126 PHYSIOLOGY OF MUSCLES AND NERVES. 



pulsates, tlius indicating that something, which we have 

 called excitement, has occurred within the nerve. While, 

 however, the current flows steadily through the nerve, 

 the muscle remains perfectly quiescent, nor is . any 

 change apparent in the nerve itself. Yet it may easily 

 be proved that the electric current has effected a com- 

 plete change in the nerve, not only in that part traversed 

 by the current, but also in the neighbouring parts above 

 and below the portion of the nerve subjected to the 

 electric current. The great importance of this lies in 

 the fact that it reveals relations between the forces 

 prevailing in the nerves and the processes of the elec- 

 tric currents, which relations are of great importance in 

 the explanation of the activity of nerves. 



Our knowledge of nerves has not as yet reached 

 a point at which it is possible to understand all the 

 changes which occur within them under the influence 

 of electric currents. Indeed, but one set of these changes 

 can as yet be described : these are the changes in the 

 excitability. Of all the vital phenomena of nerves, their 

 capacity of being brought into an active condition by 

 irritants has at present alone been studied by us. This, 

 as has been said in the previous chapter, may be quan- 

 titatively determined. Experiment shows that the ex- 

 citability may be altered by electric currents. If a 

 small portion of a nerve is placed on two wires in such 

 a way that an electric current may be caused to traverse 

 this portion, it appears that not only the portion actually 

 traversed by the current, but the nerve beyond this, 

 also suffers changes in its excitability. In order to 

 study these, let us imagine several pairs of wires ap- 

 plied to the nerve n n' (fig. 30). Through one of 

 these pairs of wires, c d, let a constant current be 



