258 THE MECHANICAL FUNCTIONS. 



the proportion of phosphate of lime is the greatest, when 

 compared with that of the animal substance which cements 

 them together; the force of mutual cohesion among its own . 

 particles being much greater than that imparted by the ce- 

 menting ingredient. The internal bony portions of the ear, 

 where, in order perfectly to transmit the sonorous vibra- 

 tions, the greatest solidity is required, are the densest parts 

 of the skeleton; and phosphate of lime enters most largely 

 into the composition of these bones. The tympanic portions 

 of the temporal bone of the whale and the cachalot, where 

 the great size of the organ gives us advantages in examining 

 them, are as dense and as hard as marble. The bony por- 

 tions of the teeth, likewise, afford instances of very hard cal- 

 careous formations; but the enamel, which consists almost 

 wholly of phosphate of lime, is harder still, and resembles 

 the siliceous stones, being, like flint, capable of striking fire 

 wdth 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 sub- 

 jected to their mechanical action. But this extreme degree 

 of crystalline hardness v/ould 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 brittleness, and 

 been productive of the worst consequences 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 inter- 

 posed between the calcareous particles. 



This composition of bone is rendered evident by subject- 

 ing it to certain chemical processes. On exposure to heat, 

 we find it first becoming black, from the development of 

 the charcoal attendant upon the destruction of the animal 

 membrane. The oil contained in the cavities exudes, and, 

 taking fire, is soon totally consumed. The bone then reco- 

 vers its whiteness, and undergoes no farther change by the 



