842 PHYSIOLOGY 



the precipitin may be looked upon therefore as a test by which 

 we may decide whether these proteins have passed through the 

 intestinal wall unchanged. In most cases it is found that, however 

 abundant the amount of protein administered in the soluble form, 

 none of it appears in the urine, 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 alimen- 

 tary 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 amino-acids, such as leucine and tyrosine, as the result 

 of digestion, they regarded their production as evidence of a waste 

 of material. Albumoses 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 the 

 physiologists were occupied with the problem as to the fate of these 

 peptones and albumoses after their entrance into the mucous mem- 

 brane. They do not pass as such into the blood. The injection of 

 small quantities of albumose 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 Schmidc 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 albumose or of peptone is to 

 be found in the blood. The observations of Hofmeister and others 



