THE SKELETON. 339 



Bone is composed of mineral matter, two thirds, and animal matter, 

 one third ; in childhood the animal matter is in larger proportion, while 

 in old age the mineral matter is in excess. 



The mineral matter is chiefly calcium phosphate, while the animal 

 matter is largely gelatin. 



Joints may be classified according to their structure as 

 follows : 



Classification of Joints. i. Immovable, such as the 

 sutures between the bones of the skull ; 



2. Mixed, such as the joints between the vertebrae ; 



3. Movable, which allow free motion between the parts ; 

 (a) Ball and socket, as in the hip and shoulder ; 



(ft) Hinge, as in the knee and elbow ; 



(c) Pivot, as in the forearm, and between the atlas and 

 axis ; 



(d) Gliding, as between the short bones of the wrist, and 

 of the ankle. 



Study of Joints. Examine these joints in the articulated skeleton, 

 and so far as possible, in fresh specimens (of rabbits). Compare the 

 ball and socket joints of the hip and shoulder. Also compare the hinge 

 joints of the knee and elbow. 



Hygiene of the Bones. Sometimes the bones of chil- 

 dren are deficient in mineral elements, and are unduly soft 

 and flexible. This condition indicates a disease called 

 rickets. Even if the bones are normal, children should 

 not be encouraged to walk early, as bow-legs may result. 

 Most bow-legged persons seem to be active, and probably 

 their muscles developed faster than the bones. Constrained 

 positions or excessive use of special groups of muscles may 

 result in lateral curvature of the spine. The height of 

 seats and desks should be carefully looked after. 



Effects of Alcohol. Alcoholic drinks are particularly 

 injurious to the young. They interfere with proper nutri- 



