DIGESTION AND ABSORPTION IN THE INTESTINES. 807 



human beings in cases of anus praeternaturalis, in which the lower 

 portion of the intestine was practically isolated, and also upon 

 lower animals, in which an artificial anus was established at the 

 end of the small intestine. These observations all indicate that 

 the secretion of the large intestine, while it contains much mucus 

 and shows an alkaline reaction, is not characterized by the presence 

 of distinctive enzymes. When the contents of the small intestine 

 pass the valve they still contain a certain amount of unabsorbed 

 food material. As was stated in the chapter on the movements of the 

 intestine, these contents remain a long time in the large intestine, 

 and since they contain the digestive enzymes received in the duo- 

 denum the digestive and absorptive processes no doubt continue as 

 in the small intestine. This general fact is well illustrated in experi- 

 ments made upon dogs, most of whose small intestine (70 to 83 

 per cent.) had been removed.* These animals could digest and 

 absorb well, and formed normal feces, provided care was taken with 

 the diet. An excess of fat or indigestible material caused diarrhea 

 and serious loss of food material in the feces. An interesting feature 

 in the large intestine is the marked absorption of water. In the 

 small intestine water is absorbed no doubt in large quantities, but 

 its loss is evidently made good by osmosis or secretion of water into 

 the intestine, since the contents at the ileocecal valve are quite 

 as fluid as at the pylorus. In the large intestine the absorption of 

 water is not compensated by a secretion; the material loses water 

 rapidly while in the ascending colon, and before it reaches the 

 descending colon it has acquired the consistency of the feces. The 

 alkaline reaction of the contents of the large intestine makes a 

 favorable environment for the growth of bacteria, particularly the 

 putrefactive bacteria that attack protein material. Putrefaction 

 is a normal occurrence in the large intestine, and much interest 

 has been shown in its extent and its possible physiological signifi- 

 cance. 



Bacterial Action in the Small Intestine. In the intestines are 

 found numerous bacteria which are able to hydrolyze the food 

 material, particularly the carbohydrates and proteins. Fermen- 

 tation of the carbohydrates gives rise to a number of organic 

 acids, such as lactic and acetic acid, but none of the products of 

 fermentation can be regarded as distinctly toxic. Putrefaction 

 of the protein molecule, on the other hand, gives rise to a number of 

 nitrogenous split products that are supposed to have a toxic action. 

 Under normal conditions, on a mixed diet, it appears that in the 

 small intestine carbohydrate fermentation is the characteristic 

 action of the bacteria, while in the large intestine, protein putre- 

 faction undoubtedly occurs. There has been considerable discus- 



* Erlanger and Hewlett, "American Journal of Physiology," 6, 1, 1902. 



