THE ELEMENTARY 

 NERVOUS SYSTEM 



CHAPTER I 



INTRODUCTION 



THE elementary nervous system is that type of nerv- 

 ous system in which the structural and functional ele- 

 ments present themselves in their simplest states. It is 

 most extensively represented in the lower multicellular 

 animals, but it is found locally in as highly differentiated 

 forms as the vertebrates. It probably represents the 

 primitive nervous system in that it reproduces much that 

 must have been characteristic of nervous organs in their 

 earliest stages of evolution. It has been an object of in- 

 vestigation and interest only within comparatively recent 

 years. Its derivative, that complex aggregate of organs 

 known as the central nervous system of the higher ani- 

 mals, attracted the attention of anatomists and physiol- 

 ogists at a much earlier period. This was doubtless due 

 to the unusual development of the central nervous organs 

 in man and other higher animals. The human brain is a 

 structure whose size and position naturally compels at- 

 tention, whereas the chief nervous organs in many of the 

 lower animals are often so insignificant as to be easily 

 overlooked. 



Although the first attempts at elucidating the struc- 

 ture and functions of the nervous system were made on 



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