1 1 8* 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- 



