ACTION OF MEDICINES. 245 



either directly or indirectly, be traced to some dis- 

 turbance in the functions of digestion. 



634. Very little is known respecting the mode in 

 •which medicines act, or the effects which they pro- 

 duce when taken into the stomach. The influence of 

 some substances may, to a certain extent at least, be 

 explained, such, for example, as dilute sulphuric acid, 

 which it is evident must react upon any common 

 salt it meets with, generating muriatic acid and sul- 

 phate of soda. Again, the manner in which one form 

 of indigestion, arising from the incipient fermentation 

 of food, is arrested by certain volatile oils, or by 

 saline draughts charged with carbonic acid, may be 

 explained ; but the mode in which bark, opium, 

 and indeed nearly all other drugs act, is quite 

 unknown. 



635. It is true that chemists have ascertained that 

 these substances derive their active powers from the 

 presence of minute quantities of various peculiar prin- 

 ciples which they contain. These substances have 

 been separated, analyzed, examined, and named ; but 

 the knowledge thus obtained has not thrown much 

 light upon the real mode in which they act. It has 

 merely shown the nature of the active agent, but 

 neither the way in which it acts, nor the principle 

 upon which its activity depends (513). 



2V 



