training. The growth and development of mental power in the individual, and the process by which, after attaining a maximum of power, the mind gradually becomes less active, until in the course of time it undergoes at least a partial decay, form the special subjects of which I propose now to treat; but, in order to form clear ideas on these subjects, it will be necessary to consider several associated matters. In particular, it will be desirable to trace the analogy which exists between bodily and mental power, not only as respects development and decay, but with regard to the physical processes involved in their exercise.
It is now a well-established physiological fact that mental action is a distinctly physical process, depending primarily on a chemical reaction between the blood and the brain, precisely as muscular action depends primarily on a chemical reaction between the blood and the muscular tissues. Without the free circulation of blood in the brain, there can be neither thought nor sensation, neither emotions nor ideas. It necessarily follows that thought, the only form of brain-action which we have here to consider, is a process not merely depending upon, but in its turn affecting, the physical condition of the brain, precisely as muscular exertion of any given kind depends on the quality of the muscles employed and affects the condition of those muscles, not at the moment only, but thereafter, conducing to their growth and development if wisely adjusted to their power, or causing waste and decay if excessive and too long continued. It is important to notice that this is not a mere analogy. The relation between thought and the condition of the brain is a reality. So far as this statement affects our ideas about actually existent mental power, it is of little importance; for it is not more useful to announce that a man with a good brain will possess good mental powers than to say that a muscular man will be capable of considerable exertion. But as it is of extreme importance to know of the relation which exists between muscular exercise and the growth or development of bodily strength, so it is highly important for us to remember that the development of mental power depends largely on the exercise of the mind. There is a "training" for the brain as well as for the body—a real physical training—depending, like bodily training, on rules as to nourishment, method of action, quantity of exercise, etc.
When we thus view the matter, we at once recognize the significance of relations formerly regarded as mere analogies between mental and bodily power. Instead of saying that, as the body fails of its fair growth and development if overtaxed in early youth, so the mind suffers by the attempt to force it into precocious activity, we should now say that the mind suffers in this case in the same actual manner—that is, by the physical deterioration of the material in and through which it acts. Again, the old adage, "mens sana in corpore sano," only needs to be changed into "cerebrum sanum in corpore sano," to