70 THE HUMAN BODY 



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 towards the 

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

 joint be circumducted, 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 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 

 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 as a hinge- 

 joint. In it the articulating bony surfaces are of such shape as to 

 permit of movement, to and fro, in one plane only, like a door on 

 its hinges. The joints between the phalanges of the fingers are 

 good examples of hinge-joints. If no movement be allowed where 

 the finger joins the palm of the hand it will be found that each can 

 be bent and straightened at its own two joints, but not moved in 

 any other way. The knee is also a hinge-joint, as is the articula- 

 tion between the lower jaw and the base of the skull which allows 

 us to open and close our mouths. The latter is, however, not a 

 perfect hinge-joint, since it permits of a small amount of lateral 

 movement such as occurs in chewing, and also of a gliding move- 



