372 THE POPULAR SCIENCE MONTHLY. 



STRUCTURAL PLAN OF THE HUMAN BRAIN. 



By Prof. CHAELES SEDGWICK MINOT, 



OF THE HARVARD MEDICAL SCHOOL. 



THE human brain is the most complicated organ known, and 

 although its anatomy has been the object of innumerable in- 

 vestigations, 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 develop- 

 ment 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 respect- 

 ively. 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 cen- 

 tral 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 nerv- 

 ous system consists of two parts, which differ so markedly in their 

 origin and differentiation that it would be hardly an exaggera- 

 tion to say that there are two nervous systems, for the original 

 duality is never obliterated. The two parts I shall term the 



