THE ABSORPTION OF THE FOOD-STUFFS 747 



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 

 of 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 ammo-acids, such 

 as leucine and tyrosine, as the result of digestion, they regarded their pro- 

 duction 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 phy- 

 siologists 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 Hofmeister 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 pro- 

 teoses, which was in itself capable of producing small traces of these sub- 

 stances. 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 



