ON SPECIFIC THERAPEUTICS. 53 



"chemo-receptors." In like manner the bodies 

 of the higher animals must possess such chemo- 

 receptors in certain organs, as they are also 

 injured by the poison. If now a mouse infected 

 with trypanosomes is injected with the arsenical 

 medicament, this substance will be distributed 

 between the parasites and the organism of the 

 mouse. If the receptivity of the parasites is the 

 stronger, they will be killed in the organism ; in 

 the alternative case, they will not be destroyed. 

 Consequently the curative result obtained when 

 experimenting under normal circumstances re- 

 presents also the differential between two 

 avidities ; i.e., if a mouse infected with the 

 normal strain of trypanosomes is injected with 

 the arsenical preparation, the " trypanotropic " 

 force of the drug is stronger than the " organo- 

 tropic " force ; we therefore obtain a curative 

 effect with doses which are not injurious to the 

 organism of the mouse. 



In the resistant strains this is not the case, 

 since the same doses show no trace of action on 

 the parasites. One might imagine that this 

 result was due to the trypanosomes having lost 

 their chemo-receptors : but it is not necessary 

 to make this assumption, and Nature has in its 

 wonderfully complex mechanism found another 



