If the reader has studied the part of this work dealing with the distribution and function of the nerves, he will understand that the whole animal machine depends upon them to keep its various organs working together in harmony and sympathy one with another. The question as to what drugs act upon the nervous system can only be answered in a limited sense. It is probable that no drug can act without its aid, but there are special agents which experience and experiment have demonstrated to have a particular and well-marked action on nervous tissue, and to these we give the title which appears above.

If we desire to affect a particular nerve or branch, we cannot select an agent that will speed like an arrow to the spot indicated and affect no other, but we must seek to obtain the desired effect through the medium of the blood. The agent will have to be taken into the circulation, and there will be distributed over the nervous system in an equal degree.