90 THE HUMAN BODY. 



of the spaces of the cancellated bone, the bony substance 

 proper, as found in the lamellae, is composed of earthy and 

 organic portions intimately combined, so that the smallest 

 distinguishable portion of bone contains both. The earthy 

 matters form about two thirds of the total weight of a dried 

 bone, and may be removed by soaking the bone in dilute 

 hydrochloric acid. The organic portion left after this treat- 

 ment constitutes a flexible mass, retaining the form of the 

 original bone; it consists chiefly of an albuminoid, ossein, 

 which by long boiling, especially under pressure at a higher 

 temperature than that at which water boils when exposed 

 freely to the air, is converted into gelatin, which dissolves 

 in the hot water. Much of the gelatin of commerce is pre- 

 pared in this manner by boiling the bones of slaughtered 

 animals, and even well-picked bones may be used to form a 

 good thick soup if boiled under pressure in a Papin's digester; 

 much nutritious matter being, in the common modes of do- 

 mestic cooking, thrown away in the bones. 



The earthy 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, retaining perfectly the shape and structural details of 

 the original bone. It consists mainly of normal calcium 

 phosphate, or bone-earth (Ca 3 , 2P0 4 ) ; but there is also pres- 

 ent 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 pres- 

 ent 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 may be distorted by any con- 

 tinued 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 voluntarily) the thigh-bones will al- 

 most certainly be bent over the edge of the seat by the weight 

 of the legs and feet, and a permanent distortion may be pro- 

 duced. 'For the same reason it is important that a child 

 be made to sit straight while 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 



