50 



CONSCIOUS NERVOUS OPERATIONS 



Fig. 35. Hinge joint 

 of the elbow. 



1 hum eras. 2 ulna. 



The ball and socket joint seen in the shoulder and the 

 hip has the end of one bone fitted into the hollow of 

 another, and provides for motion in 

 any direction (Figs. 34 and 86). 



The pivot joint is that in which one 

 bone rotates round another, as in the 

 atlas and axis joint (Fig. 32), already 

 described, and in the rotation of the 

 ulna on the radius at their junction 

 with the wrist. 



The hinge joint permits of motion 

 in one plane only, as in the joints of 

 the fingers. Some hinge joints have 

 provision for additional movements, as 

 in the elbow (Fig. 35) and in the 

 articulation between the lower jaw 

 and the skull. 



57. Synovial Membrane. The broad, thin ligament sur- 

 rounding a joint forms a closed sac. This sac is lined 

 with the synovial membrane, which secretes a fluid whose 

 purpose is to lubricate the joint, as oil lubricates the parts 

 of a machine which move upon one another (Fig. 36). 



58. Structure of Bone. A living bone is tough, strong, 

 and slightly elastic. Burned in a fire it retains its size 

 and shape, but becomes brittle and easily crumbles to pow- 

 der. Soaked for a few days in dilute muriatic acid, it also 

 retains its shape, but becomes so flexible that, if one of the 

 long bones, it may be tied in a knot. The fire destroys 

 the 33 per cent of animal matter in the bone, leaving the 

 calcium phosphate, calcium carbonate, and the small quan- 

 tities of other salts which constitute the earthy or mineral 

 portion of bone. The acid dissolves out the earthy salts 

 and leaves the animal tissues. 



