240 THE SKELETON 



Irregular bones. — The general features of the irregu- 

 lar bones, such as the vertebrae (see Ex. XLIX., c, and 

 p. 225), have already been discussed. AH irregular bones 

 show adaptations for the attachment of muscles or articu- 

 lations. They have an outer covering of either cartilage 

 or periosteum, with inner coats of hard and spongy bone. 

 They are without cavities except for the spaces between 

 the spongy tissue. 



Chemical composition of bonel — If we place a clean rib in 

 a bottle with a 20% solution of hydrochloric acid and let it 

 stand for a few days, we shall find, upon removing it from 

 the acid, that its character has changed. (See Ex. L.) It 

 has the same shape as before, but is so soft as to be easily 

 cut with a knife, and so flexible as to permit of its being 

 tied in a knot. The substance of which it is now composed, 

 is cartilage or gristle, and this substance is identical in 

 structure with the cartilage that formed the pads for the 

 articulations. E-\adently, the acid has taken away the 

 stiffening material. 



If we burn a rib in a fire we note another change. The 

 bone is again without change in form, but is very white 

 and brittle. If we put this brittle structure into hydro- 

 chloric acid it entirely dissolves. 



These experiments demonstrate that bones are made 

 of two substances: one, cartilage which wiU burn but is 

 insoluble in acid; the other, a mineral salt, of lime forma- 

 tion, which will not burn but dissolves readily in acid. 

 Another feature that the experiments teach us is that these 

 two substances are so mixed in the living bone that the 

 removal of one or the other does not alter the shape of the 

 bone. To the cartilage component of bone is due evidently 

 . its flexibility, while the lime salt is responsible for its stiff- 



