184 MANUAL OF PHYSIOLOGY. 



kind of diet, being greater with a vegetable than meat diet, 

 averaging about 150 grammes a day (60-250 grms.). This 

 amount may be greatly increased by largely partaking of indi- 

 gestible forms of food. The more rapid the passage of the ingesta 

 through the intestine the greater is the amount of fluid remaining 

 with the faeces, so that any stimulant to the intestinal movements 

 reduces the consistence of the faeces and facilitates the evacuation. 

 The factor depends in a great measure on the presence of indol, 

 which is an outcome of pancreatic digestion, and also upon the 

 presence of certain volatile fatty acids. The color depends upon 

 the amount of the bile pigment and the degree of change the latter 

 has undergone. 



The faeces are composed of (1) the undigested parts of the food, 

 and (2) the useless or injurious parts of the secretions of the various 

 glands. In the first category we find perfectly indigestible stuffs 

 such as yellow elastic tissue, horny structure, portions of hairs from 

 animal food, and cellulose woody fibre and spiral vessels from 

 plants, and also masses of digestible substances which have been 

 swallowed in too large pieces to be throughly acted on by the secre- 

 tions. All forms of food may thus appear in the faeces, but most 

 commonly vegetable substances are conspicuous. 



In the second category we find a variable quantity of mucus 

 and the decomposed coloring matter of the bile, together with 

 some cholic acid, cholesterin, etc. 



A few inorganic substances are found, mainly those which diffuse 

 with difficulty, as calcium salts and ammonio-magnesium phos- 

 phate. 



Putrefactive Fermentations in the Intestine. 



With the air and saliva which are swallowed mixed with the food, 

 large numbers of the lower organisms existing in them are intro- 

 duced into the alimentary canal. 



The effect of these organisms is to produce certain fermentative 

 changes quite distinct from the action of the special ferments 

 peculiar to the digestive fluids. 



This is proved by the composition of the gases found in the in- 

 testine. Atmospheric air only is introduced from without, and 



