ALIMENTATION AND DIGESTION 131 



life and growth, and we accordingly find that the number of 

 microbes is much greater in the small and large intestines. 

 It is not the microbe itself, however, which is of importance 

 to the organism as a whole, but the substances which it pro- 

 duces from the foods. Most of these substances are either 

 harmless themselves or else are readily changed into harm- 

 less substances either before or soon after entering the 

 blood; others are poisons, but are normally present in such 

 minute quantities as to be entirely negligible; more rarely 

 they are produced in large quantities and may cause various 

 ill effects either locally or upon the body as a whole. 



The production of undue quantities of such harmful sub- 

 stances, most of which are derived from proteins, is chiefly 

 dependent upon the food supply of the bacteria. This is 

 normally kept low by the speedy and efficient removal of the 

 peptones. Native 1 proteins are acted on comparatively slowly 

 by bacteria and, in any case, must first be changed into pep- 

 tones or simpler peptids before they can be further broken 

 down into harmful bodies. If, however, the processes of absorp- 

 tion quickly and efficiently remove the digestive products, sub- 

 sequent harmful decomposition of the food is prevented, for 

 there are normally no bacteria in the blood. It is therefore of 

 great importance to maintain the efficiency of absorption. This 

 can be done in general only by leading a normal life by tak- 

 ing sufficient muscular exercise, by proper habits of sleep and 

 rest, by proper feeding, and so on. The hygienic conduct of 

 life tends to maintain all functions of the body in proper work- 

 ing condition, those of the digestive organs included; and 

 nothing else can be depended on, in the long run, to do this. 

 To this subject we shall return in the chapters on hygiene, 

 when dealing directly with the personal conduct of life. 



1 A "native" protein is a protein as it occurs in nature before being 

 changed by digestion or other chemical action. The proteins in food are 

 largely native proteins or else, what amounts to the same thing, as far 

 as the action of bacteria is concerned, native proteins coagulated by heat 



