When a person, eager to give expression to an impulse, restrains himself till an opportune moment (for example, a runner awaiting the start of a race), one is dealing with an experiment of Nature which duplicates, in principle, Pavlov's experiments on "delay." Excitations are roused by a conditioned stimulus but are held in check by inhibitions pending arrival of the proper moment. The sleepiness which man and lower animal alike sometimes show under these conditions signifies that the inhibitions have, so to speak, overshot the mark and have irradiated widely over the cerebral cortex.Abstract Teaser