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 active tissues (muscles and glands), and in the course of such 

 transmission to make whatever 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 an- 

 other for the passage 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 which are 

 such important features of modern life. 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. 



135 



