PART IX 

 THE CENTRAL NERVOUS SYSTEM 



CHAPTER I. WW VI 

 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 trad The 

 nerve trad and centers are located mainlj in the spinal cord and brain, 

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

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

 of tlie tracts with the centers are problems which, under t lie title of 

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

 larly by the anatomisl 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 thes,. .-enters may, however, be played upon by in- 

 fluences coming from differenl regions of the body, it is evident that tl 

 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 

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

 words, we must investigate tin functional uses <>t tin avaUabU paths. 



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

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

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

 any outgoing wire, but a knowledge of how each wire runs does aot I 

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

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

 gist can tell ns how the tracts and centers run, hut not the conditi 

 under which they may art together. This it is the duty ^'.' the physiolo( 

 to ascertain. 



Since it is the degree of development of the central nen .- system 

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

 Information concerning the relative importance of the various parts of 



781 



