656 
ON THERAPEUTICS. 
head. Although the agents belonging to each sub-division 
are connected by a similarity of action, they nevertheless 
produce their effects in various ways: a consideration of the 
actions belonging to each will be a proper preliminary to 
their application to the cure of disease. 
THE CONDITIONS OF MEDICINAL ACTION. 
Medicines we have found to possess a power of influencing 
the animal economy, and for the perfect development of this 
power that certain conditions are necessary. First, we are to 
assume a susceptibility to the action of the agent, as it is by no 
means a necessary consequence that what acts with energy in 
the body of one animal shall equally or at all affect the organism 
of another: the determination of this very important point is 
due to experiment. As a matter of philosophic interest, it may 
be legitimate to reason upon the causes of such idiosyncrasies of 
the system, but no speculation can supply the place of absolute 
proof, with which alone science can be satisfied. Secondly, 
the medicine is to be appropriated by the system, through the 
medium of some part or organ. Some agents act more 
quickly and decidedly when applied to the surface of the skin, 
others only produce their effects when introduced into the 
stomach. Again, injection into the circulation, or into the 
intestinal canal, are methods by which some most powerful 
action may be obtained. Either, or all of these methods are 
at the service of the experimenter, and the selection of either 
must depend upon a variety of circumstances. It would 
seem that something analogous to digestion is necessary 
before the action of' a drug can be developed, as a certain 
time always elapses between the administration and the es¬ 
tablishment of its characteristic effects. This period differs 
according to the method of administration, and the nature of 
the agent: in some cases the time is exceedingly short. The 
effects of chloroform are quickly apparent when the agent is 
given as a draught or enema, or in the form of vapour; or 
more decidedly when injected into a large vein : the inter¬ 
vening time differs in all these methods of administration, 
but in no case do many minutes pass before some action 
is perceived. Aloes given to the horse requires to 
remain in the system from eight to twenty hours before 
purgation results, according to the solubility of the drug, and 
its method of preparation, as well as the condition of the 
system. This great difference in the time of incubation 
must depend upon the degree of ease with which the agent 
is assimilated and carried into the circulation. Agents which 
