UTILISATION OF PROTEIDS IN THE ANIMAL 167 



is at work [Greenwood and Saunders]. Significant, too, as 

 Kutscher and Seeman have pointed out, is the circumstance 

 that, in plants, the reserve proteid — stored in seeds, tubers, 

 and bulbs — is utilised by the germinating organism only after 

 complete breakdown, whereas simple solution would suffice for 

 mere transport. 



Such considerations as these should not bias us into a too 

 ready acceptance of the experimental evidence bearing on what 

 actually is formed in the intestine, which is admittedly still 

 imperfect. Yet all the indications of modern research are in 

 favour of the belief that intact proteid molecules do not 

 normally enter the paths of assimilation. Thus Kutscher and 

 Seeman, confirming the less complete observations of Kuhne 

 and Lea, found that the material passed through an opening 

 made in the middle of the small intestine of a dog, in course 

 of full proteid digestion, contained amino-acids in quantity. 

 They failed to detect them in the blood, as have all observers ; 

 but this is no puzzle to any one acquainted with the conditions 

 of the circulation, and certainly does not in itself necessitate 

 a belief in the reconstitution of proteid from amino-acids 

 during their passage through the gut wall. The quite recent 

 experiments of Leathes and Cathcart show, indeed, that during 

 absorption of proteid material from the gut, non-proteid nitro- 

 genous substances are found in the blood in a definitely 

 increased amount. 



Confirmatory evidence for the probability of a complete 

 breakdown of proteid during digestion, is offered from another 

 side. An interesting line of experimentation initiated by Loewi 

 has shown that an animal maintains its normal equilibrium 

 when fed with no proteid other than what has been previously 

 digested as completely as possible. Whatever may be the 

 amount of intact proteid necessary for presentation to the 

 tissues, animals so fed must obtain it by a resynthesis, for their 

 food contained none. 



But certain very cogent facts have led recent thought to go 

 even further than has yet been indicated in departure from the 

 teaching of Liebig, and have suggested the view that a large 

 proportion of the amino-acids liberated in digestion may suffer 

 yet further breakdown before entering the domain of internal 

 metabolism as strictly defined. It is a fact long familiar, but 

 always puzzling, that the rate of excretion of nitrogen from the 



