284 THE POPULAR SCIENCE MONTHLY. 



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, 

 theii, 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 feel- 

 ing 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 dis- 

 comfort 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. 



