PART IX 

 THE CENTRAL NERVOUS SYSTEM 



CHAPTER LXXXVI 

 THE EVOLUTION OF THE NERVOUS SYSTEM 



The nervous system of the higher animals consists of the nerve cen- 

 ters, and the nerves with their various interconnecting tracts. The 

 nerve tract and centers are located mainly in the spinal cord' and brain, 

 where, by their interlacement, they form an extremely complex struc- 

 ture. The exact position of the centers and the course and connections 

 of the tracts with the centers are problems which, under the title of 

 neurology, have during recent years been contributed to more particu- 

 larly by the anatomist and the pathologist. The information thus 

 gathered tells us the possible tract or tracts of nerve fibers through which 

 the various centers may communicate either with one another or with 

 the structures outside the central nervous system upon which they 

 act. Since each of these centers may, however, be played upon by in- 

 fluences coming from different regions of the body, it is evident that there 

 must remain, as an equally important aspect of the subject, the investi- 

 gation of the means by which the various available centers and tracts are 

 brought into communication and action at the proper time. In other 

 words, we must investigate the functional uses of the available paths. 



We may compare the central nervous system with a telephone system, 

 the exchanges representing the nerve centers, and the wires the nerve 

 trunks. Any incoming wire may be connected by the operator with 

 any outgoing wire, but a knowledge of how each wire runs does not tell 

 us under what conditions the various wires will be connected for trans- 

 mission of messages. It is the same with the nervous system ; the neurolo- 

 gist can tell us how the tracts and centers run, but not the conditions 

 under which they may act together. This it is the duty of the physiologist 

 to ascertain. 



Since it is the degree of development of the central nervous system 

 which determines an animal's position in the evolutionary scale, much 

 information concerning the relative importance of the various parts of 



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