CH. XXXVI.] THE MCES 549 



succus entericus, and finally because the larger food residue favours 

 the development of bacteria. On an average, from one-third to 

 one-fifth (varying with the diet) of the weight of dried faeces 

 consists of bacteria. The average weight of dried bacteria excreted 

 daily is 8 grammes ; this contains 0-8 gramme of nitrogen, or about 

 half the nitrogen of the faeces. Strasburger estimated that about 

 128,000,000,000,000 bacteria are evacuated in the fasces of a man 

 every day. The vast majority of these are dead. 



When cellulose is absent from the diet, the faeces contain from 

 65-75 per cent, of water; the dry residue contains about 7 per cent, 

 of nitrogen, and the non-nitrogenous material consists of about 

 equal quantities of ash and substances soluble in ether, with small 

 quantities of stercobilin and other bile residues. The ash contains 

 mainly calcium phosphate, with small amounts of iron and 

 magnesium. The ethereal extract contains cholesterin, lecithin, fatty 

 acids, soaps, and a very small amount of neutral fat. The proteins 

 are chiefly mucin and nucleo-protein, and are derived not from the 

 food, but from the intestinal wall, or are contained in the bacteria ; 

 no doubt a large part of the ethereal extract is also supplied by 

 the bacteria. 



Cellulose is thus the only important constituent of the food 

 which is unaffected by the digestive juices, although a variable 

 amount, which is largest in herbivorous animals, undergoes bacterial 

 decomposition. The presence of cellulose also interferes with the 

 absorption of proteins, for the digestive juices have difficulty in 

 penetrating the cellulose membranes of vegetable cells. Thus Voit 

 found that 42 per cent, of the nitrogen in the food were lost in the 

 faeces of a vegetarian. This is due solely to the cellulose and not 

 to any difference in the digestibility of animal and vegetable proteins, 

 for if vegetable food is finely subdivided, and then thoroughly cooked 

 and softened, this loss is lessened, and if vegetable protein is 

 entirely freed from cellulose, it is as thoroughly absorbed as animal 

 protein. Fifteen per cent, of the dry substance of green vegetables 

 and brown bread, 20 per cent, of carrots and turnips, and a still 

 larger amount of beans are lost in the faecal residue. 



The intestinal contents travel more rapidly when vegetables are 

 present, for the indigestible cellulose stimulates peristalsis, and there- 

 fore a large quantity of water escapes absorption in the colon. Thus 

 on an ordinary mixed diet 35 grammes of dry substance and 100 

 grammes of water are daily excreted in the faeces, whereas on a 

 vegetable diet the quantities are 75 and 260 grammes respectively. 



