io THE POPULAR SCIENCE MONTHLY 



the blood in this ability to be massed or delivered where needed. The 

 phagocytic mechanism of defense operates through all the orders of the 

 metazoa; and while it can hardly have been developed originally as a 

 protective system against parasites, and doubtless represents a mech- 

 anism for disposing of effete and useless particulate matter in the body 

 by a process of intracellular digestion, yet it has reached through 

 evolutionary selection a high state of perfection and must have exer- 

 cised no small influence in protecting from extinction certain living 

 species. 



There is good reason to believe that in the final disposal of bacteria 

 intruded into the body the phagocytes play the terminal role: i. e., 

 under favorable conditions they are attracted through chemical stimuli 

 furnished by the bacteria to which they respond to englobe them, after 

 which the bacteria are often disintegrated. But there is equally good 

 reason to believe that, with few exceptions, this engulfing can not take 

 place until the bacteria have been acted on by certain plasmatic con- 

 stituents that prepare the bacteria to be taken into the body of the 

 phagocytes. The further the phenomena of bacterial destruction in the 

 body are probed the more certain does it become that there is no single 

 and uniform process of their disposal. The humoral doctrine of bac- 

 terial destruction contains much of fact, the phagocytic doctrine much 

 of fact, and it is quite certain that the practical defensive activities of 

 the body constantly imply the use of both mechanisms. 



And when we push the analysis of the manner in which bacteria 

 injure the body and enumerate the various bactericidal substances which 

 have now been determined as existing in the plasma and in the cells, 

 we find that this interaction must be supposed to take place. Plasmatic 

 bactericidal action and phagocytic inclusion are cooperative functions; 

 plasmatic antitoxic action and phagocytic detoxication are cooperative 

 functions; plasmatic opsonization and phagocytic ingestion are com- 

 plemental functions; plasmatic agglutination and phagocytic engulfing 

 are also complemental, although less essential functions. And although 

 in intending infections the toxic action of the bacteria to be dealt with 

 is less a matter of great consequence, yet in principle the disposal of a 

 few bacteria is not different from the disposal of many; and in dealing 

 with the poison or toxic elements of bacteria, the plasma possesses 

 distinct power of direct neutralization as the phagocytes possess distinct 

 ability to transform poisonous into non-poisonous molecules. 



I desire now to refer again to the subject of racial and species 

 immunity for which the humoral factors of bacterial destruction af- 

 forded an imperfect explanation, in order that I may point out that 

 the introduction of bacteria, incapable of causing infection, into immune 

 species is followed by immediate phagocytic ingestion and destruction of 

 the microorganisms. The rapidity and perfection of the phagocytic 

 reaction in insusceptible animals are very impressive and might readily 

 lead to the decision that they suffice to explain the resistance or 



