_Counter-irritants_ are substances which are applied to the surface of the body for the purpose of communicating to the superficial sensory nerves impulses (irritation) which may serve to inhibit the morbid functions of distant or central nervous structures. Various explanations of this process have been presented; notably a most ingenious argument by Dr. T. Lauder Brunton[40] in favor of the theory of inhibition by interference of vibrations--impulses from the periphery interfering with vibrations originating in morbid conditions of the central nervous organs--very much as darkness is produced by the interference of luminous undulations, and silence is effected by interference of sonorous waves in the atmosphere. This hypothesis is the one that is most conformable to the present requirements of science, and is hereby accepted as the most satisfactory explanation of the action of counter-irritants. The drugs in general use for the purpose of counter-irritation are _cantharides_, _capsicum_, _mustard_, _thapsia_, _turpentine_, _aconite_, and _croton oil_. _Cups_, _leeches_, and certain special applications of _electricity_, also owe much of their virtue to the same power of modifying functions at a distance. Under the influence of all these agents the molecular vibrations of the brain may be profoundly influenced; pain may be annulled, and sleep be procured. The circulation of the blood is so powerfully affected by these measures that their good effects are usually attributed to the circulatory changes thus produced. But, as in all other modifications of the circulation, the nervous tissues and the vascular channels must first be reached by influences unconnected with the blood itself before its current can be diverted from one region to another. The inhibitory action upon the nervous organs is the primary effect. The diversion of the circulation is a secondary consequence, by which, undoubtedly, the degree of nervous change can be estimated, and by which the inhibitory effects in the brain are intensified and sustained.