356 PHYSIOLOGY CHAP. 



the starch introduced with the cereals or other important vegetable 

 food is completely digested and absorbed. 



Starch is never found in the faeces on feeding with wheat, 

 rye, or wholemeal bread, or with rice, potato, or pulse. It only 

 occurs when fresh vegetables, salad, and leguminous products 

 cooked whole are ingested ; or in the faeces of diarrhoea, when the 

 digestive apparatus is not able to perform its normal functions. 

 The cell-membranes, which consist of cellulose, are less digested 

 in proportion as they are thicker and harder : and these protect 

 the starch, protein, and fat from the action of the digestive juices. 

 The thin walls of the cell, on the contrary, can be dissolved by the 

 digestive juices. In any case, the amount of starch, protein, and 

 fat which escape the action of the digestive juices, owing to the 

 enclosing cellulose membrane, and are found undigested in the faeces, 

 is extremely small on an ordinary diet, since salad and fresh 

 vegetables do not usually form the staple food, but only supplement 

 it, cereals being ingested only after they have been ground and 

 converted into bread or farinaceous foods. 



Kermauner studied the amount of undigested residues of meat 

 found in the faeces with a full flesh diet. Nearly every one agrees 

 that the faeces of normal individuals constantly contain more or 

 less modified muscle fibres, in an amount that varies with the 

 quantity of meat ingested. The residues of meat found in the 

 faeces are thus not confined to indigestible parts, such as elastic 

 fibres, tendon, and cartilage, but also include a certain quantity 

 of undigested muscle. Kermauner shows, however, that the total 

 amount of these substances, under normal conditions of digestion, 

 is always very small, and seldom exceeds 1 per cent of the meat 

 ingested. In an ordinary mixed diet, therefore, the non-digested 

 food residues are a negligible quantity. The same may practically 

 be stated for man, as was determined by Voit for the dog, i.e. that 

 in a strict flesh diet no food residues occur in the faeces, the meat 

 being almost entirely digested and assimilated. 



From the physiological and hygienic standpoint, the chemical 

 investigation of human faeces methodically undertaken by Prausnitz 

 is more important. He attempted to ascertain the variations in 

 percentage composition of the content of nitrogen, ethereal extracts 

 (fats), and ash constituents, on different diets. 



In five normal individuals (two doctors, a medical student, and 

 two laboratory servants) he analysed the faeces formed during an 

 exclusively vegetable or mainly animal diet, both represented by 

 foods that can be almost entirely absorbed (meat, rice, fine wheat, 

 bread, butter). The values obtained were then compared with the 

 faecal analysis of a vegetarian. 



The following table sums up the results : 



