1 10 THE MECHANICAL FUNCTIONS. 



gives them a determinate shape and firmness. 

 In all these cases bone, which, besides being 

 three times as heavy, is devoid of elasticity, and 

 liable to fracture, wotdd have been much less 

 suitable. Cartilage is often employed as an 

 intermedium for connecting different bones, as 

 for instance, between the ribs and the sternum, 

 or breast-bone ; whereby, besides the advantage 

 of greater lightness, the pliancy of the material 

 diminishes those jars which are incident to the 

 frame in all its violent actions. 



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 consti- 

 tuents. But substances of still greater hard- 

 ness, 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 framework of the 

 body, and for constructing levers of various kinds 

 to be employed in the more energetic move- 

 ments of the higher animals. For all these pur- 

 poses it was necessary to superadd a material 

 endowed with stronger cohesive powers, and 

 capable by its dense concretion of forming solid 

 and inflexible organs. The substances which 

 nature has selected for this office are the salts of 

 lime. Sometimes the Carbonate, and sometimes 



