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Page:Popular Science Monthly Volume 4.djvu/352

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advanced in years find themselves indisposed to undertake original research, not from any want of power, but because they recognize the fact that sufficient time does not remain for them to bring such work to a satisfactory issue. They feel that they would have to leave to others the rearing of their mental offspring.

It cannot be questioned, however, that with old age there comes a real physical incapacity for original work, while the power of maturing past work remains comparatively but little impaired. Dr. Carpenter has shown how this may partly be explained by the physical changes which lead in old age to the weakening of the memory; or perhaps we should rather say that in the following passage his remarks respecting loss of memory serve to illustrate the loss of brainpower generally, and especially of the power of forming new ideas, in old age. "The impairment of the memory in old age," he says, "commonly shows itself in regard to new impressions; those of the earlier period of life not only remaining in full distinctness, but even it would seem increasing in vividness, from the fact that the eye is not distracted from attending to them by the continued influx of impressions produced by passing events. The extraordinary persistence of early impressions, when the mind seems almost to have ceased to register new ones, is in remarkable accordance with a law of nutrition I have formerly referred to. It is when the brain is growing that the direction of its structure can be most strongly and persistently" (query, lastingly) "given to it. Thus the habits of thought come to be formed, and those nerve-tracks laid down which (as the physiologist believes) constitute the mechanism of association, by the time that the brain has reached its maturity; and the nutrition of the organ continues to keep up the same mechanism in accordance with the demands upon its activity, so long as it is being called into use. Further, during the entire period of vigorous manhood, the brain, like the muscles, may be taking on some additional growth, either as a whole or in special parts; new tissue being developed and kept up by the nutritive process, in accordance with the modes of action to which the organ is trained. And in this manner a store of 'impressions' or 'traces' is accumulated, which may be brought within the 'sphere of consciousness' whenever the right suggesting strings are touched. But, as the nutritive activity diminishes, the 'waste' becomes more rapid than the renovation; and it would seem that, while (to use a commercial analogy) the 'old-established houses' keep their ground, those later firms, whose basis is less secure, are the first to crumble away—the nutritive activity which yet suffices to maintain the original structure not being capable of keeping the subsequent additions to it in working order. This earlier degeneration of later-formed structures is a general fact perfectly familiar to the physiologist."

One of the most remarkable features of mental development, characteristic, according to circumstances, of mental growth and of mental