In opening the study of muscle and nerve we need to form a conception of the nature of these three functions—irritability, conductivity, and changeableness of form. Not that the functions are as distinct as the ideas to which the three terms give rise. They are three aspects of a common function; although this is a reflection which will carry more weight when the ways in which protoplasm reacts to external forces have been considered. A stimulus may be mechanical, something in the nature of a blow which displaces the particles of protoplasm; or it may be chemical or thermal, disintegrating a portion of its substance; or electrical, divorcing the ions of its molecules. Only the last in any way resembles a natural stimulus; since electrical stimulation alone can be repeated without the substance stimulated showing any evidence of injury in the process. Mechanical, thermal, chemical stimuli destroy a portion of the protoplasm upon which they act. Yet even the weakest of electric currents is a gross disturbance as compared with natural stimuli, such as touch, warmth, sound, light. The essential and most distinguishing quality of living matter is its return to its original state immediately after stimulation. It does not even wait until the stimulator has ceased to act. An effective influence is a sudden change in the environment. It is answered by a sudden response, followed by a return of protoplasm to the state in which it was before the impact of the external force. The change progresses through the protoplasm as a transitory alteration of state, the particles concerned in conducting it returning to their original condition the moment it has passed. No non-living matter responds to force in this way. If a stone is dropped into a pond, a wave circles outwards from the spot it strikes; but this is a wave of displacement, not a change of state. Suppose the pond contained a solution of sugar which the impact of the stone changed into vinegar, and that the zone of vinegar spread outwards, the liquor returning to the condition of sugar and water as it passed. Here we should see some analogy to the progress of an impulse. But no non-living matter behaves like this. A product of the laboratory may be so unstable as to explode when shaken, passing on the slightest provocation into a more stable state. It does not return after the explosion to its previous strained condition. Having thrown away its energy, it continues on a lower plane. Protoplasm parts with energy to recover it again. It returns to instability after assuming a more stable form.