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II 







THE ABSORPTION OF THE FOODSTUFFS 793 



nor is any precipitin formation aroused. Ascoli has however observed such 

 events occasionally to follow the administration of large doses of egg white, 

 and it has been shown that there is a difference in the behaviour of animals to 

 the introduction of soluble protein into their alimentary canal, according as 

 they are new born or are more than a few days old. It seems that during the 

 first few days of life the cellular lining of the alimentary canal is permeable 

 to foreign proteins, whereas later on any protein which is taken up unchanged 

 from the gut does not arrive in the same unchanged condition in the blood 

 stream. 



The absorption however of unchanged proteins can play but a small 

 part in the assimilation of protein as a whole. Animals very rarely take 

 coagulable proteins in a condition in which they will arrive at the small 

 intestine in a state of solution unchanged. Even in the carnivora the living 

 tissues taken into the stomach will undergo coagulation by the acid, and will 

 then be dissolved by the gastric juice. In man practically all the proteins of 

 the food are either insoluble or are rendered insoluble by the process of 

 cooking. For absorption to take place it is therefore necessary that this 

 insoluble or coagulated protein should be brought into solution, and this 

 process is accomplished, together with hydration, by means of the ferments 

 f the gastric and pancreatic juices. 



This process of solution has long been regarded as the chief object of the digestive 

 ferments. Although both Kiihne and Schmidt Miilheim were aware of the production 

 of amino-acids such as leucine and tyrosine as the result of digestion, they regarded 

 their production as evidence of a waste of material. Proteoses and peptones are soluble, 

 diffusible, and rapidly absorbed from the alimentary canal, and there is no doubt that 

 a large proportion of the products of protein digestion are taken up by the absorbing 

 membrane in this form. For many years physiologists were occupied with the problem 

 as to the fate of these peptones and proteoses after their entrance into the mucous 

 membrane. They do not pass as such into the blood. The injection of small quantities 

 of proteose and peptone into the blood gives rise to the excretion of these substances by 

 the kidneys ; injection of larger quantities has pronounced poisonous effects, which were 

 first studied by Schmidt Miilheim and Fano. If samples of blood be taken either from 

 the portal vein or from the general circulation after a heavy protein meal, no trace 

 either of proteose or of peptone is to be found in the blood. The observations of Hof- 

 meister and others to the contrary depend on the fact that these observers employed a 

 method for the separation of coagulable protein, as an antecedent to the testing for 

 proteoses, which was in itself capable of producing small traces of these substances. 

 Hofmeister showed that during the absorption of a protein meal the mucous membrane 

 either of the stomach or of the intestine, if rapidly killed by plunging into boiling water 

 directly it was taken from the animal, always contained a considerable amount of 

 peptone, and similar observations were made by Neumeister. If however the mucous 

 membrane was kept warm for half an hour after removal from the body, the peptone 

 disappeared. Salvioli, under Ludwig's guidance, introduced peptone into a loop of 

 gut which was kept alive by passing defibrinated blood through its vessels. At the 

 end of some hours the loop was found to contain a certain amount of coagulable protein, 

 but no trace of peptone, nor was any trace of the latter substance found in the blood 

 which had been passed through the vessels. These observations were interpreted as 

 pointing to a regeneration in the intestinal wall of coagulable protein from the proteose 

 and peptone taken up from the gut, and opinions were divided whether the most 

 important part of this regeneration was to be ascribed to the leucocytes of the villi 

 (Hofmeister) or to the epithelial cells of the mucous membrane itself. 



It is evident that such a conclusion was not justified by the experiments. All that 



