408 THE NEKVOUS SYSTEM 



Thus, an impulse started at the end of an axon is trans- 

 mitted directly to the protoplasm of the cell body and 

 through that to the extreme ends of the dendrites and 

 any other axones that may arise from that cell body. 

 Similarly, an impulse arising in the dendrites or cell body 

 is likewise transmitted over the axones to their termina- 

 tions. Bearing in mind that impulses usually originate in 

 one of three places, the cell body, the dendrites, or the ter- 

 minations of an axon, it will follow that the principal func- 

 tion of the nerve fiber is conduction. This characteristic 

 is still further evidenced by the fibrillar character of the 

 axon protoplasm. 



When two points are connected by two nem-a the arrange- 

 ment is as follows: The dendrites of one neuron are so 

 placed as to be brought in contact with the terminations 

 of the axon of the second. In this relation the course of 

 the impulse takes one of two directions, and this direction 

 may be formulated in the third law. 



Third law. An impulse which originates in the termi- 

 nations of the axon of one neuron, and has been conducted 

 to the dendrites of the nerve cell of that neuron, may pass 

 from those dendrites into the terminals of the axon of 

 the second neuron, and thus on to the dendrites in the 

 final cell of the series. Similarly, an impulse originating 

 in the cell body or dendrites of one neuron may pass, by 

 way of the axon terminations, into the dendrites of the 

 connecting neuron, and thus on to the final axon termi- 

 nations of the series. 



These three laws enable us to explain the method of 

 impulse transmission in all forms of nerve action; while 

 neura may differ in the numl:)er of axones, in the length of 

 the axones, and in location, the laws of action given above 



