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Chapter XII. 



SECRETION. 



The capability of effecting certain chemical changes 

 in the crude materials introduced into the body, is 

 one of the powers which more especially charac- 

 terize life; but although this power is exercised 

 both by vegetable and by animal organizations, we 

 perceive a marked difference in the results of its 

 operation in these two orders of beings. The food 

 of plants consists, for the most part, of the simpler 

 combinations of elementary bodies, which are ela- 

 borated in cellular or vascular textures, and con- 

 verted into various products. The oak, for ex- 

 ample, forms, by the powers of vegetation, out of 

 these elements, not only the green pulpy matter of 

 its leaves, and the light tissue of its pith, but also 

 the densest of its woody fibres. It is from similar 

 materials, again, that the olive prepares its oil, and 

 the cocoa nut its milk ; and the very same ele- 

 ments, in different states of combination, compose, 

 in other instances, at one time the luscious sugar of 

 the cane, at another the narcotic juice of the 

 poppy, or the acrid principle of the euphorbium ; 

 and the same plant which furnishes in one part 

 the bland farina of the potatoe, will produce in 

 another the poisonous extract of the nightshade. 

 Yet all these, and thousands of other vegetable 

 products, differing widely in their sensible quali- 

 ties, agree very nearly in their ultimate chemical 

 analysis, and owe their peculiar properties chiefiy 



