Page:Popular Science Monthly Volume 1.djvu/288

276 quality, and, having separated it, parts with it again, according to the demands made upon it. So this force, specialized by the various portions 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 observation 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 observations I shall try to illustrate the theory that a nerve-cell is called into action by stimulation applied to it from without, and that, according 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 condition 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 information 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 physiology of the nervous 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