218 PHYSIOLOGY OF MUSCLES AND NERVES. 



mitted only toward the periphery or also toward the 

 centre ; or as to whether, when a sensory nerve alone is 

 irritated, the excitement is transraitted only toward the 

 centre or also toward the periphery. For as the sensory 

 nerves do not pass at the periphery into muscles, by 

 means of which their actions could be expressed, there 

 is no means of telling whether the excitement in them 

 is transmitted to the periphery. But our knowledge of 

 the electric changes which occur during activity affords 

 a means of determining this question. For these 

 changes are observable in the nerve itself, independently 

 of the muscles and other terminal apparatus. If a 

 purely motor nerve is irritated, and is then tested at a 

 central point, negative \ariation is found to occur in 

 this also; and similarly, if a purely sensory nerve is 

 irritated, negative variation may be shown in a part of 

 the nerve lying between the irritated point and the 

 periphery. This, therefore, shows that the excitement 

 in all nerve-fibres is capable of propagation in l^oth 

 directions ; and that if action occurs only at one end, 

 this is due to the fact that a terminal apparatus capable 

 of expressing the action is present only at that end.^ 



4. If negative variation in the nerve current is 

 really a necessary and inseparable sign of that condition 

 within the nerves which is called the ' activity of the 

 nerves,' it must, like the excitement, propagate itself 

 within the nerve at a measurable speed. Bernstein 

 succeeded in proving this, and measured the speed at 

 which the propagation occurs. If one end of a long 

 nerve is irritated, the other end being connected with 

 a midtiplier, a certain time must elapse before the 

 irritation, and consequently also the negative variation, 

 ' See Notes and Additions, No. IL 



