COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 47 



of proteins called in Chapter I the albuminoids or the sclero- 

 proteins (51 e) accompanied by varying amounts of mineral 

 matter, and, of course, a considerable proportion of water. 



80. Bone. Bone is the most familiar example of the sup- 

 porting tissues of the animal body. In the young embryo the 

 bones first appear as cartilaginous structures consisting of 

 rounded cells imbedded in a homogeneous intercellular substance 

 containing also fibers and consisting mainly of collagen (51 e). 

 As development advances, the process of ossification begins, the 

 homogeneous substance of the cartilage taking up inorganic salts, 

 chiefly calcium phosphate, while the fibers of the cartilage are 

 stated not to take part in this process. In addition to mineral 

 matter, the bones store up also a variable amount of fat. Ma- 

 ture bone, therefore, aside from its fat, consists of a basis of 

 organic matter largely impregnated with mineral matter. The 

 presence of these two classes of constituents is readily demon- 

 strated by the familiar experiments in which, on the one hand, 

 the mineral matter is removed by immersion in dilute acid 

 leaving behind the flexible cartilage, or, on the other hand, 

 the organic basis of the bone is destroyed by heating, leaving 

 the so-called bone ash. 



Ossification has not been completed at birth but continues to a 

 greater or less extent up to full maturity Moreover, it is not carried 

 to the same extent in all bones nor in different parts of the same bone. 

 Consequently, both the percentages of ash and of fat and the propor- 

 tion of water to dry matter in bones may vary within wide limits, so 

 that it is impossible to state an average composition. The extremes 

 of 1 5 per cent and 44 per cent have been found for the average water 

 content of the entire skeleton of the dog and even wider variations 

 have been reported in the case of man. Compact bones contain less 

 water than more spongy ones. 



In general it may be said that from one-half to two-thirds of 

 the dry, fat-free bone consists of ash. About three-fourths of 

 the remainder is stated to consist essentially of albuminoids, or 

 collagens, yielding gelatin when treated with hot water, es- 

 pecially under pressure. It is evident, therefore, that the 

 skeleton of an animal contains not only a large share of the 

 total ash of the body but a not inconsiderable portion of its 

 nitrogenous constituents as well. On the average of the ten 

 animals analyzed by Lawes and Gilbert (97), 77.78 per cent of 



