92 THE HUMAN BODY. 



calcium phosphate, or bone earth (Gas, 2P04); but there is 

 also present a considerable proportion of calcium carbonate 

 (CaC0 3 ) and smaller quantities of other salts. 



Hygiene of the Bony Skeleton. In early life the bones 

 are less rigid, from the fact that the earthy matters then 

 present in them bear a less proportion to the softer organic 

 parts. Hence the bones of an aged person are more brittle 

 and easily broken than those of a child. The bones of a 

 young child are in fact tolerably flexible and will be dis- 

 torted by any continued strain; therefore children should 

 never be kept sitting for hours, in school or elsewhere, on a 

 bench which is so high that the feet are not supported. If 

 this be insisted upon (for no child will continue it volunta- 

 rily) the thigh-bones will almost certainly be bent over the- 

 edge of the seat by the weight of the legs and feet, and a. 

 permanent distortion may be produced. For the same- 

 reason it is important that a child be made to sit straight in 

 writing, to avoid the risk of producing a lateral curvature- 

 of the spinal column. The facility with which the bones, 

 may be moulded by prolonged pressure in early life is well 

 seen in the distortion of the feet of Chinese ladies, pro- 

 duced by keeping them in tight shoes; and in the extraor- 

 dinary forms which some races of man produce in their 

 skulls, by tying boards on the heads of the children. 



Throughout the whole of life, moreover, the bones re- 

 main among the most easily modified parts of the Body; 

 although judging from the fact that dead bones are the 

 most permanent parts of fossil animals we might be in- 

 clined to think otherwise. The living bone, however, is 

 constantly undergoing changes under the influence of the 

 protoplasmic cells imbedded in it, and in the living Body is 

 constantly being absorbed and reconstructed. The expe- 

 rience of physicians shows that any continued pressure, 

 such as that of a tumor, will cause the absorption and dis- 

 appearance of bone almost quicker than that of any other 

 tissue; and the same is true of any other continued pres- 

 sure. Moreover, during life the bones are eminently plas- 

 tic; under abnormal pressures they are found to quickly 

 assume abnormal shapes, being absorbed and disappearing 



