DIGESTION AND ABSORPTION OF PROTEIN 15 



many previous workers, they concluded that it had been broken down. 

 They suggested, as the result of some of their experiments, that the 

 mammary gland might be looked on as a channel of excretion for 

 caseinogen. They found later (283) that, if horse serum were injected 

 into dogs, the nitrogen equilibrium could be maintained and they con- 

 cluded therefore that the tissue cells could to a certain extent take on 

 the specific function of the intestinal mucous membrane. They only 

 got excretion of protein in the urine after a very large dose of serum. 

 Heilner (183) has suggested that this utilization of injected protein is 

 brought about by the generation of a special ferment He found that 

 the injected serum was well utilized. In this connexion the recent 

 work of Abderhalden and his pupils is of interest (see p. 18). 



Freund and Popper (143) carried out a series of interesting experi- 

 ments in which they examined the blood of animals with and without 

 the intestine cut out of the circulation. They found five minutes after 

 the intravenous injection of a solution of peptone or other product of 

 protein digestion that about 50 per cent, of the injected material could 

 not be recovered, due simply to the distribution throughout the body. 

 Of the other 50 per cent, they found that, if the intestine were in the 

 circulation, only 1 5 to 20 per cent, was recoverable from the blood after 

 twenty minutes, whereas with the intestine out of circulation practically 

 the whole 50 per cent, was recovered. They demonstrated further that 

 in the first instance about 32 per cent, of the material recovered was so 

 far broken down that it no longer gave a precipitate with tannic acid, 

 whereas in the case with the intestine absent only some 12 to 18 per 

 cent, was thus changed. They could obtain no direct evidence to show 

 that any part of the injected material was changed into a coagulable 

 form. This work, of cours'e, is strong evidence in favour of the conten- 

 tion that the intestine plays some important part in the preparation even 

 of parenterally introduced protein before it is utilized by the tissues. 



