Page:Collected Physical Papers.djvu/279



Certain changes take place when a living muscle is pinched or an electric shock passed through it. A responsive twitch is produced; the muscle is changed in form, becoming shorter and broader; the particles of the living substance are strained under the stimulus. The effect of the shock then disappears, and the muscle relaxes into its usual form.

This sudden change of form then, is one, but not the only mode of response of a living substance to external stimulus. Under external stimulation the muscle is thrown into a state of strain. On cessation of the stimulus it automatically recovers. As long as it is alive, so long will it respond and recover, being ready again for a new response. This brief disturbance of the living poise, to be immediately restored to equilibrium of itself, is quite unlike the rolling of a stone downhill from a push. For the stone cannot regain its original position; but the living tissue at once reasserts its first stable poise on the cessation of stimulus. Thus a muscle, as long as it is alive, remains ever-responsive. It is in intimate relation with the forces by which it is surrounded, always responding to, and recovering from, the multitudinous stimuli of its physical environment.

The living body is thus affected by external stimuli—mechanical shock, electrical stimulus, and the stimuli