2GS ADVANCED PHYSIOLOGY 



it will soon hit these projections, and its movement in that 

 direction will consequently be stopped. The round ends 

 of the bones are rendered smoother by being covered with 

 cartilage, as in the hinge joint, and a membrane around the 

 joint secretes a synovial fluid for moistening the joint and 

 reducing friction. The bones at the shoulder joint are bound 

 together by ligaments, but these are not so numerous as at 

 the knee joint. The only important one, the capsular liga- 

 ment, is attached to the scapula around the socket, and then 

 extends out over the head of the humerus in such a way that 

 it has a wide, extended fastening to that bone. It is so loose 

 as 'to make it possible for the bone to move in any direction 

 without hindrance. If it is cut the bones come apart at 

 once. 



The ball-and-socket joint at the hip differs from that at the 

 shoulder, in that the muscles are much more massive and 

 powerful and the socket is much deeper. The joint is thus 

 firmer but has less freedom of movement; Fig. 135. 



Pivot Joints. In the case of a pivot joint, the two bones 

 concerned rotate on one another. In the movements of the 

 head, for example, all forward and backward tilting occurs 

 between the occipital bone of the skull, and the first, or atlas, 

 vertebra, the joint there being essentially a hinge joint. All 

 turning from right to left (not tilting from side to side) occurs 

 between the first, atlas, and second, or axis vertebra, one 

 bone rotating on top of the other and thus forming a pivot 

 joint. The turning of the radius bone of the lower arm on 

 the end of the humerus bone of the upper arm is another good 

 example of a pivot joint; Fig. 128. 



INJURIES TO JOINTS 



There are two kinds of accidents, not counting broken 

 bones, which occur, and frequently occur together, in joints, 

 These are sprains and dislocations. 



Sprains. A sprain is due to the stretching of some of the 



