THE SUPPORTING TISSUES. 109 



apparent that in young persons the food should contain suf- 

 ficient amounts of mineral salts, otherwise the bones will 

 not acquire their intended hardness. Milk is peculiarly 

 adapted for this, containing quite a large amount of earthy 

 matter. It not infrequently happens that children, in 

 spite of an apparently rich diet, are really starving for 

 want of the mineral matters which are needed to give con- 

 sistency to these supporting tissues. The final hygienic 

 recommendation that no unusual strains of any kind shall 

 be placed upon any of them while they are in their forma- 

 tive period, is too plain to need further comment. 



JOINTS. 



The supporting tissues not only serve to give form and 

 outline to the body, but in the case of bones they serve to 

 make a system of levers to accomplish the various move- 

 ments of which the body is capable. To permit such move- 

 ments, bones are articulated together to form more or less 

 movable joints. In the body there are found four kinds of 

 movable joints. They are, first, the gliding joints; second, 

 the pivot joints; third, the hinge joints, and fourth, the 

 ball-and-socket joints. In addition to these movable joints, 

 bones articulate in such a way as to preclude motion. In- 

 stances of such articulation are plain in the bones of the 

 skull, where the bones remain separate until the skull has 

 reached its mature size. Such articulations are called 

 sutures. 



In the gliding joints a number of bones glide over each 

 other to such an extent as to permit quite a little motion. 

 Instances of such joints are in the wrists. and ankles. Here 

 the carpal and tarsal bones gliding over each other permit 

 all the different motions of which the wrist and ankle are 

 possible. There are but two instances of pivot joints in the 

 body. The plainest instance is the joint between the atlas 

 and the axis, where the odontoid process of the axis forms 

 the pivot, around which the atlas rotates. A second illus- 

 tration is in the case of the forearm, where, by the peculiar 



