312 THE TISSUES AND MECHANISMS OF DIGESTION. 



takes place we do not know ; the amount probably varies according to the 

 amount of carbohydrates eaten, the condition of the alimentary canal, and 

 other circumstances. It may be under certain circumstances simply a' part 

 of normal digestion ; under other circumstances it may be excessive and 

 give rise to troubles. 



That fermentative changes may occur in the small intestine is further 

 indicated by the facts that the gas there present may contain free hydrogen, 

 and that chyme, after removal from the intestine, continues at the tempera- 

 ture of the body to produce carbonic acid and hydrogen in equal volumes. 

 This suggests the possibility of the sugar of the intestinal contents under- 

 going the butyric acid fermentation, during which, as is well known, car- 

 bonic anhydride and hydrogen are evolved. By this change the sugar is 

 removed from the carbohydrate group into the fatty acid group ; it is thus, 

 so to speak, put on its way to become fat. We shall see hereafter that sugar 

 may be somewhere in the body converted into fat ; this conversion, however, 

 takes place chiefly if not wholly in the tissues, and such change as may take 

 place in the alimentary canal is to be regarded as suggestive rather than as 

 important. 



The hydrogen thus occurring in the intestine may also arise from the 

 proteid decompositions spoken of above. However arising it may act as a 

 reducing agent reducing sulphates, for instance and thus giving rise to 

 sulphides and to sulphuretted hydrogen ; as a reducing agent it assists in 

 the formation of the fecal and urinary pigments. 



Thus, during the transit of the food through the small intestine, by the 

 action of the bile and pancreatic juice, and possibly to some extent of the 

 succus entericus, assisted by various microorganisms," the proteids are largely 

 dissolved and converted into peptone and other products, the starch is 

 changed into sugar, the sugar possibly being in part further converted into 

 lactic and other acids, and the fats are largely emulsified and to some extent 

 saponified. These products as they are formed pass into either the lacteals 

 or the portal bloodvessels, so that the contents of the small intestine, by the 

 time they reach the ileo-csecal valve, are largely but by no means wholly 

 deprived of their nutritious constituents. So far as water is concerned, the 

 secretion of water into the small intestine maintains such a relation to the 

 absorption from it that the intestinal contents at the end of the ileum, 

 though much changed, are about as fluid as in the duodenum. 



In the Large Intestine. 



244. The contents, whether alkaline or not, in the ileum now become 

 once more distinctly acid. This, however, is not caused by any acid secre- 

 tion from the mucous membrane ; the reaction of the intestinal walls in the 

 large as in the small intestine is alkaline. It must, therefore, arise from 

 acid fermentations going on in the contents themselves ; and that fermenta- 

 tions do go on is shown by the appearance of marsh gas as well as hydrogen 

 in this portion of the alimentary canal. The character and amount of fer- 

 mentation probably depend largely on the nature of the food, and probably 

 also vary in different animals. 



Of the particular changes which take place in the large intestine we 

 have no very definite knowledge ; but it is exceedingly probable that in the 

 voluminous caecum of the herbivora a large amount of digestion of a pecu- 

 liar kind goes on. We know that in herbivora a considerable quantity of 

 cellulose disappears in passing through the alimentary canal, and even 

 in man some is digested. It seems probable that this cellulose digestion 

 takes place in the large intestine, and is the result of fermentative changes 



