310 INFECTION AND IMMUNITY 



Welch, and Eisenberg, and is, in fact, but an axiom which would be 

 recognized immediately by any trained biologist — that all micro- 

 organisms will adapt themselves so far as is permitted by their physio- 

 logic peculiarities to the stress of the environment, the exact direction 

 which this adaptation will take being detei-mined by the charactei- of 

 the environment, chemical and physical, and the physical, chemical, and 

 physiologic characteristics of the germ involved. 



Thus far, in considering the means of offense and defense at the com- 

 mand of the bacteria, we have largely left out of consideration the ani- 

 mal organism against which these are directed, or by the changes in 

 whose functions, metabolism, tissues, cells, and fluids, we are largely 

 made aware of their existence. 



The internal defenses of the animal body — and with these alone we 

 are concerned — have largely been elucidated, as we have seen, through 

 morphologic investigation of cellular activities taking place in the ani- 

 mal body or under controlled conditions in the test tube, and by visible 

 reactions taking place in test tubes between the fluids of normal or im- 

 munized animals and the bacteria and their products, and, finally, by 

 the more purely physiologic tests of the protecting power and mechan- 

 ism of action of animal fluids or extracts when introduced into another 

 animal of the same or different species, along with the bacteria or their 

 products. 



Such studies have, as is well known, afforded a vast amount of in- 

 formation. Through them the soluble secreted bacterial poisons have 

 been demonstrated and have been found to stimulate the production of 

 neutralizing bodies, the antitoxins; bacteria and their culture filtrates 

 liave been shown to call forth bodies which are present in the serum of 

 animals treated with them, and which cause a precipitation of certain 

 bacterial constituents of the filtrate — the precipitins; and injections of 

 animals with bacteria or their products have been found to cause the 

 production of bodies which are present in the serum and which have 

 the power of agglutinating the bacteria when brought into contact with 

 them — the agglutinins; and other bodies are likewise produced which 

 are capable under proper conditions of killing the bacteria — the bac- 

 tericidal substances — or even of dissolving them as we have seen in 

 some instances — the bacteriolytic substances. All of these bodies may 

 be demonstrated in the serum of certain normal animals and may be 

 shown to be increased during the immunization of these animals with 

 bacteria or their products. The complementing body, however, which 

 is necessary for the activation of the bactericidal and bacteriolytic 



