ABSORPTION AND THK ROUTES OF FOOD. 355 



is not readily absorbed. Traces of peptones and sugars, 

 and possibly salts may be absorbed when one speaks mathe- 

 matically, but for practical physiological purposes we have 

 to turn to the small intestine for this function. 



THE ABSORPTION OF THE PEPTONES. 



It will be remembered that the various proteids taken in 

 the body are by the digestive changes of the pepsin and 

 trypsin finally converted into peptones, leaving out of con- 

 sideration for the present small bits of peptones which have 

 been disintegrated still further into leucin and tyrosin. 

 These peptones are dialyzable, and so there seems at first 

 no difficulty in understanding how they might enter the 

 blood. But the difficulty presents itself quite seriously when 

 it is recalled that peptones are not found in the blood any- 

 where, not even in the blood coming directly from the in- 

 testines. Evidently these peptones are changed after leaving 

 the intestine and before reaching the blood. In fact pep- 

 tones injected into the blood are poisonous. The blood 

 coming from the intestines and carrying the absorbed food 

 contains not a trace of peptone, but contains those albumens 

 of the blood treated at length in the chapter on coagu- 

 lation. It is, therefore, evident that the peptones were 

 changed back into albumens in the act of passing through 

 the intestinal walls. Experiments have been made to dem- 

 onstrate this fact. Portions of fresh intestine have been 

 removed from the body, filled with a solution of peptones, 

 and the ends tied. This intestine was then immersed in a 

 liquid containing not a trace of peptones and allowed to 

 remain there until most of the peptones had disappeared in 

 the intestine, but not a trace of peptone was found in the 

 outside solution. Here it was present in the form of albu- 

 mens. It would be interesting to know in what manner this 

 change was effected. Is it due to the action of the epi- 

 thelium cells? Is it a change brought about by the lym- 

 phatic tissue which is so plentiful in the walls of the 

 intestine? These questions are, however, much more easily 



