442 STUDIES IN ADVANCED PHYSIOLOGY. 



these impulses themselves. That the sensation of sight 

 was produced by the optic nerve as such, and that a muscle 

 contracted in obedience to a peculiar motor nervous im- 

 pulse. Soon after, this very sweeping distinction was nar- 

 rowed down to a disvision into two kinds of nervous im- 

 pulses those which produced motion and those which re- 

 sulted in sensation. That such impulses were inherently 

 different was occasioned by the observation that when a 

 motor nerve was cut and the end connected with the muscle 

 pinched a contraction followed, but that when the central 

 end was pinched no sensation followed. While on the other 

 hand, when the sensory nerve was cut, no motion resulted 

 upon the distal end being pinched, but a decided sensation 

 followed the stimulation of the central end. There is, how- 

 ever, every reason to believe that these impulses do not dif- 

 fer among themselves, but that the difference in results is 

 due to the different endings which these nerves have. A 

 sensory nerve cannot produce motion for the simple reason 

 that it is not in contact with muscles, while a motor nerve 

 cannot produce sensation for the simple reason that it does 

 not run to centers where such sensations are received. 



That all nerve fibers are physiologically alike seems 

 further evident from the following reasons: (1.) A micro- 

 scopic and chemical analysis shows no differences whatever 

 between the nerve fibers. (2.) All nerve fibers carry im- 

 pulses in both directions. For instance, when a nerve is 

 stimulated in the middle of its course the impulse runs from 

 this point towards both ends with equal facility and rapidity, 

 and this is true whether the nerve be a sensory or a motor 

 one. (3.) The nature of the nervous impulse seems to be 

 the same no matter what may have been the occasion for 

 its excitation. Thus a nervous impulse resulting from the 

 pinching of a nerve is, as far as we are able to detect, per- 

 fectly identical with the impulses produced by an electric 

 current, or even by the natural end organ itself. Thus, the 

 impulse which runs along the optic nerve, regularly occa- 

 sioned by light falling on the retina, is identical with the 



