84 HUMAN PHYSIOLOGY 



close comparison between them and telephone wires. 

 A wire can be cut and spliced, when it will conduct 

 nearly as well as before. A nerve cannot be reunited 

 after cutting so as to resume its service. 1 



The velocity at which the impulse passes along the 

 nerve has been measured quite accurately. By suitable 

 apparatus it is possible to show that when a considerable 

 length of nerve is used to excite contraction in a muscle 

 the response is distinctly less prompt than when the 

 stimulus is applied close to the muscle. The difference 

 in time between the two trials must be attributed to the 

 employment of an extra length of nerve in the first case. 

 By a simple calculation it appears that the rate of trans- 

 mission in the nerve of a frog at room temperature is in 

 the neighborhood of 100 feet per second. This is the 

 speed of an express train running 70 miles an hour. 

 Higher rates seem to bo reached in the warm-blooded 

 animals but the maximum is less than the velocity of 

 sound waves in the air, and is insignificant by com- 

 parison with the speed of light. 



The term nerve-impulse which we usually use to denote 

 the energy carried along a nerve does not bind us to any 

 particular theory as to its nature. Whatever it is, it is 

 rapid enough to insure prompt reactions in animals of 

 ordinary size. Furthermore, it is not appreciably ex- 

 hausting to the fibers concerned in forwarding it. 

 Muscles fatigue with use, as we have seen; nerves seem 

 nearly if not quite proof against fatigue. The reader 

 should recognize that this statement is limited to the 

 fibers on which our attention for the present is fixed. 

 There are other elements in the nervous system which 

 probably suffer wear and tear when in use. The sus- 

 ceptibility of these structures to fatigue will be dis- 

 cussed at another time. 



1 Surgeons often reunite nerves which have been cut and eventually 

 secure a renewal of action. But this does not contradict the statement 

 above ; new fibers must grow across the gap and extend to the terminations 

 before the result can be obtained. (See p. 90.) 



