The remedies employed for the purpose of external irritation are divisible into two groups.

1.  Rubefacients;

2.  Epispastics.

A rubefacient is a remedy which causes heat and redness; but, if the contact with the skin be sufficiently prolonged, vesication may be produced. An epispastic is a remedy which excites inflammation and vesication. The first group of remedies are restricted in their application to such therapeutical results as can be attained by a superficial and temporary action in the skin. The second group are intended for more permanent action and a deeper impression on internal organs. These remedies differ not only in the degree, but in the character of the effects produced. An impression on the periphery induces some kind of molecular modification at the center. According as the impression is slight or severe are the centric modifications localized to the point of reception or transferred to distant points (reflex impressions). According to the severity of the peripheral impression are the resulting local centric disturbance and the reflex changes (trophic alterations). Thus a slight peripheral rubefaction may cause a trivial centric vascular spasm, but an extensive burn on the body may induce vaso-motor paresis, and consequent inflammatory changes in remote organs.