358 A MANUAL OF PHYSIOLOGY 



to other than the soluble ferments of the digestive tract, 

 although we have incidentally mentioned the action of the 

 lactic acid bacilli on carbohydrates and of the fat-splitting 

 bacteria on fats. It is now necessary to recognise that the 

 presence of bacteria is an absolutely constant feature of 

 digestion ; and although their action must in part be looked 

 upon as a necessary evil which the organism has to endure, 

 and against the consequences of which it has to struggle, it 

 is not unlikely that in part it may be ancillary to the processes 

 of aseptic digestion. But bacteria are not essential, as some 

 have supposed. For it has been shown that a young guinea- 

 pig, taken by Caesarean section from its mother's uterus 

 with elaborate antiseptic precautions, and fed in an aseptic 

 space on sterile milk, grew apparently as fast as one of 

 its sisters brought up in the orthodox microbic way. The 

 alimentary canal remained free from bacteria (Nuttall and 

 Thierfelder). 



Among the more important actions of bacteria on the 

 proteid food-products in the intestines may be mentioned 

 the formation of indol, phenol, and skatol, the first having 

 tyrosin for its precursor, and being itself after absorption 

 the precursor of the'indican'in the urine; the second being 

 to a small extent thrown out with the faeces, but chiefly 

 absorbed and eliminated by the kidneys as an aromatic 

 compound of sulphuric acid ; the third passing out mainly in 

 the faeces. 



The large intestine is the chosen haunt of the bacteria of 

 the alimentary canal; they swarm in the faeces, and by their 

 influence, especially in the caecum of herbivora, but also to 

 a small extent in man, even cellulose is broken up, the final 

 products being carbon dioxide and marsh gas. Cellulose- 

 dissolving enzymes of great activity have been found in the 

 hepatic secretion of the snail and the hepato-pancreas of the 

 carp. Filter-paper is rapidly dissolved by the latter. The 

 contents of the large bowel are generally acid from the pro- 

 ducts of bacterial action, although the wall itself is alkaline. 



Faeces. In addition to mucin, secreted mainly by the large 

 intestine, the faeces consist of indigestible remnants of the 

 food, such as elastic fibres, spiral vessels of plants, and in 



