CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 505 



tions going on in the contents themselves ; and that fermentations 

 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. 



So far as observations go at present, such secretion as is 

 furnished by the walls of the large intestine does not contain any 

 unorganized ferment capable of acting on any of the constituents 

 of food. If this be the case all the changes which take place in 

 the large intestine, except those which the ferments brought down 

 from the small intestine can effect before they are stopped by the 

 acid-reaction, must be carried out by means of organized ferments, 

 by means of micro-organisms. 



Concerning the exact nature of these changes 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 peculiar kind goes on. We know that in herbivora a con- 

 siderable 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 carried out by 

 means of micro-organisms, marsh gas being one of the products 

 formed at the same time. 



Be this as it may, whether digestion, properly so called, is all 

 but complete at the ileo-caecal valve, or whether important changes 

 still await the chyme in the large intestine, one great characteristic 

 of the work done in the colon is absorption. By the abstraction of 

 all the soluble constituents, and especially by the withdrawal of 

 water, the liquid chyme becomes as it approaches the rectum con- 

 verted into the firm solid faeces, and the colour shifts from the 

 bright orange, which the grey chyme gradually assumes after 

 admixture with bile, into a darker and dirtier brown. 



The Fceces. 



284. These consist in the first place of the indigestible and 

 undigested constituents of the meal : shreds of elastic tissue, hairs 

 and other horny elements, much cellulose and chlorophyll from 

 vegetable, and some connective tissue from animal food, fragments 

 of disintegrated muscular fibre, fat-cells, and not unfrequently 

 undigested starch-corpuscles. The amount of each must of course 

 vary very largely according to the nature of the food, and the 

 digestive powers, temporary or permanent, of the individual. In 

 the second place, to the above must be added substances not dis- 

 tinctly recognisable as parts of the food but derived for the most 

 part from the secretions of the alimentary canal ; these, indeed, 

 as we have said, may be alone, as in an isolated portion of the 

 intestine, sufficient to give rise to a faecal mass. The faeces contain 

 F. ii. 33 



