BACTERIA OF THE INTESTINE 141 



the bacteria and spores introduced with the food thrive and multiply 

 abundantly. The Sarcinae which occur in almost every drinking-water 

 are -very -prone to make their appearance. They include several species, 

 which were formerly all classed together as 5. ventriculi and supposed to 

 be slightly pathogenic. 



The richest hotbed for bacteria in the animal body is the intestine. 

 Its contents are alkaline and the prevailing temperature is high and constant, 

 so that putrefactive and fermentative bacteria of all kinds, both aerobic 

 and anaerobic, find optimal conditions of existence. Fresh human faeces 

 contain 75 per cent, water and about 1 per cent, bacteria. These in- 

 clude spores and rods of all kinds, among which may be often recognized 

 the large cells of B. buccalis maximus that have been swallowed with food. 

 It is calculated that from twelve to fifteen milliards of bacteria pass out 

 of the body daily in the faeces (119). Many of them are dead, and 

 stain very badly, others are still alive, and in some spores may be seen. 

 As these spores have not been set free from the rods, there is no doubt 

 they are of recent origin and formed in the intestine. 



The nature of the putrefactive and fermentative changes wrought by 

 the bacteria on the remnants of undigested food is, of course, dependent 

 largely on the composition of the food. With a meat diet, putrefactive 

 changes predominate, and products like tyrosin, leucin, indol, skatol, 

 sulphuretted hydrogen, and ammonia arise (see Ch. XI). With vegetable 

 nourishment rich in carbohydrates, fermentation takes the upper hand, par- 

 ticularly the methane fermentation of cellulose. If the food and habits of 

 the individual be regular, an intestinal bacterial flora of fairly constant 

 composition arises, the leading species being generally the pleotrophic 

 Bacterium coli commune (120) which is both zymogenic and saprogenic 

 (Ch. XVI). Other species (e. g. B.putrificus coli) occur, but have not yet been 

 thoroughly investigated. In the interior of the intestine, towards the middle 

 of the mass that fills it, only anaerobic processes can go on, but at the 

 sides, in contact with the mucous membrane, which itself is thickly covered 

 with bacteria, aerobic changes can occur. 



It is not necessary to discuss the means by which bacteria obtain 

 entrance into the alimentary canal. We swallow myriads every day with 

 our food, and it is only natural that they should multiply in the very 

 favourable environment that the intestine affords. Their occurrence is 

 a necessary consequence of their wide distribution in nature, and must 

 not be taken as indicating a symbiotic connexion between them and 

 their host. It has been imagined that they were perhaps useful auxiliaries 

 in the process of digestion, but there is no foundation for such a supposition. 

 We are fortunately not so miserably dependent upon bacteria as this. 

 This has been experimentally proved by feeding new-born animals upon 

 sterilized food (121). In this way it is possible to exclude bacteria, and 



