368 THE MECHANICAL FUNCTIONS. 



steel. It is scarcely necessary to point out the 

 obvious intentions which are fulfilled by this 

 peculiarity of structure, conferring extraordinary 

 hardness on a part of which the appropriate office 

 is that of breaking down hard bodies subjected 

 to their mechanical action. But this extreme 

 degree of crystalline hardness would be ill suited 

 to other parts of the frame. In ordinary bones, 

 absolute rigidity is not the quality which is 

 alone wanted ; for, in general, the hardest bodies 

 are also the most fragile. An excess of rigidity, 

 therefore, would have been attended with brittle- 

 ness, and been productive of the worst conse- 

 quences to parts exposed to sudden and violent 

 concussions. It is in order to guard against this 

 evil that an elastic animal matter is employed as 

 the basis of the structure, acting as a strong 

 cement interposed between the calcareous par- 

 ticles. 



This composition of bone is rendered evident 

 by subjecting it to certain chemical processes. 

 On exposure to heat, we find it first becoming 

 black, from the developement of the charcoal 

 attendant upon the destruction of the animal 

 membrane. The oil contained in the cavities 

 exudes, and, taking fire, is soon totally con- 

 sumed. The bone then recovers its whiteness, 

 and undergoes no further change by the action 

 of the fire. If it be now examined, it will 

 be found to have lost nearly half its original 



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