THEIR MODE OF ACTION.. 359 



the vital principle, and does not exercise a chemical 

 action upon the living organ. 



Another class of bodies change the direction, the 

 strength, and intensity of the resisting force (the 

 vital principle), and thus exert a modifying influ- 

 ence upon the functions of its organs. They^. 

 produce a disturbance in the system, either by their 

 presence, and by themselves undergoing a change ; 

 these are medicaments. 



A third class of compounds are called poisons, 

 when they possess the property of uniting with 

 organs or with their component parts, and when 

 their power of effecting this is stronger than the 

 resistance offered by the vital principle. 



The quantity of a substance and its condition 

 must, obviously, completely change the mode of 

 its chemical action. 



Increase of quantity is known to be equivalent to 

 superior affinity. Hence a medicine administered 

 in excessive quantity may act as a poison, and a 

 poison in small doses as a medicine. 



Food will act as a poison, that is, it will produce 

 disease, when it is able to exercise a chemical action 

 by virtue of its quantity ; or, when either its con- 

 dition or its presence retards, prevents, or arrests 

 the motion of any organ. 



A compound acts as a poison when all the parts 

 of an organ with which it is brought into contact 

 enter into chemical combination with it, while it 



