SECT. 96.] OSSEOUS SYSTEM. I 83 



also, broken off from exuberant growths around the articular extremities, may 

 escape into the interior of the joint. 



§ 96. Physical and Chemical Peculiarities of Ihe Bones, and 

 their Accessory Organs. — The bones, besides containing a small 

 quantity of water (according to Stark, 3 to 7 per cent, in the 

 compact substance) and fat (2 to 3 per cent., Bibra), principally 

 consist of a substance yielding gelatine and of inorganic con- 

 stituents. In the adult, the latter constitute about two-thirds 

 (6882, Bibra) of the dry bone, and are almost all obtained by 

 calcination. In this case, when proper precaution is taken, the 

 bone completely retains its external form, but very readily breaks 

 down into a white, opaque, brittle, heavy powder — the so-called 

 bone-earth. This consists chiefly of 57 to 59 per cent, of tribasic 

 phosphate, and 7 to 8 per cent, of carbonate of lime, some fluoride 

 of calcium (traces), phosphate of magnesia, silica (traces), and 

 alkaline salts. The substance of the bones yielding gelatine is the 

 so-called bone-cartilage (cartilago ossium), and constitutes about 

 one-third of the dry bone. This is obtained by treating a bone 

 with diluted hydrochloric or nitric acid, at alow temperature; it 

 is a soft, elastic, yellowish, cartilaginous-like substance, which 

 corresponds exactly to the shape of the bone, and yields, when 

 boiled, ordinary gelatine. 



The physical properties of the bones depend upon their com- 

 position. Their hardness, denseness, and firmness are due to the 

 earthy, their elasticity and flexibility to the organic constituents. 

 In healthy adult bones, the two main components are united in 

 such proportions, that the bones, whilst sufficiently hard and firm, 

 possess a certain, though small degree of elasticity, so that they 

 have a considerable power of resistance, and do not so very readily 

 break when subjected to mechanical violence. In early age, when 

 the organic part is in larger proportion, their hardness, and, con- 

 sequently, their capability of bearing weight, is much less; in this 

 condition, they are more disposed to bend under pressure, whilst, 

 on the other hand, their greater elasticity tends much to save 

 them from fracture. This is the case, in a much higher degree, in 

 rickets, where the organic constituents vary from between 70 and 80 

 per cent. In old age, on the contrary, the bones become harder, 

 but, at the same time, more brittle, so that they break very readily, 

 although their greater fragility may be partly owing to the rare- 

 faction of the tissue which takes place at this period. The com- 

 bustibility of the bones is due to their organic basis ; their opacity, 



