UTILISATION OF PROTEIDS IN THE ANIMAL 165 



But in what form does ingested proteid leave the cavity of 

 the alimentary canal ? How is it presented to the absorbing 

 mechanism ? Owing to the conditions which prevail in the 

 animal, the endeavour to obtain an experimental answer to 

 this question has proved extraordinarily difficult ; and the 

 increasing ingenuity displayed recently in attacking it has 

 only emphasised the elusive nature of the problem. We have 

 no complete answer, but evidence has accumulated which 

 is extremely suggestive, and it points to a conclusion very 

 different from that arrived at by earlier workers. So far from 

 the proteid being (as Liebig thought) scarcely altered in its 

 essential composition during physiological digestion, it is 

 probably broken down in the completest way, and leaves the 

 gut lumen in the form of the liberated individual amino- 

 acids. 



A priori considerations have earlier stood in the way of 

 accepting this conclusion — considerations which we now see 

 have no weight. In the first place, a belief, due originally to 

 the influence of Liebig, that the animal body has very small 

 power of determining chemical synthesis, made such a conception 

 of proteid metabolism unacceptable, for synthesis in the tissues 

 would have to follow such a breakdown in the bowel. But 

 increased knowledge has made us well able to credit the body 

 with such synthetic powers as these. The animal cannot 

 emulate the primary magic of the green plant, which, by the 

 aid of the sun's energy, calls forth organic material with high 

 potential energy from inorganic stuff containing minimal 

 potential energy ; but once this step has been taken, the animal 

 cell seems to have a wide range of synthetic capacities. Another 

 apparent objection had teleological bearings. Such decom- 

 position of proteid in the gut would be " a waste of chemical 

 potential energy which would serve no purpose " [Bunge]. 

 But we now know that hydrolytic breakdown involves only 

 minimal changes in potential energy, and the mixture of amino- 

 acids, derived from a proteid, contains well-nigh the whole of 

 the energy of the original substance [Rubner, Loewi]. 



Many considerations now prepare us indeed for a ready 

 belief in the completeness of the breakdown. 



We have come first of all to realise more fully how efficient 

 is the organisation of proteoclastic enzymes in the alimentary 

 tract. We know that the pepsin of the gastric juice does not 



