2 THE VITAL FUNCTIONS. 



of the internal organs, we may trace the operation 

 of many causes inevitably leading to their ultimate 

 destruction. Continued friction must necessarily 

 occasion a loss of substance in the harder parts of 

 the frame ; and evaporation is constantly tending 

 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 con- 

 traction, and which, if suffered to continue, would, 

 in no long time, render them incapable of exer- 

 cising their proper functions ; and the same obser- 

 vation applies also to the nerves, and to all the 

 other systems of organs. Provision must accord- 

 ingly be made for remedying these constant causes 

 of decay by the supply of those peculiar materials, 

 which the organs require for recruiting their de- 

 clining energies. 



It is obvious that the developement of the organs, 

 and general growth of the body, 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 pur- 

 pose, be incorporated with those which previously 

 existed, and become identified with the living sub- 

 stance. Thus we often find structures forming in 

 the bodies of animals, of a nature totally different 

 from that of the part from which they arise. 



In addition to these demands, a store of materials 

 is also wanted for the reparation of occasional 

 injuries, to which, in the course of its long career, 

 the body is unavoidably exposed. Like a ship 



