The operation of medicines is either primary or secondary; the primary operation being that which results from their immediate influence on the system; the secondary, that which follows their original and characteristic impression, in consequence of certain physiological laws. Thus, purgation is the primary operation of a cathartic; derivation of blood from the head, with the attendant relief of any existing cerebral congestion, is a secondary operation, and a simple consequence of the first, just as the same result would follow an equal amount of intestinal irritation with diarrhoea from other causes.