LACTIC ACID. 103 



Finally, after the discovery made by Redtenbacher, that 

 glycerine is convertible into metacetonic acid, there seems to be 

 something attractive in the hypothesis that glycerine, which, in the 

 metamorphosis of the fats, obviously undergoes an independent 

 change, is converted into lactic acid, which, as we have already 

 shown, is allied to metacetonic acid. As we have no probable 

 conjectures regarding the further course of the haloid base of the 

 fats in the animal body, it is possible that these substances may 

 contribute, through their base, to the formation of lactic acid. 



We have endeavoured, in the above sketch of the occurrence of 

 lactic acid in the animal body, to restrict ourselves most rigidly to 

 established facts, and we have rejected all those of our own experi- 

 ments on which the slightest doubt appeared to rest: without 

 referring to authorities, we have allowed the facts to speak for 

 themselves, and have attached as little credit to the negative 

 assertions of Liebig, as to the older experiments of Berzelius, 

 regarding the occurrence of lactic acid in bile, sweat, &c., with that 

 impartiality which becomes every one wishing to be an honest 

 scientific observer. We shall now consider the advantages which 

 may accrue to the animal organism from the occurrence of lactic 

 acid in this or that organ, without any reference to the views and 

 errors which we formerly maintained. Although we no longer 

 regard lactic acid as one of the most important elements in relation 

 to the metamorphosis of the animal tissues, it is yet of sufficient 

 importance to attract the attention of physiologists. It is more- 

 over obvious that questions regarding the function of a substance 

 in the animal body, can never receive more than a hypothetical 

 answer; for purposes may indeed be conjectured or understood, 

 but they cannot be palpably demonstrated. If, therefore, we judge 

 of the physiological importance of an animal substance on hypo- 

 thetical grounds, we do not necessarily adopt lax and untenable 

 illusions of the fancy, but shall confine ourselves to logical con- 

 clusions. 



Uses. In ascribing to lactic acid an essential influence on the 

 digestion of nitrogenous food, our opinion is based, not on a mere 

 conjecture derived from the constant occurrence of this acid in 

 the gastric juice, but on the result of direct experiments* with 

 artificial digestive fluids, from which it appears that lactic and 

 hydrochloric acids cannot be replaced in the process of digestion, 

 by any other animal or organic acids. The question how the acid 

 acts, will be entered into in our observations on " Digestion." 



* Berichte der Gesellsch. der Wiss. zu Leipzig. 1849. 



