pleasure to extreme pain. The first phenomenon is expressed in the language of every-day life, when we say there is nothing that we may not "get used to." We get used to sights, to sounds, to tastes, to smells, to the endurance of bodily pain. It may be stated as an almost constant fact that the same thing repeated, the centre again stimulated with the same stimulus, always loses somewhat of its effect, and consequently less force is expended. We endure the excitation better and feel less fatigued, whatever it be. If by any chance, however, through illness or other cause, our stock of force becomes lessened, we find that we cannot so well endure our habitual stimuli, and they become painful instead of pleasant. Our feelings, then, are regulated partly by the amount of stimulation, partly by the condition in which our centres are when stimulated; and that which applies to pain applies also to pleasure. Pleasurable excitations when repeated lose their charm, or they fail to please us when a disordered liver or a headache makes us dismal.
The second phenomenon is different. Although an excitation repeated loses its effect, yet an excitation prolonged without cessation passes from pleasure to pain without this process ever being reversed.
There is no voluntary action, whether mental or bodily, which does not in time cause fatigue; but it will be found that actions accompanied with direct emotion fatigue the soonest. Almost all bodily or mental processes are accompanied by some amount of feeling or emotion. They are pleasant to us or distasteful; we may be wearied of doing them, or wearied by doing them, according as the mind or the body is fatigued. In either case the process is the same, though the centre which experiences the discomfort is different. The pleasantest occupations or amusements may cause such sheer bodily fatigue that we can do them no longer, and to attempt it causes pain. It would appear that every thing carried to this point—to the extent of exhausting the nerve-force of the centre stimulated—causes discomfort or pain, which is only to be removed by cessation of the particular stimulus, and the substitution of another, stimulating other centres, or by the rest of the whole nervous system. This brings me to the consideration of another point, namely, that violent stimulation of a centre exhausts the nerve-force, not of that centre only, but of the whole nervous system. A terrible shock may so use up the nerve-force that the individual falls senseless, or, short of this, he may yet be so paralyzed with fear or grief that he loses all muscular power, or he may be so violently moved that the great exertions which he makes only last for a short time. We all know that for a long-sustained muscular effort the mind must be tranquil, and free from emotion, and the muscular movements must be regular and even, and free from spasmodic and violent action. How it is that the nerve-force of the whole system is poured out in this or that form of emotion, or idea, we see, but cannot trace the process.
