132 ON NERVE-EXCITATION BY THE NERVE-CURRENT. 



or with induction currents, the currents of the nerve itself are of 

 great importance, especially such as can be shown to exist in the 

 neighbourhood of the section or of branches, as well as in that of 

 the physiological terminations peripheral and central such currents 

 adding themselves algebraically to the exciting currents, and so 

 increasing or diminishing them. In this wise are explained the state- 

 ments concerning the differences of excitability at different points 

 in the course of a nerve-trunk, as well as the varying results of 

 excitation with weak currents.' 



Since a more detailed account may be expected from Griitzner at 

 some future time, I will here confine myself to the record of a 

 particular instance which belongs to this class of phenomena, and 

 which is of interest because a contraction actually caused by closure 

 of a derived nerve-current appears as if caused by opening of a 

 weak battery-current, and conversely a contraction caused by opening 

 of a branch of a nerve-current appears as if due to closure of a weak 

 battery-current ; a case then in which, as a matter of fact, a make- 

 contraction may be mistaken for a break-contraction, and vice versa. 



Assuming that a rheochord is used to send by derivation a fraction 

 of a battery-current through the nerve, and that the key is intro- 

 duced between the rheochord and the unpolarisable electrodes, then, 

 if the nerve is in contact with the electrodes by a transverse and 

 by a longitudinal surface, the key will short-circuit the nerve- 

 current, and possibly a contraction will ensue, even when the 

 rheochord is not in connection with the battery. 



Under similar conditions, but more seldom, there may be a 

 contraction when one opens the nerve-rheochord circuit, or, as I 

 shall term it, 'nerve-circuit.' For this to happen it is necessary to 

 choose a preparation which, under the above conditions, gives break- 

 as well as make-contraction with its nerve-current. 



If now the rheochord is connected with the battery, so that the 

 derived current is ascending in the nerve, this current will be 

 opposed to the nerve-current in the nerve-circuit, and may com- 

 pensate it exactly if of appropriate strength 1 , which particular case 

 we will now assume. The consequence will be that closure of the 

 nerve-circuit will no longer give any contraction, because, in spite 

 of the closure in the conductors between nerve and rheochord, no 



1 I need hardly remark that I speak here of compensation of the nerve-current not 

 in any absolute sense, but only in accordance with the conventional sense of the 

 word. For there is no question here of a true compensation of current in the nerve, 

 but only of a compensation of the branch current in the conductors which connect 

 the nerve and rheochord. 



