1890.] Study of Natural and Artificial Digestions. 



197 



Leaving tyrosin out of account for the present, the more usual view 

 has been that any degradation of the nitrogen of proteids into the 

 amide form in the alimentary canal, accompanied by the probable 

 immediate conversion of the leucin into urea in the liver, and its 

 direct excretion as urea, implies a waste of energy which is improbable 

 on teleological grounds. And this view is supported by the state- 

 ments as to the absence of leucin (and tyrosin) from the alimentary 

 canal. Now, however, we have to deal with the fact that these amid- 

 ated acids are formed during proteid digestion. There are two possible 

 ways in which their formation may be of importance to the animal 

 economy. Although at first sight the conversion of part of the 

 proteid into leucin and its speedy elimination as urea seems to imply 

 a waste of energy to the body, it is not improbable, as Foster has 

 suggested, that this may be of real use to the economy. We know 

 that in artificial digestions the amount, nature, and digestibility of a 

 proteid, and the activity of the ferment which acts upon it, deter- 

 mine to some extent the course and results of the digestion. These 

 factors will probably make themselves still more felt in a natural 

 digestion in the body. Thus leucin may make its appearance to very 

 varying extents at different times in the same animal, and so provide 

 a sort of safety valve to the organism by diverting from the tissues 

 what would otherwise frequently be an unnecessarily large burden of 

 proteid metabolism. A second view is one which opens up the whole 

 question of the physiological significance of the amides in the animal 

 economy. It is known that the amides play an all-important part in 

 the nitrogenous metabolism of plants. Leucin and tyrosin occur 

 widely spread in many of the tissues and fluids of animals. In many 

 deranged conditions, notably of the liver, these substances occur in 

 largely increased amounts, so that a discharge of them takes place 

 from the body. It is thus possible that normally their occurrence in 

 smaller amounts is due to the fact that in the healthy organism 

 they are being continually used up in the chemical cycle of tissue 

 metabolism. Thus the real significance of their formation during 

 digestion may well be for the supply of new amidated compounds, to 

 take the place of those portions which must be always becoming 

 useless for further metabolic processes by continual wear and tear. 

 Examined in the light of all the facts which can be brought to bear 

 upon it, there is much evidence in support of this view. It is, 

 however, incompatible with the requirements of a preliminary com- 

 munication to enter fully here into a discussion and application of 

 this evidence. 



