CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 479 



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 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 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 

 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 these must be added substances not distinctly 

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

 from the secretions of the alimentary canal. The faeces contain 

 mucus in variable amount, sometimes albumin, cholesterin, butyric 

 and other fatty acids, lime and magnesia soaps, colouring matters, 

 and inorganic salts, especially earthy phosphates, crystals of 

 ammonio-magnesia phosphates being very conspicuous. The 

 reaction is generally but not always acid. They also contain a 

 ferment similar in its action to pepsin, and an amylolytic ferment 

 similar to that of saliva or pancreatic juice. The bile salts are 

 represented by a small quantity of cholalic acid, or some product 

 of that body, and sometimes a very small quantity of taurin. The 

 glycin and most or all of the taurin have been absorbed from the 

 intestine, and the cholalic acid has been partly absorbed and partly 

 decomposed. The fact that the faeces become ' clay-coloured ' 



