288 MICROBES AND TOXINS 



He has disarticulated protoplasm into atomic groups, both 

 structural and functional, which act like the organs of the 

 cells; he has extended the anatomy and physiology of the 

 tissues and organs by imagining a sort of molecular anatomy 

 and physiology for the cell. 



The cell is a microcosm in which multiple functions take 

 place by continual exchanges between the protoplasm and the 

 external world. Each of these functions is represented by a 

 group or " side-chain," capable of entering into (chemical) com- 

 binations wijh corresponding groups in foreign substances, 

 food-stuffs, poisons, and also drugs. All the vital phenomena 

 can be reduced to a series of such exchanges, which are nutri- 

 tion phenomena and retain this character, whether it is a 

 protein which is being assimilated, or a serum producing 

 immunity, or quinine killing the malaria parasite. 



For a substance to act on a cell, whether an organ cell or a 

 microbe, it must fix itself on the protoplasm. Did not a 

 philosopher in the middle ages declare that drugs ought to 

 have points or hooks to enable them to seize upon the organs ? 

 There is scarcely any phenomenon which haunts the mind of 

 the biologist more than this fixation, which, according to some, 

 is a physical phenomenon, one of molecular adhesion, according 

 to others the chemical interplay of the side-chains. 



The antitoxin injected into a patient has affinities only for 

 the corresponding toxin. But the arsenic which we inject into 

 an individual suffering from sleeping-sickness possesses affinities 

 both for the parasites and for the cells of certain organs. It is 

 a double-edged weapon. We must suppress, or at least atten- 

 uate as much as possible, the dangerous affinity in favour of 

 the useful one, a problem in chemical substitution. The task 

 is chiefly met with in connection with protozoal diseases, but 

 it is not impossible to aim in a similar fashion at the bacterial 

 infections. 



We already know examples of these, we may call them, 

 elective affinities. Ehrlich himself showed long ago, that 

 methylene-blue possesses a special affinity for the living nerve- 

 fibres ; there has even been derived from this in vivo staining, a 



