96 THE HUMAN BODY. 



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 

 movement than any other. Through movements occurring 

 in it the thigh can be flexed, or bent so that the knee ap- 

 proaches the chest; or extended, that is moved in the oppo- 

 site direction. 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 circumdticted, that is made to describe a cone of 

 which the base is at the foot and the apex at the hip. Fi- 

 nally 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 answer- 

 ing to the round ligament of the hip-joint. Another ball- 

 and-socket joint exists between the carpus and the meta- 

 carpal bone of the thumb; and others with the same variety, 

 but a much less range, of movement between each of the 

 remaining metacarpal bones and the proximal phalanx of 

 the finger which articulates with it. 



Hinge-Joints. Another form of synovial joint is known 



