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animals, which is capable of sustaining the growth or 

 continued integrity of such tissues ; and that the non- 

 azotized constituents of the food, may be capable of con- 

 version into fat or gelatine, or may serve an important 

 purpose connected with the function of respiration, and the 

 maintenance of the elevated temperature of animal bodies, 

 which will be afterwards explained, — or are, otherwise, merely 

 excrementitious. This, however, seems to be a hasty and 

 questionable conclusion. When we consider that carrots, 

 turnips, and other esculent roots, and more especially the 

 tubers of the potato, consist almost exclusively of non- 

 azotized principles, together with small proportions of earthy 

 salts, — that these are found to be decidedly nutritious, — 

 and that the potato forms so important an article of food 

 to the poor, particularly the Irish peasantry, who have 

 muscles and sinews like other persons, and are not more 

 remarkable for their fat than those who enjoy a more 

 highly azotized food, — we shall perhaps be inclined to ques- 

 tion the validity of Liebig's theoretical conclusion. It will 

 also appear the more questionable, when we find a direct 

 proof that animal bodies do possess a converting power 

 over the materials of their food, by which an azotized prin- 

 ciple is formed, differing from any present in the food. It 

 is a remark of Pereira's* upon this point, that Liebig 

 himself acknowledges that the chief constituent of nervous 

 matter is a peculiar chemical principle, which is formed in the 

 animal body exclusively, not being found in any vegetable 

 substance, and which the purely herbivorous animal must, 

 therefore, be constantly forming de novo. Urea, likewise, 

 and other azotized excrementitious matters, must be con- 

 stantly forming in the body, from the elements of effete 

 particles previously having some other arrangement. 



* Dr. Pereira on Food and Diet, p. 189. 



