DIGESTION. 689 



the same process. The pepsin action is exerted mainly to effect the 

 earlier actions, i. e., the change from protein through primary and 

 secondary proteose to peptone ; the erepsin is concerned mainly with 

 the later transformation from peptone to polypeptide and amino- 

 bodies (arnino-acids), while the action of trypsin is important through- 

 out the entire decomposition. The general course and ultimate results 

 of protein digestion are now fairly clear, and it is universally accepted 

 that proteins are absorbed in the form of the amino-bodies or, perhaps, 

 in part as fairly simple polypeptides, the protein nuclei of Abder- 

 halden. The digestion of protein is believed to have a deeper signifi- 

 cance than (as in the case of the carbohydrates and fats) the mere pro- 

 duction of soluble and dialyzable substances. As the protein material 

 absorbed by the intestine is converted again to protein, and as the 

 ingested (foreign) protein must give rise to a different (native) 

 protein, it is easy to see that this change must be much more readily 

 carried out if the foreign protein is first split into its component parts, 

 the amino-bodies. Abderhalden believes that not all the protein is 

 necessarily split entirely to amino-acids, but part may remain in the 

 polypeptide form and serve as a nucleus to which the other amino- 

 bodies may be added which are needed for the production of the 

 native protein. The native protein thus formed is conveyed by the 

 portal system to the liver. 



Absorption in large intestine. As the large intestine secretes no 

 enzymes, the only digestive action here is due to the enzymes which 

 have been brought down from above. This is probably not of any 

 great moment, as experimental work has shown that, with the excep- 

 tion of water, there is little absorption by the large intestine. It is, 

 however, true that clinical work with rectal feeding shows that per- 

 sons may be sustained to a certain extent by rectal injections of pre- 

 digested food mixtures. 



Feces consist of the unabsorbed material from the food, the waste 

 material excreted from the blood, detached epithelium, and the secre- 

 tions of the intestine. The odor depends largely on the indole and 

 skatole, to a less degree on valeric and butyric acids, and on hydro- 

 gen sulphide present. The quantity and composition of feces passed 

 depend on the nature of the food and the energy of the digestive 

 powers. A grown person in normal condition discharges from 100 

 to 250 grammes (4 to 9 ounces) daily. A diet rich in animal proteins 

 causes the quantity of feces to be small, while a diet rich in vegetable 

 44 



