To secure continued activity the clock must be rewound, the gun must be recharged, more coal must be supplied to the engine. In like manner the continuation of muscular and in- <p 172> tellectual activity depends upon the restoration of muscle and brain cells. The necessity for renewal is greater or less according to the amount stored in reserve and the rapidity of consumption. A maximum head of steam may keep the engine running for a long time unless the load is too heavy or the speed too great. Though under certain conditions the amount of muscle and brain energy stored in reserve is large, continuous or rapid activity of necessity expends the reserve and leads to exhaustion. It is a simple process to rewind the clock, to reload the gun, and to replenish the fuel. To restore muscular and nerve cells is a very delicate process. So wonderful is the human organism, however, that the process is carried on perfectly without our consciousness or volition except under abnormal conditions. Food and air are the first essentials of this restoration. Indirectly the perfect working of all the bodily organs contribute to the process --especially deepened breathing, heightened pulse, and increase of bodily volume due <p 173> to the expansion of the blood vessels running just beneath the skin. _Here pleasure enters. Its effect on the expenditure of energy is to make muscle and brain cells more available for consumption, and particularly to hasten the process of restoration or recuperation_. The deepened breathing supplies more air for the oxidation of body wastes. The heightened pulse carries nourishment more rapidly to the depleted tissues and relieves the tissues more rapidly from the poisonous wastes produced by work. The body, the machine, runs more smoothly, and fewer stops for repairs are made necessary. In addition to these specific functions, pleasure hastens all the bodily processes which are of advantage to the organism. The hastening may be so great that recuperation keeps pace with the consumption consequent on efficient labor, with the result that there is little or no exhaustion. This is in physiological terms the reason why a person can do more when he ``enjoys'' his work or play, and can <p 174> continue his efforts for a longer period without fatigue. The man who enjoys his work requires less time for recreation and exercise, for his enjoyment recharges the storage battery of energy.