74 THE HUMAN BODY 



ticular cartilages and form the synovial membrane. This, which 

 thus forms the lining of the joint, is always 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 mois- 

 tened ; 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 main- 

 tained 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 sur- 

 face 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 movement 

 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 direction. It can be abducted, so 

 that the knee moves outwards; and adducted, or moved back to- 

 wards the other knee again. The limb can also by movements at 

 the hip-joint be 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 com- 

 pared 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 



