218 PHYSIOLOGY OF Mt'SCLES AND NERVES. 



milled only toward the periphery or also toward tin- 

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

 irritated, the excitement is transmitted 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 apparatu.-. If a 

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

 central point, negative \ariatiou is found to occur in 

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

 irritated, negative variation may he shown in a part of 

 the nerve lying between the irritated point and the 

 peripherv. This, therefore, shows that the excitement 

 in all nerve-fibres is capable of propagation in both 

 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. 1 



4. If negative variation in the nerve current is 

 really a neces.-ary and inseparable -i^n of that condition 

 within the nerves which is called the 'activity of the 

 nerves,' it must, like the cxeilement. propagate itself 

 \\illiin the ner\e at a measurable speed. lierustein 

 succeeded in proving this, and measured the .-p.-ed at 

 which the preparation occurs. If one end of a Ion-,' 



nerve is irritated, tl (her end bein^ connected with 



a multiplier, a certain time must elapse before the 

 irritation, and consequent 1\ also the negative variation, 



1 Sn- Null-sand Addition-, N.I. 11. 



