26 THE BIOLOGY OF BACTERIA 



we shall further have to keep clear in our minds that their action is 

 complex, and not simple. In the first place, we have an infection of 

 the body due to the bacteria themselves. It may be a general and 

 widespread infection, as in anthrax, where the bacilli pass, in the 

 blood or lymph current, to each and every part of the body ; or it 

 may be a comparatively local one, as in diphtheria, where the invader 

 remains localised at the site of entrance. But, be that as it may, the 

 micro-organisms themselves, by their own bodily presence, set up 

 changes and perform functions which may have far-reaching effects. 

 It is obvious that the wider the distribution the wider is the 

 area of tissue change, and vice versd. Yet there is something of far 

 greater importance than the mere presence of bacteria in human or 

 animal tissues, for the secondary action of disease-producing germs — 

 and possibly it is present in other bacteria — is due to their poisonous 

 products, or toxines, as they have been termed. These may be of the 

 nature of ferments, and they become diffused throughout the body, 

 whether the bacteria themselves occur locally or generally. They 

 may bring about very slight and even imperceptible changes during 

 the course of the disease, or they may kill the patient in a few hours. 

 Latterly bacteriologists have come to understand that it is not so 

 much the presence of organisms which is injurious to man and other 

 animals as it is their products, which cause mischief ; and the 

 amount of toxic product bears no known proportion to the degree of 

 invasion by the bacteria. The various and widely differing modes of 

 action in bacteria are therefore dependent upon these three elements 

 (1) the tissues or medium, (2) the bacteria or agents, and (3) the 

 products of the bacteria or toxins ; and in all organismal processes 

 these three elements act and react upon each other. 



Seed and Soil. — It is of essential importance to the right under- 

 standing of the rdle which bacteria play in the production of disease 

 to give full place to the part taken by the soil on which they are 

 implanted. Few ideas in bacteriology are more erroneous, or likely 

 to lead to graver misconception, than to suppose that bacteria 

 produce the same effect under all conditions, and that the human 

 tissues play a small part. One might equally well expect seed to 

 behave in the same way in all kinds of soil. We know that as a 

 fact, seeds only flourish under certain conditions, and that the soil 

 is only second in importance to the seed-life itself. It is somewhat 

 the same in the production of disease. The early school of pre- 

 ventive medicine declared for the health of the individual and laid 

 the emphasis upon predisposition ; the modern school have declared 

 for the infecting agent, and have laid emphasis upon the bacillus. 

 The truth is to be found in a right perception of the action and 

 interaction of the tissues and the bacillus. B. diphtherice in one 

 person's throat (A) sets up diphtheria, in another person's throat 



