64 DISORGANISING- AGENTS. tive medicines affect, each in its own way, the composition of an organ, one alterative may, after a time, lose its influence, while the organ thus saturated, as it were, with the remedy, may still be suscept¬ ible of the influence of another. The practice of medicine affords-, in innumerable cases, a confirmation of this statement. By the con¬ tinued use of an alterative medicine, the composition of the organ will have suffered such a chemical change, that the same affinity for this substance no longer exists in the organism, while an affinity for another substance may still remain. Imponderable matters also are in this way alterative; thus the eye, after being long fixed on a gfeen surface, loses gradually its sensibility for this colour, which becomes dull and grey. At the same time, however, the sensibility for the red rays is increased. So, also, a long exposure of the retina to the red rays makes it susceptible of the green. In the same way, by fixing the eye for some time on yellow, the sensibility for that colour is lost, while the perception of violet becomes more intense, and vice versa; the same relation exists between blue and orange. 3. Agents which destroy the organic composition. (Decom¬ posing agents.)—These are substances which, without first pro¬ ducing a stimulant or simply alterant effect, directly destroy the essential composition of the organised tissues. Some of the agents which are “stimulants” when they operate gently, produce by a more violent action too great a disturbance of the powers of the part; such are heat, electricity, &c. Others are “alteratives,” which by an extreme degree of their action produce great changes in the composition of the tissues, forming with the organic matter combi¬ nations which the organic force is not able to counterbalance. It is in this way that the narcotic alterants have a destructive action; and those alterants which modify the formation of the fluids of the body, and the organic changes effected in them by different organs, —for example, the antimonial and mercurial preparations, and the mineral acids and alkalies,—have, when in a concentrated state, an equally destructive influence on the organic composition. Stimu¬ lants can produce disorganisation in two ways. Some agents are stimulants only when their action does not surpass a certain degree of intensity; and, when their action is more violent, instead of reno¬ vating the organic composition and force, or even favouring this renovation by exciting new affinities, they produce immediately an essential change of composition. In this case no irritation or reac¬ tion precedes the local or general death; the disorganisation is imme¬ diate, as in death from electricity, lightning, &c. Other stimuli, which under certain conditions have a renovating action, may have a destructive effect by exciting the action of an organ during too long a period: more force being exhausted than can be restored again in an equal space of time. This action is called over-excitement. An organ thus over excited, as, for example, the eye by light, is rendered permanently weaker. The decomposing agents are used in medicine only when it is wished really to produce destruction of a part,