In this division, remedies are employed with a view to their influence over the functions of the nervous system. They do not immediately or necessarily affect the function of nutrition; they do not enter into the formation of tissues; and, having modified the functions of the nervous system, they are excreted from the organism in the form in which they entered it. It is probable that the selective action on this system is due to the fact that the nervous tissue is the most highly specialized in function, and therefore most susceptible to such impressions.

The different parts of the nervous system are so closely united in function that a disturbance at any point is differentiated to other and often widely-separated points, and the complexus of effects is made up of many minor disturbances. For this reason it is quite impossible, in the present state of our knowledge, to make a classification which will sharply define the limits of activity of any particular remedy. Nevertheless, physiological experiment and clinical experience have furnished us sufficiently accurate information with regard to the most important actions of the remedies of this division, to justify an arrangement based on their most conspicuous qualities.