Page:Popular Science Monthly Volume 51.djvu/664

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HE results of late researches in physiological and experimental psychology contribute much toward a rational explanation of the causes of abnormal and deficient mental characteristics in childhood. To begin with, it is now satisfactorily shown that mental action is accompanied by the expenditure of energy derived from the breaking down of highly unstable chemical compounds in cerebral nerve cells. The brain has come to be regarded as a storehouse of nerve power; and when too severe or prolonged demands have been made upon it the cells become much depleted of their contents, and there results a condition of brain tire or fatigue. The process of depletion has been studied with great care in the case of the frog by Dr. Hodge, of Clark

 showing changes observed in the nucleus of the living sympathetic nerve cell of the frog, as the result of direct electrical stimulation (Hodge, after Donaldson ). After six hours and forty-nine minutes of stimulation, the nucleus (n) is seen to be reduced to less than one half its size when the stimulation began.

University, and some of his results are shown in the accompanying Diagram A.

While no similar observations have been made upon human beings, for obvious reasons, yet it seems reasonable that the same law holds here in regard to the expenditure of nerve energy by physical or mental work.

It is evident that the law of the conservation of energy is implied in this phenomenon, and the accompanying diagrams may serve to make clear the method of its application. In Diagram B there is shown leading to the very heart of the nerve cell a filament, N, whose office is to carry messages in the form of stimuli from the world without; and leading from each cell are a number of avenues or passageways, D, through which may run off to other cells or muscles the energy set free by the advent of some sense stimulus or by the processes of thought and feeling. A very slight stimulus may in certain instances unloose a relatively