Protoplasm is responsive to external force. It conducts the impulses to which stimulation gives rise. Eventually the impulses, which travel along strands of tissue highly specialized for the purpose of conduction—nerves—reach collections of protoplasm which are so disposed that when they dissociate energy is set free. A comprehensive term is much needed for the connotation of this third essential property of protoplasm, the capacity of liberating energy which characterizes “effectors.” An external force, so small in intensity as to be negligible when we are dealing with the body’s accounts, acts upon the protoplasm of a receptor. A change in state results. The change is conducted to an energy-liberating organ. This organ is supplied with blood which brings it food. Food is its store of energy, the raw material from which it manufactures its ammunition. When an impulse reaches an energy-liberating organ its protoplasm dissociates. But here the protoplasm is so disposed—the cells which contain it have such a form—that when it dissociates a change in the cell follows; it alters in shape, or it discharges into its environment heat, or electricity, or light. The dissociation and reassociation of the protoplasm of an effector involves chemical change. Molecules of water and of carbonic acid are cast off. The energy sacrificed in letting matter fall into these very stable forms is the energy made visible, as it were, in lifting a weight or dispersing heat. It must be replaced if the organ is to retain its power of acting when next an impulse reaches it. To replace it, protoplasm takes up food and oxygen from the blood. The liberation of energy which occurs when a muscle contracts is not a special phenomenon—something which does not occur when the muscle is at rest. It is an intensification of a process which is always taking place. The substance of muscle, like that of nerve and every other tissue, is always combining with oxygen and giving off water and carbonic acid. When we are auditing the body’s accounts, we enter so and so much food and oxygen on the debit side, we credit it with the same weight of water and carbonic acid; or we debit it with the energy potential in the food, and enter to its credit the mechanical work done and the heat set free by the oxidation of this food. Food is the petrol the combustion of which causes the movement of the car. The external force which stimulates a receptor is too insignificant in amount to be carried to account. Physiologists neglect it, just as engineers neglect the energy liberated by the sparking-plug which ignites the petrol, when they are estimating the efficiency of a motor.