94 THE HUMAN BODY. 



moistened in health by a small quantity of glairy synovial 

 fluid, something like the white of a raw egg in consistency, 

 and playing the part of the oil with which the contiguous 

 moving surfaces of a machine are moistened; it makes all 

 run smoothly with very little friction. 



In the natural state of the parts, the head of the femur and 

 the bottom and sides of the acetabulum lie in close contact, 

 the two synovial membranes rubbing together. This contact 

 is not maintained by the ligaments, which are too loose and 

 serve only to check excessive movement, but by the numerous 

 stout muscles which pass from the thigh to the trunk and 

 bind the two firmly together. Moreover, the atmospheric 

 pressure exerted on the surface of the Body and transmitted 

 through the soft parts to the outside of the air-tight joint 

 helps also to keep the parts in contact. If all the muscles 

 and ligaments around the joint be cut away, it is still found in 

 the dead Body that the head of the femur will be kept in its 

 socket by this pressure, and so firmly as to bear the weight of 

 the whole limb without dislocation, just as the pressure of 

 the air will enable a boy's " sucker " to lift a tolerably heavy 

 stone. 



Ball-and-socket Joints. Such a joint as that at the hip is 

 called a ball-and-socket joint and allows of more free move- 

 ment than any other. Through movements occurring in it 

 the thigh can be flexed, or bent so that the knee approaches 

 the chest; or extended, that is, moved in the opposite direc- 

 tion. It can be abducted, so that the knee moves outwards; 

 and adducted, or moved back towards the other knee again. 

 The limb can also by movements at the hip-joint be circum- 

 ducted, that is, made to describe a cone of which the base is 

 at the foot and the apex at the hip. Finally, rotation can 

 occur in the joint, so that with knee and foot joints held 

 rigid the toes can be turned in or out, to a certain extent, by 

 a rolling around of the femur in its socket. 



At the junction of the humerus with the scapula is another 

 ball-and-socket joint permitting all the above movements to 

 even a greater extent. This greater range of motion at the 

 shoulder-joint depends mainly on the shallowness of the 

 glenoid cavity as compared with the acetabulum, and upon 

 the absence of any ligament answering to the round ligament 

 of the hip-joint. Another ball-and-socket joint exists between 

 the carpus and the metacarpal bone of the thumb; and others 



