32 INTRODUCTION. 



of the animal body are composed of chemical ele- 

 ments which the blood contains, each of such fluids 

 and solids being distinguished by the various propor- 

 tions it possesses of such elements ; whence we dis- 

 cover that their formation depends upon certain sub- 

 tractions from the elementary composition of the 

 blood with perhaps, in a very few cases, the addition 

 of some foreign element. 



These operations, by which the nutritive fluid 

 sustains or renovates the solid or liquid materials of 

 the various parts of the body, might justly receive the 

 general name of secretions ; this appellation, however, 

 is usually restricted to the production of liquids, 

 while that oi nutrition is more especially appropriated 

 to the production and deposition of the materials, 

 which are necessary for the sustenance of the 

 solids. 



The composition of every solid organ, and of every 

 fluid, is suitable to the peculiar functions of each, 

 and is preserved entire, as long as health continues, 

 by the repairing operation of the blood. The blood 

 itself, from the constant supplies which it thus fur- 

 nishes, is undergoing a perpetual alteration, but is 

 in its turn restored to its proper state by digestion, 

 which renews its material, by respiration, which 

 deprives it of the superfluous hydrogen and carbon, 

 and by perspiration, $c, which relieve it from other 

 superabundant particles. 



These perpetual transformations of chemical com- 

 position, constitute a no less essential part of the 

 vital action than the visible movements and alimen- 



