CHEMICAL COMPOSITION OF BONE. 307 



been selected for this purpose from its forming 

 much harder compounds with animal membrane 

 than the carbonate. Wherever great strength 

 and rigidity are required, this is the material 

 depended on for imparting these qualities ; and 

 it has accordingly been employed for the osseous 

 structures, which are among the most elaborate 

 results of organization. The densest and hardest 

 of these structures are those in which the pro- 

 portion of phosphate of lime is the greatest, 

 when compared with that of the animal sub- 

 stance which cements them together ; the force 

 of mutual cohesion among its own particles 

 being much greater than that imparted by the 

 cementing ingredient. The internal bony por- 

 tions of the ear, where, in order perfectly to 

 transmit the sonorous vibrations, 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 ex- 

 amining them, are as dense and as hard as 

 marble. The bony portions of the teeth, like- 

 wise, afford instances of very hard calca- 

 reous formations ; but the enamel, which con- 

 sists almost wholly of phosphate of lime, is 

 harder still, and resembles the siliceous stones, 

 being, like flint, capable of striking fire with 



