The direct action of a drug is the effect it produces on any organ with which it comes in contact. Thus sulphuric acid applied to the skin, or taken into the stomach, will, according to its degree of concentration, irritate or destroy the mucous membrane which it touches. Its direct action upon them is therefore that of an irritant or caustic.

Curare, when applied to the ends of a motor nerve in a muscle, paralyses them. It does this either when the muscle is soaked in a solution of curare, or when the curare is carried through the substance of the muscle by means of the blood circulating in it.

Paralysis is therefore the direct effect of curare on the motor nerves.

The convulsions which sometimes occur in poisoning by curare are caused by its indirect action. It has no stimulating effect on the nerve-centres, when applied to them directly or caried to them by the blood, but by paralysing the muscles of respiration, and thus causing asphyxia, it indirectly irritates the nerve-centres, and causes convulsions.