CHAP, i.] DIGESTION. 299 



In the large intestine, the contents become once more distinct- 

 ly 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 fermentations 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. 



Of the particular changes which take place in the large 

 intestine we have no 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 probably 

 digested. It seems probable that this cellulose digestion is carried 

 on in the large intestine, though we know nothing of the nature of 

 the agency by which it is effected, and possibly the conversion may 

 take place elsewhere as well ; indeed recent evidence goes to shew 

 that in ruminants the change takes place in part in the stomach 

 and that it is effected by the saliva. The other digestive changes 

 are probably of a fermentative kind. 



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

 but complete at the ileo-csecal 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. 



In the faeces there are found in the first place the indigestible 

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

 hairs and other corneous elements, much cellulose and chlorophyll 

 from vegetable, and some connective tissue from animal food, frag- 

 ments of disintegrated muscular fibre, fat-cells, and not unfre- 

 quently 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 

 introduced as food, but arising as part of, or as products of, the 

 digestive secretions. The fasces contain a ferment similar to 

 pepsin, and an amylolytic ferment similar to that of saliva or 

 pancreatic juice. They also contain mucus in variable amount, 

 sometimes albumin, cholesterin, butyric and other fatty acids, lime 

 and magnesia soaps, excretm (a non-nitrogenous crystalline body, 



