. A machine is a contrivance for direct- 

 ing 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 lhat energy is not wasted in the pro- 

 duction of useless motion. The places in machines where parts rub or 



