194 FUNCTIONS OF NUTRITION. 



in the caecum of herbivorous animals, although the mucous surface of 

 the large intestine is constantly alkaline. 



As the remnants of the alimentary mass pass the ileo-caecal valve 

 into the large intestine, they acquire a pasty consistency and a repulsive 

 odor. Both these changes become more marked in the middle and lower 

 part of the gut, until all the superfluous fluids have disappeared, and the 

 consistency and odor of the feces are fully developed. This is not a 

 putrefactive odor, but is characteristic of the contents of the large in- 

 testine. Its source may be either a peculiar transformation of some 

 of the ingredients of the food, or an excretory action of the intestinal 

 mucous membrane. It is probably in great part the result of an excre- 

 tion, since in different animals, whatever the nature of their food, the 

 feces have usually a distinct odor characteristic of the species. 



The average daily quantity of feces in man is 150 grammes, of which 

 about 75 per cent, is water and 25 per cent, solid residue. They consist, 

 first, of undigested remnants of the food, and secondly, of excreted mate- 

 rial from the alimentary canal. The undigested substances derived from 

 the food are mainly animal or vegetable tissues, which, from their con- 

 stitution, are incapable of digestion. These are elastic fibres, or bits of 

 elastic tissue, which nearly always pass the intestine unchanged ; shreds 

 of tendon or fascia not sufficiently softened by cooking ; horny epidermic 

 tissues, both animal and vegetable ; and the spiral tubes and ducts of 

 vegetable substances. The excreted materials are the mucus of the large 

 intestine and probably also the volatile substances which produce the 

 fecal odor. The coloring matters of the bile are present in a more or 

 less altered form. 



The mineral salts in the feces amount to a little over one-tenth of 

 the solid ingredients. They are for the most part the same with those 

 of the animal fluids in general, but are mingled in different proportions ; 

 only about 4 per cent, consisting of the soluble chlorides and sulphates, 

 while fully 80 per cent, are composed of lime and magnesium phos- 

 phates. They are regarded as derived partly from the unabsorbed 

 mineral ingredients of the food, and partly from the intestinal secretions. 



