CHEMISTRY OF DIGESTION AND NUTRITION. 255 



found normally in the feces. Some of this carbohydrate and proteid under- 

 goes destruction by bacterial action, as has already been explained (p. 249), 

 but some of it is absorbed, or may be absorbed, before decomposition occurs. 

 The power of absorption in .the large intestine has been strikingly demon- 

 strated by the fact that various substances injected into the rectum are 

 absorbed and suffice to nourish the animal. Enemata of this character are 

 frequently used in medical practice with satisfactory results, and careful 

 experimental work on lower animals and on men under conditions capable of 

 being properly controlled has corroborated the results of medical experience 

 and shown that even in the rectum absorption takes place. Without giving 

 the details of this work, it may be said that it is now known that proteids in 

 solution, or even such things as eggs beaten to a fluid mass with a little salt, 

 are absorbed from the rectum, and this notwithstanding the fact that no 

 proteolytic enzyme is found in this part of the alimentary canal. The 

 theoretical bearing of this fact upon the general process of absorption will be 

 brought out in the next paragraph. Fats also (such as milk-fat) and sugars 

 can be absorbed in the same way. 



Absorption of Proteids. As we have seen in the preceding paragraphs, 

 absorption of proteids takes place in the stomach and the small and large 

 intestines, but in all probability mainly in the small intestine. The end- 

 products of the digestion of proteids by the proteolytic enzymes are proteoses 

 and peptones. Tryptic digestion produces also leucin, tyrosin, and the related 

 amido- bodies, but so far as proteid has undergone decomposition to this stage 

 it is no longer proteid, and does not have the nutritive value of proteid. The 

 logical conclusion from our knowledge of proteid digestion should be that 

 all proteid is reduced to the form of proteoses or peptones before absorption, 

 and that the great advantage of proteolysis is that proteids are more readily 

 absorbed in this form than in any other. Tn the main we must accept this 

 conclusion. The process of proteid digestion would seem to be without mean- 

 ing otherwise. But we must not shut our eyes to the fact that proteid may be 

 absorbed in other forms than peptones or proteoses. This has been demon- 

 strated most clearly for the rectum and the lower part of the colon, as was 

 stated in the preceding paragraph. Enemata of dissolved muscle-proteid 

 (myosin), egg-albumin, etc. are absorbed from this part of the alimentary canal 

 without, so far as can be determined, previous conversion to peptones and 

 proteoses, and we must admit that the same power is possessed by other 

 parts of the intestinal tract. It is probable, for instance, that the very first 

 product of pepsin-hydrochloric digestion, syntonin, is capable of absorption 

 directly. This fact, however, does not weaken the conclusion that peptones 

 and proteoses are absorbed more easily than other forms of proteids, and that 

 they constitute the form in which the bulk of our proteid is absorbed. 

 Opinions as to why these forms of proteids are more easily absorbed than any 

 other must vary with the theory held as to the nature of absorption. It was 

 formerly believed that absorption is entirely a process of imbibition and 

 osmosis through the mucous membrane. The fact that proteoses and peptones 



