134 ON NERVE-EXCITATION BY THE NERVE-CURRENT. 



a contraction is obtained. If with previously closed battery-circuit 

 the nerve-circuit is opened, there is no contraction, assuming always 

 that the ascending- battery-current in the nerve is not so strong as 

 to give an opening contraction in spite of its partial compensation 

 by the nerve-current. If, on the other hand, with previously closed 

 nerve-circuit the battery-circuit is opened, the nerve-current finds by 

 this closure, a new channel of derivation, and a contraction follows, 

 which is, in this case, further strengthened by the influence of the 

 voltaic alternative. Then, if we may assume that this is due to the 

 circumstance that a spot at which a current enters becomes more 

 excitable to a current which makes its exit by it, the closing exci- 

 tation of the nerve -current will thereby be rendered all the stronger. 

 As to the influence which may be exerted by polarisation currents, 

 I will here entirely pass it over, since such currents are also closed 

 in the nerve-circuit whenever the battery-circuit is opened. 



All the above-mentioned circumstances must be taken into ac- 

 count, if the preparation does not give any contraction to simple 

 closure or opening of the nerve -circuit in the absence of any 

 battery-current. For, in this case also, the nerve-current is present, 

 and its influence manifests itself in the same direction. 



One thus obtains, with the wealcest branch currents from the battery 

 provided that they leave the nerve ly its transverse section) an ' opening 

 contraction ' first at opening of the battery -circuit, and only with con- 

 siderably stronger currents at opening of the nerve-circuit ; and in 

 correspondence with this a ' closure contraction ' shows itself, first at 

 closure of the battery -circuit, and only with stronger currents at closure 

 of the nerve-circuit. 



Up to this point we have assumed that the weak branch- 

 current from the battery leaves the nerve by its transverse section. 

 In the opposite case the consequence will be different according as 

 the battery-circuit is closed after previous closure of the nerve- 

 circuit, or vice versa. For, in the former case, a current that of 

 the nerve namely already enters by the electrode which is in 

 contact with the longitudinal surface, and this current will simply 

 be increased by the additional branch current of the battery. But 

 if the nerve-circuit is not closed until after the battery-circuit has 

 been closed, the nerve- and battery-currents add themselves together 

 at the moment of closure, and consequently the effect of such 

 closure will be greater. Similarly, by opening the nerve-circuit 

 both currents are simultaneously broken. 



