What I have found so apparent in physical exertion is equally true in intellectual labor. Writing or research work which progresses satisfactorily leaves me relatively fresh; unsuccessful efforts bring their aftermath of weariness.
Intellectual work which is pleasant is stimulating and does not fag one, while intellectual work which is uninteresting or displeasing is depressing and exhausting.
We can readily trace the source of energy in mechanical devices. The hands of a clock continue in their course because of the energy <p 171> locked up in a compressed spring or elevated weight. The gun projects the bullet because of the sudden chemical union of carbon with saltpeter and sulphur. The steam engine takes its energy from the steam secured by combustion of coal or other fuel.
The work of the human organism is usually classified as muscular or intellectual. In either the expenditure of energy is as dependent upon known causes as is the activity of the mechanical devices mentioned above.
Every muscular activity is dependent upon muscular cells ready for combustion; without such combustion no muscular work is performed.
Every intellectual process is likewise dependent upon brain cells ready for combustion, and no intellectual work can be performed without combustion of these brain cells.
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.