Page:Popular Science Monthly Volume 43.djvu/386

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THE human brain is the most complicated organ known, and although its anatomy has been the object of innumerable investigations, often by observers of the highest ability, we are still far from understanding its organization. Within recent years, however, embryologists have turned to the study of the development of the brain, and have succeeded in elucidating many of the obscure features. Here, as in so many other cases, embryology has furnished the master-key to unlock the mystery of the adult anatomy. The series of conceptions which we have derived from our present knowledge of the development of the brain are so clearly established that I regard them as impregnable. They are so far in advance of all previous achievements in the study of the brain that they may be called almost revolutionary, and they are of so fundamental a character that the entire anatomy of the brain and the entire physiology of the brain must be recast to agree with our embryological results.

The present article is an attempt to summarize, as simply as possible, the principal conclusions of recent researches on the nervous system.

Physiologists have long been accustomed to divide nerve fibers into two classes: efferent, or those which carry out impulses; and afferent, or those which carry in nerve impulses to the nervous system. Not infrequently the less accurate terms sensory and motor are used as synonymous with afferent and efferent respectively. The nerves are bundles of nerve fibers, and each nerve is supposed to have typically two roots—one sensory, by which all the sensory fibers enter, and the other motor, by which all the efferent fibers leave, the nervous system. It was supposed that every nerve fiber was connected with a nerve cell in the central nervous system, and that the nerve fibers grew out from the central nervous system. It has long been known that various nerves have thickenings at certain points; the thickenings are the so-called ganglia and they contain nerve cells. The cells in these ganglia were supposed to have migrated from the central parts along the nerves.

The preceding recapitulation of familiar elementary facts will serve to emphasize the following new conclusions: 1. The nervous system consists of two parts, which differ so markedly in their origin and differentiation that it would be hardly an exaggeration to say that there are two nervous systems, for the original duality is never obliterated. The two parts I shall term the