276 THE POPULAR SCIENCE MONTHLY. 



quality, and, having separated it, parts with it again, according to the 

 demands made upon it. So this force, specialized by the various por- 

 tions of the human brain, exists in every cell and centre, in greater or 

 less degree, and upon the condition of its existence depends, it would 

 seem, the pleasure or pain experienced when the part is called upon 

 to act. 



A cell, when it sets in action other cells, or other organs of the 

 body, appears to deprive itself of this force, and in time to become 

 exhausted, so that rest and repose are necessary for its renewal. If it 

 be too metaphysical to talk of the conversion of bodily heat into force, 

 and of force into muscular motion, it is, nevertheless, a fact of observa- 

 tion that a nerve-centre becomes exhausted by over-excitation and 

 over-action, and, being exhausted, becomes incapable of energizing till 

 its power or force is renewed by rest or food. In the following obser- 

 vations I shall try to illustrate the theory that a nerve-cell is called 

 into action by stimulation applied to it from without, and that, accord- 

 ing to its special quality, it will then energize and act upon other cells 

 or structures. The amount of action, and the feeling attending it, 

 depend on the condition in which it is at the time. And this con- 

 dition will vary' in proportion to its rest, nutrition, and heat, and also 

 in proportion to the strength of the stimulation and the length of time 

 during which it is carried on. 



The first question is, By what method are we to gain any informa- 

 tion upon these points ? Absolute proof of what I have asserted is 

 not to be expected ; were it forthcoming, we should have learned it 

 long ago. We shall have to apply the methods of observation and 

 experiment, and, of these, observation will aid us most. We can 

 observe the phenomena of Feeling in infants from the commencement 

 of life, in children, in adults, in the aged. From mere sensations we 

 can trace the dawn of what are called Emotions, or, to use an older 

 terminology, Passions. We can observe them also in the lower 

 animals, and in the varieties, so to speak, of man in the savage, the 

 insane, the idiot. And, in observing the feelings, we are compelled 

 also to observe the outcome of them in the shape of bodily and facial 

 motion, which is often the only evidence of their existence. Also, we 

 shall observe the same individual under the various conditions of 

 hunger and repletion, of sleep or want of sleep, of cold or heat, of 

 health or disease. And we shall see how all the phenomena, which 

 our inductive observation can collect, agree or disagree with the laws 

 laid down by those who have by experiment investigated the physi- 

 ology of the nervoiis system. From one method dissection of the 

 actual brain we shall not learn much. When the action is over and 

 the force departed, the actual structure teaches us little about the 

 working. The greatest discoveries have been made by experimenting 

 on living animals. 



If we observe the life of an infant, we find it spent chiefly in sleep 



