vi INTESTINE AS AN OEGAN OF EXCBETION 345 



Apart from the nature of the diet, the daily amount ingested, 

 in excess of certain physiological limits, alters the amount of the 

 faeces. A superabundant meal, although it may consist wholly of 

 digestible substances, gives rise to more excreta, because a more or 

 less considerable portion escapes the action of the digestive enzymes, 

 and fails to come in contact with the absorbing surface of the 

 intestine. In a mixed diet of normal amount, the weight of the 

 human faeces is about -f~|- that of the ingested food (Liebig), 

 i.e. 120-150 grrns. with 30-37 grms. solid substances (C. Yoit). 



The consistency of the faeces varies with their water content, 

 which generally fluctuates between 68 and 82 per cent, within 

 physiological limits, i.e. excluding cases of diarrhoea, in which there 

 is much more water. It depends less on the quantity of water 

 drunk than on the vigour of intestinal peristalsis, the tone of the 

 intestinal vessels, and the state of the epithelium by which intestinal 

 absorption is regulated. 



The chemical reaction of the faeces varies greatly with the 

 scope and activity of the fermentative and putrefactive processes 

 in the different parts of the intestine, conditions which are not easy 

 to determine for individual cases. In the last part of the ileum 

 the reaction may be alkaline, neutral, or slightly acid; in the 

 large intestine it is usually distinctly acid. This depends, not on 

 acidity of the secretion from the mucous membrane (which is 

 alkaline, both in the small and in the large intestine), but on 

 acid fermentations in the faecal content ijdue to the intestinal 

 bacteria which decompose the carbohydrates. This is proved by 

 the fact that the acidity of the faeces is greatest when the diet is 

 rich in starchy and saccharine substances. 



The acidity of the faecal masses is mainly due to the presence 

 of lactic acid, derived from the lactic fermentation of sugar, the 

 vigour of which probably varies with the amount of carbohydrates 

 ingested and with other conditions of the intestinal tract, which 

 have not been exactly determined. 



The neutral or alkaline reaction sometimes exhibited by the 

 faeces depends essentially on the putrefactive processes of the 

 proteins, which give rise to a development of ammonia. 



The abundant secretion of mucus in the large intestine also 

 favours the neutral or alkaline reaction of the faeces. 



The colour of the faeces varies considerably according to the 

 nature of the food. Contrary to the general opinion, the bile 

 pigments and their decomposition products have little influence on 

 the normal colour of the dejecta. 



On an exclusive flesh diet the faeces, independent of the bile, 

 are blackish, owing to the presence of haematin and ferrous 

 sulphide. On an exclusive diet of brown or wholemeal bread they 

 are lighter in colour. On a diet rich in fat they are yellowish or 

 clay-coloured. In infants the greenish-yellow colour of the excreta 



