138 THE BODY AT WORK 



the case, to the combination of these two factors. The relation 

 of fermentation to alimentation can be shown by counting 

 the microbes in a specimen of the contents of the large intes- 

 tine. In a particular case it fell from 65,000 per milligramme 

 upon a mixed diet to 2,000 per milligramme upon a diet of milk. 

 In the world at large bacteria perform many offices of the 

 utmost usefulness to other living things. They fix nitrogen in 

 the soil, sweeten polluted rivers, reduce animal and vegetable 

 matter to a condition in which it is available as plant-food. 

 Their presence within the alimentary canal is inevitable ; but 

 it is somewhat doubtful whether, with the exception of the 

 fermentation of cellulose, they do the economy any service 

 with which it could not dispense. As parasites of the alimen- 

 tary canal, some kinds are less desirable than others. Recently 

 a method of limiting their variety has been introduced and 

 advocated with much enthusiasm, as favourable to the hygiene 

 of the digestive tract. In countries in which the cows are 

 driven, in summer, to mountain pastures, the peasants of the 

 plains live during their absence largely upon milk brought down 

 at intervals, and allowed to turn sour. Sour milk, in Bulgaria, 

 develops a bacterium of extraordinary vigour. It can live in 

 a medium containing as much as 10 per cent, of lactic acid, a 

 concentration fatal to other forms of Bacterium acidi lactici. 

 It is easily cultivated, and when ingested continues to multiply 

 in the alimentary canal. So peculiarly lusty is this bacterium 

 that it makes life impossible for other germs. As it dies out 

 after two or three months, it seems unlikely that a man who 

 swallows the Bulgarian milk-germ runs a risk of inviting a 

 repetition of the tragedy which followed the acclimatization 

 of the mongoose in Jamaica. Its supremacy has been attri- 

 buted to its capacity of developing a concentration of lactic 

 acid too high for the well-being of other bacteria ; but it is 

 improbable that it has the opportunity of doing this in the ali- 

 mentary canal of a person living on a mixed diet. The extinc- 

 tion of other bacteria (if they are extinguished) is more likely 

 to be due to an antagonism of a more subtle kind, at present 

 inexplicable, but not without parallel. The purifying influence 

 of the water of the Ganges has for ages been an article of faith. 

 Pilgrims from fever-stricken districts bathe in it, foul it, drink 

 it, with the corpses of their fellows floating down the stream. 



