THE HUMAN SKELETON. 91 





Bones of the hand. 



51. (The bones of the lower extremity consist of the thigh, 

 leg and foot.! The thigh bone (os femora,) is the longest 

 bone of the human skeleton, and is remarkable for its great 

 strength, supporting, as it does, the whole body, and often 

 several hundred pounds in addition. ^ The hip joint is a perfect 

 specimen of the ball and socket joint. This is for the pur- 

 pose of giving great extent and variety of motion to the legs, 

 as in walking. The end of the hip bone is perfectly round 

 like a ball, and covered with a smooth, shining cartilage ; 

 and this is received into a deep cup, also lined with cartilage 

 and moistened with the synovial fluid. Besides all this, 

 there are strong ligaments all around, binding the bone 

 firmly in its place, so that it is a very rare thing for it to be 

 dislocated 1 . 



52.^ The knee is a hinge joint of singular construction. The 

 rubbing parts are flat and shallow, and therefore would 

 easily get out of place, were it not for the very strong liga. 

 ments which surround it. When the ligaments on the inside 

 of the joint are too weak, a person is said to be knock-kneed, 

 because the knees knock together. In weakly children, this 

 deformity may frequently be cured by exercise. It can also 

 be shown that, owing to the air being completely shut out of 

 the joint, forming what is called a vacuum, the bones of the 

 knee are held together by a constant pressure of the atmos- 

 phere equal to sixty or seventy pounds. On the fore-part of 



