396 FAECES. [Book ii. 



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 ap- 

 proaches the rectum converted 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. 



§ 234. 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 un- 

 frequently 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 sub- 

 stances not distinctly recognizable as parts of the food but de- 

 rived for the most part from the secretions of the alimentary 

 canal ; when a portion of the intestine is isolated from the rest 

 so that no food enters into it, a quantity of material accumulates 

 in the interior and this in the course of time assumes a faecal 

 appearance.- 



The faeces contain "mucus in variable amount, sometimes al- 

 bumin, cholesterin, butyric and other fatty acids, lime and mag- 

 nesia 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 decom- 

 posed. The fact that the faeces becomes 'clay-coloured' when 

 the bile is cut off from the intestine shews that the bile-pigment 

 is at least the mother of the faecal pigment ; and a special pig- 

 111 Mt, which has been isolated and called stercobilin, is said to 

 b3 i lentical with the substance called urobilin, which may be 



