CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 395 



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 fsecal and urinary pigments. 



Thus during the transit of the food through the small intes- 

 tine, 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- 

 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. 



233. 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 fermen- 

 tations going on in the contents themselves ; and that fermen- 

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

 well as hydrogen in this portion of the alimentary canal. The 

 character and amount of fermentation 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 in- 

 testine we have no very definite knowledge ; but since such 

 secretions as are afforded by the walls of the intestine itself do not 

 seem to contain any ferments, we may conclude that the changes 

 which do take place are effected by micro-organisms. It is 

 exceedingly probable that in the voluminous csecum of the her- 

 bivora a large amount of digestion of a peculiar kind goes on. 

 We know that in herbivora a considerable quantity of cellulose 

 disappears in passing through the alimentary canal, and even in 



