2 THE VITAL FUNCTIONS. 



every other mechanical contrivance, is subject 

 to wear and deteriorate by constant use. Not 

 only in the greater movements of the limbs, but 

 also in the more delicate actions of the internal 

 organs, we may trace the operation of many 

 causes inevitably leading to their ultimate des- 

 truction. Continued friction must necessarily 

 occasion a loss of substance in the harder parts 

 of the frame ; and evaporation is constantly tend- 

 ing to exhaust the fluids. The repeated actions 

 of the muscles induce certain changes in these 

 organs, both in their mechanical properties and 

 chemical composition, which impair their powers 

 of contraction, and which, if suffered to continue, 

 would, in no long time, render them incapable 

 of exercising their proper functions; and the 

 same observation applies also to the nerves, and 

 to all the other systems of organs. Provision 

 must accordingly be made for remedying these 

 constant causes of decay by the supply of those 

 peculiar materials, which the organs require for 

 recruiting their declining energies. 



It is obvious that the developement of the 

 organs, and general growth of the body, must 

 imply the continual addition of new particles 

 from foreign sources. Organic increase consists 

 not in the mere expansion of a texture previously 

 condensed, and the filling up of its interstices 

 by inorganic matter ; but the new materials that 

 are added must, for this purpose, be incorporated 

 with those which previously existed, and become 



