80 CHEMICAL COMPONENTS OF THE HUMAN BODY. 



the muscular substance, but is one of the products of the direct transformation 

 of that substance consequent upon the exertion of its vital powers, appears from 

 this, that it is found in the muscles of purely carnivorous animals in no smaller 

 amount than in those of the animals that have consumed amylaceous or saccha- 

 rine matters as food ; and although it has not yet been produced artificially, either 

 by fermentation or otherwise, from any nitrogenous animal matter, yet, as is 

 pointed out by Lehmann, there are strong indications that such a conversion is 

 by no means hypothetical ; and all that has been said of the generation of sugar 

 in the body ( 46), of course applies ecjually to lactic acid. The occasional pre- 

 sence of lactic acid in the urine, appears to have been fully proved by the re- 

 searches of Lehmann; who has shown that, although it cannot be considered as 

 a normal constituent of that excretion, it is liable to appear there, whenever the 

 quantity introduced into the blood in a given time, whether by the transforma- 

 tion of the amylaceous and saccharine constituents of the food, or by the meta- 

 morphosis of muscular tissue, is greater than the respiratory process can carry 

 off; and thus it may be habitually present in the urine of individuals whose re- 

 spiration is obstructed, notwithstanding that no actual excess of it has been gene- 

 rated within their bodies. Lactic acid seems also to be occasionally present in 

 the sweat (which owes its sour smell, however, to acetic acid) ; and its presence 

 has been suspected also in the bile, though with respect to this it would not be 

 safe to make a positive statement. 



50. On the whole, then, it may be positively affirmed, that Lactic acid is a 

 normal constituent of the Human body, and that it is to be looked on under two 

 aspects, both as to its origin and its destination. Its origin may be attributed : 

 1st, to the direct transformation of the amylaceous and saccharine constituents 

 of the food ; and 2d, to the metamorphosis of muscular and (probably) other 

 azotized tissues. On the other hand, its destination may be considered as being : 

 1st, to supply a pabulum for the combustive process, and thus to contribute 

 in maintaining the heat of the body ; and 2d, to take part in the reduction of 

 the albuminous and other constituents of food in the stomach, either by itself 

 acting as the solvent, or by decomposing the chlorides of calcium or sodium 

 contained in the gastric fluid, and by thus setting free hydrochloric acid. Its 

 presence in the urinary secretions may be regarded as exceptional ; the kidneys 

 affording (so to speak) a safety-valve, whereby the accumulation of lactic acid 

 in the blood is prevented. 



5. Excrementitious Substances. 



51. Although it might seem more correct to proceed, in the next place, to 

 speak of the Mineral or Inorganic constituents of the Human body, yet it will 

 better serve the purpose of illustrating the chemical metamorphoses which take 

 place in the economy, if we next direct our attention to those Organic compounds, 

 which may be regarded as the products of the disintegration of the tissues, or 

 of the decomposition of superfluous alimentary matter, and which, wherever 

 they are found within the living body, may be considered as on their way to be 

 eliminated from it by the action of the excretory organs. Of these excremen- 

 titious matters it may be stated generally, that, although their composition is 

 such as (with very few exceptions) the Chemist is unable to imitate by the 

 artificial union of their components, yet it is far simpler than that of the Histo- 

 genetic substances ; the number of the combining equivalents of their elements 

 being smaller, and the mode in which they are united (as indicated by the de- 

 compositions of which these compounds are susceptible) being usually much 

 more apparent. Moreover, several of them are remarkable for their capability 

 of assuming a crystalline form ; which, as long since pointed out by Dr. Prout, 

 is peculiarly indicative of their incapacity for serving as materials for the con- 



