96 THE MECHANICAL FUNCTIONS. 



In the construction of cartilage, nature seems to 

 have attained the utmost degree of density which 

 could be given to an internal texture composed 

 merely of the usual animal constituents. But sub- 

 stances of still greater hardness, united with perfect 

 rigidity, are wanted, in numberless instances, for 

 giving effectual protection to soft and delicate 

 structures, for supplying a firm basis to the frame- 

 work of the body, and for constructing levers of 

 various kinds, to be employed in the more ener- 

 getic movements of the higher animals. For all 

 these purposes it was necessary to superadd a ma- 

 terial endowed with stronger cohesive powers, and 

 capable by its dense concretion of forming solid 

 and inflexible organs. The substances which na- 

 ture has selected for this office are the salts of lime. 

 Sometimes the Carbonate, and sometimes the Phos- 

 phate of lime is employed for forming these hard 

 and unyielding structures ; and often both these 

 calcareous substances are united together in dif- 

 ferent proportions in the same solid fabric. When 

 the carbonate of lime predominates, or is the sole 

 earthy ingredient, it constitutes Shell: when there 

 is a greater proportion of the phosphate, it is called 

 a Crust, as is the case with the coverings of the 

 Lobster and the Crab : when the earthy matter 

 consists almost wholly of phosphate of lime, it com- 

 poses the different forms of Hone. I shall have 

 occasion to describe the formation and properties 

 of each of these structures in the sequel. 



Tiie protection of the delicate structure of the 

 fabric from the injurious influence of external 

 agents is an object of great importance in the ani- 

 mal economy, and is one which nature has shown 



