478 CHANGES IN THE LARGE INTESTINE. [BOOK n. 



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 temperature of the body to produce 

 carbonic acid and hydrogen in equal volumes. This suggests the 

 possibility of the sugar of the intestinal contents undergoing the 

 butyric acid fermentation during which, as is well known, carbonic 

 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 con- 

 verted 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 nf 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 faecal 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 micro-organisms, 

 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 or 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 blood vessels, so that the contents of the small intestine, by 

 the time they reach the ileo-caecal 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. 



283. The contents, whether alkaline or not in the ileum 

 now become once more distinctly acid. This, however, is not 

 caused by any acid secretion 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 fermenta- 

 tions going on in the contents themselves; and that fermentations 

 do go on is shewn by the appearance of marsh gas as well as 



