THE SUPPORTING TISSUES 47 



pressure in a Papin's digester; much nutritious matter being, in 

 the common modes of domestic cooking, thrown away in the bones. 



The inorganic salts of bone may be obtained free from organic 

 matter by calcining a bone in a clear fire, which burns away the 

 organic matter. The residue forms a white very brittle mass, re- 

 taining perfectly the shape and structural details of the original 

 bone. It consists mainly of normal calcium phosphate, or bone- 

 earth [Ca 3 (PO 4 ) 2 ]; but there is also present a considerable propor- 

 tion of calcium carbonate (CaCO 3 ) and smaller quantities of other 

 salts. 



Hygienic Remarks. Since in the new-born infant many parts 

 which will ultimately become bone consist only of cartilage, the 

 young child requires food which shall contain a large proportion of 

 the lime-salts which are used in building up bone. Nature provides 

 this in the milk, which is rich in such salts (see Chap. XXXIV), 

 and no other food can thoroughly replace it. . Long after infancy 

 milk should form a large part of a child's diet. Many children 

 though given food abundant in quantity are really starved, since 

 their food does not contain in sufficient amount the mineral salts 

 requisite for their healthy development. 



At birth even those bones of a child which are most ossified are 

 often not continuous masses of osseous tissue. In the humerus, 

 for example, the shaft of the bone is well ossified and so is each end, 

 but between the shafts and each of the articular extremities there 

 still remains a cartilaginous layer, and at those points the bone in- 

 creases in length, new cartilage being formed and replaced by bone. 

 The bone increases in thickness by new osseous tissue formed 

 beneath the periosteum. The same thing is true of the bones of 

 the leg. On account of the largely cartilaginous and imperfectly 

 knit state of its bones, it is cruel to encourage a young child to 

 walk beyond its strength, and may lead to " bow-legs" or other 

 permanent distortions. Nevertheless here as elsewhere in the 

 animal body, moderate exercise promotes the growth of the tissues 

 concerned, and it is nearly as bad to wheel a child about forever 

 in a baby-carriage as to force it to overexertion. 



The best rule is to let a healthy child use its limbs when it feels 

 inclined, but not by praise or blame to incite it to efforts which are 

 beyond its age, and so sacrifice its healthy growth to the vanity of 

 parent or nurse. 



