Page:Popular Science Monthly Volume 84.djvu/122

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118
THE POPULAR SCIENCE MONTHLY

THE ORIGIN AND EVOLUTION OF THE NERVOUS SYSTEM
By Professor G. H. PARKER
HARVARD UNIVERSITY

TO the ancients what we designate as personality was a more or less general attribute of the human body rather than an aggregate of functions having a strictly nervous source. In fact Aristotle, who was such an accurate observer and profound thinker in so many fields of biology, denied positively that the brain was in any direct way con- cerned with sensation and declared the heart to be the sensorium com- mune for the whole body. To Galen is ascribed the belief that the brain was the seat of the rational soul, the heart the location of courage and fear, and the liver that of love. This distribution of the element of personality over the physical body finds its expression in the common speech of to-day, particularly in relation to the heart, which is widely accepted by the popular mind as the source of the more tender emotions. It was chiefly through the anatomists and physiologists of the early Renaissance that the modern movement, which has tended to limit per- sonality to the nervous system, was seriously begun, a movement which, with the increase of knowledge, has gained support to such an extent that it can now be maintained beyond any reasonable doubt. Human personality is in no true sense the outcome of the non-nervous organs, such as the digestive or the circulatory organs, but is the direct product of the nervous system. This system, to be sure, is embedded among the other organs of the body and the environment thus provided influences profoundly its condition and action, but what is meant by individual personality, acuteness or dullness of sense, quickness or slowness of action, temperamental traits, such as a gloomy or bright disposition, incapacity, shiftlessness, honesty, thriftiness or sweetness, are all, strictly speaking, functions of the nervous organs. Although only the higher animals can be said to possess personality in this sense, traces of it occur in the lower forms and its evolution is indissolubly connected with that of the nervous system. It is the object of this paper to trace in broad outlines the development of those organs which in the higher animals come to be the seat of personality.

The nervous organs of the higher animals, including man, consist of enormously intricate systems of interwoven nerve cells or neurones whose unique character was first fully grasped some twenty years ago by Wal- deyer. These neurones, like other cells, possess a nucleated cell-body, the ganglion cell of the older neurologists, from which extremely at-