PHARMACOLOGY 251 



cology, which quantitatively estimates the exact 

 effect of each drug upon every organ of the body. 

 It early brought to light the ' selective' nature of 

 drug action, and with the recognition of this began 

 the rationalisation of therapeutics. A substance 

 which exerts an influence in the body shows nearly 

 always a greater affinity for one tissue and lesser 

 affinity for others. Its action is selective. Clearly 

 this gives the first indication for its rational use 

 as a drug. The exact nature of its local action 

 must next be determined, and then its minor and 

 accessory effects. The extraordinary synthetic 

 powers of modern organic chemistry enable us to 

 obtain new compounds with selective actions on 

 the body; and by gradually modifying in detail 

 the general construction of a substance of known 

 action we can intensify or modify that action as 

 we will. 



We cannot here follow up all that these efforts 

 have meant to practical medication, but we can 

 indicate what is their logical goal. In the case, 

 for instance, of a disease due to a parasite we may 

 hope to find or to make a drug which is so selective 

 as between one living tissue and the others as to 

 be poisonous and absolutely fatal to the parasite, 

 and yet wholly harmless to the host. This would 

 be a specificum absolutum, and its use would illus- 

 trate what we mean by wholly rational therapeutics. 

 The ideal is not yet reached, but it has been 

 approached in the employment of such drugs as 



