PARASITIC BACTERIA. 153 



tion, and the corpuscles leave the blood-vessels 

 and sometimes form a solid phalanx completely 

 surrounding the invading germs. Their collec- 

 tion at these points may make itself seen exter- 

 nally by the phenomenon we call inflammation. 



There is no question that the corpuscles en- 

 gage in conflict with the bacteria when they thus 

 surround them. There has been not a little dis- 

 pute, however, as to the method by which they 

 carry on the conflict. It has been held by some 

 that the corpuscles actually take the bacteria into 

 their bodies, swallow them, as it were, and subse- 

 quently digest them (Fig. 33 c, d, e). This idea 

 gave rise to the theory of phagocytosis, and the 

 corpuscles were consequently nariied phagocytes. 

 The study of several years has, however, made it 

 probable that this is not the ordinary method by 

 which the corpuscles destroy the bacteria. Ac- 

 cording to our present knowledge the method is 

 a chemical one. These cells, when they thus col- 

 lect in quantities around the invaders, appear to 

 secrete from their own bodies certain injurious 

 products which act upon the bacteria much as do 

 the alexines already mentioned. These new bod- 

 ies have a decidedly injurious effect upon the 

 multiplying bacteria ; they rapidly check their 

 growth, and, acting in union with the alexines, 

 may perhaps entirely destroy them. 



After the bacteria are thus killed, the white 

 blood-corpuscles may load themselves with their 

 dead bodies and carry them away (Fig. 33 d, e). 

 Sometimes they pass back into the blood stream 

 and carry the bacteria to various parts of the 

 body for elimination. Not infrequently the white 

 corpuscles die in the contest, and then may ac- 

 cumulate in the form of pus and make their way 



