CHAPTER IX 

 ANATOMY OF THE NERVOUS SYSTEM 



General Statement. In Chapter III the special function of 

 the nervous system was outlined, and was shown to involve the 

 transmission of stimuli from the sensory regions of the body to 

 the muscles, and in the course of such transmission to make what- 

 ever modifications are necessary to the production of the best 

 results. The sensory regions of the body are numerous; there are 

 likewise many muscles. Successful adaptation of the individual 

 to his surroundings may call at one time or another for the pas- 

 sage of stimuli from any sensory region to any muscle, or for the 

 combination of stimuli from several sensory regions to form 

 stimuli to go to any group of muscles. A somewhat analogous 

 situation occurs in the telephone systems with which the country 

 is dotted. In these communication may be desired between any 

 pair of instruments in the system. To make this possible all the 

 telephones in any one system are led into a central exchange 

 where provision is made for connecting any instrument with any 

 other. Flexibility of communication between sensory and motor 

 regions in the Body is secured in somewhat similar fashion. All 

 nerves from sensory regions are led into a central " exchange" 

 from which start all nerves to the motor organs. 



Nerve Impulses. Since it is impossible to describe the nervous 

 system without frequent reference to the messages which nerves 

 carry it is desirable before proceeding farther to state that it has 

 become the custom to call these messages nerve impulses. When 

 we speak of a nerve impulse we have in mind the process by which 

 the message is transmitted along the nerve. The situation cor- 

 responds to that in a telephone wire. When the latter is trans- 

 mitting a message the words spoken into the transmitter are 

 not carried along, but an electrical disturbance which they set 

 up. So the nerve does not transmit the exact stimulus which acts 

 upon it, but a nerve impulse which the stimulus arouses. 



Neurons. The nervous system as a whole is made up of struc- 



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