SKELETAL MUSCLE AND THE NERVOUS SYSTEM 83 



One must distinguish clearly between a nerve fiber 

 and a nerve. The nerve is a cable and may have 

 bound up in it many thousand fibers. Nerves are 

 traced by the dissector and with the unaided eye; fibers 

 require high powers of the microscope to reveal their 

 character and numbers. For the present our interest is 

 in such a fiber as leads to a moderate number of end- 

 plates in a skeletal muscle. The axon of this fiber is a 

 conductor of something to the dependent muscle fibers, 

 but what is it that it conveys? How is the stimulation 

 made effective? These are questions that we cannot 

 answer very fully. It will be helpful to begin by ex- 

 cluding certain conceptions. 



Nature of the Nerve-impulse. First of all, that which 

 passes along fibers of the nervous system is not matter but 

 energy. We do not have to do here, as in the blood 

 system, with tubular conduits. An early and crude 

 notion was to the effect that fluid pulses travel along 

 the nerves. The idea was quite elaborately developed 

 and had its value but only as a provisional symbol. 

 We must likewise avoid thinking of nerves as though 

 they were cords to be pulled upon. The expressions 

 " nerve strain" and " nerve tension" encourage this 

 view but they are figurative. A man is said to have 

 tense nerves when the tension is really in his muscles. 

 Under such circumstances the muscular state manifests 

 a corresponding nervous condition but is not to be 

 identified with it. 



Another temptation is to think, of the nerves as con- 

 ductors of electricity. There is, in fact, nothing of 

 human construction so suggestive of the nervous system 

 as is the telephone equipment of a community. But it is 

 generally held that the form of energy which nerves 

 transmit cannot be the electric current as ordinarily 

 understood. It does have electric accompaniments and 

 it has lately been urged that the impulse itself is essen- 

 tially electric. If this is true, however, the conditions 

 prevailing in nerve fibers are still so peculiar as to forbid 



