4 THE VITAL FUNCTIONS. 



responding to these ever varying changes of con- 

 dition. The artificer, whose aim is to construct 

 a machine for permanent use, and to secure it as 

 much as possible from the deterioration arising 

 from friction or other cases of injury, wouhi, of 

 course, make choice for that purpose of the most 

 hard and durable materials, such as the metals, 

 or the denser stones. In constructing a watch, for 

 instance, he would form the wheels of brass, the 

 spring and the barrel-chain of steel ; and for the 

 pivot, where the motion is to be incessant, he 

 would employ the hardest of all materials,— the 

 diamond. Such a machine, once finished, being 

 exempt from almost every natural cause of decay, 

 might remain for an indefinite period in the 

 same state. Far different are the objects which 

 must be had in view in the formation of organized 

 structures. In order that these may be qualified 

 for exercising the functions of life, they must be 

 capable of continual alterations, displacements, 

 and adjustments, varying perpetually, both in 

 kind and in degree, according to the progressive 

 stages of their internal developement, and to the 

 different circumstances which may arise in their 

 external condition. The materials which nature 

 has employed in their construction, are, there- 

 fore, neither the elementary bodies, nor even 

 their simpler and more permanent combinations ; 

 but such of their compounds as are of a more 

 plastic quality, and which allow of a variable 



