304 PHYSIOLOGY OF MUSCLES AND NERVES. 



uiiclianged, but tlie pulsations must gradually decrease in 

 strength, and all effect must finally disappear, just when 

 the maximum of excitability, and the deatli which follows 

 this, pass the lower limit of the excited port'on. 



6. Difference between Closing and Opening Induc- 

 tive Currents. Helmholtz's Arrangement (p. 151). 



When an electric current is suddenly closed in a spiral, 

 tins not only acts inductively on a neighbouring spiral, but 

 the individual coils of the ])rimary spiral act inductively on 

 each other ; an analogous effect would occur on the opening, 

 but that the sudden interruption of transmission prevents the 

 development of this opening inductive current in the primary 

 coil. The inductive current which orisjinates on the closing 

 of the current being in an opposite direction to the closed 

 i current itself, the former must weaken the latter ; the cur- 

 rent can therefore attain full strength, not at once, but only 

 gradually ; but on the opening the current suddenly ceases. 

 This difference in the duration of the closing and opening 

 of the primary current corres[)onds with differences in the 

 currents induced by them in the secondary spiral, which are 

 used for the irritation of the nerve. Figure 73 exhibits these 

 characters. The upper part of the figure represents the tem- 

 poral course of the main current in the primary spiral of an 

 inductive apparatus ; the lower part represents the temj)oral 

 course of the induced currents in the secondary spiral. The 

 line . . .0 . . .t represents the duiation. The primary current 

 is closed at the moment o. Were the retardatory influence 

 which has been mentioned not present in the piimary spiral, 

 the current w^ould at once attain its full strength J ; but 

 owin^ to that influence it attains this strength onlv firraduallv, 

 somewhat as shown by the crooked line 3. With this gradu- 

 ally occurring cuiTent corresponds a closing inductive curi-ent 

 in the secondary spiral, as is represented by the curve 4 ; 



