INFLUENCE OF POLARISATION CURRENTS. 509 



the battery circuit key, K^ is kept closed, then the closure of K l simply 

 completes the circuit for two equal and opposite derivations, one from 

 the rheochord, the other from the nerve ; it has therefore no exciting 

 effect either on closure or on opening. With the electrode key, K x , closed, 

 and lu open, a circuit for the nerve derivation is completed ; although 

 the closure of K % may not evoke contraction, its opening evokes one. 

 The closure under these conditions brings about, in reality, the sudden 

 cessation of the nerve current through the oppositely directed battery 

 current, and the opening causes the reinstatement of the nerve current 

 in its full intensity plus polarisation currents. 



With a descending current since this is in the same direction as the 

 nerve current, both the closure and the opening of K x evoke effects 

 far more readily than the closure of K 2 . 



Influence of polarisation currents. — A second means for increasing 

 the exciting efficiency of the opening of a current is closely related 

 to the above. It is efficacious in any region of the nerve, and in un- 

 injured nerves {i.e. connected with the spinal cord) as well as in those 

 with a cross section. It is best illustrated by an experimental descrip- 

 tion. A nerve having been prepared, is excited by the closure of a 

 current too weak to excite on its cessation ; an intense current is now 

 used, which excites both at closure and cessation ; and it is then found 

 that, following this, the former weak current now excites both at closure 

 and at opening. The polarisation after-current is here increased in 

 its intensity by summing with that caused by its predecessors ; and on 

 opening it is unmasked, and, being suddenly established, excites at the 

 anode, which is now its cathode. 



The above facts show that there are circumstances under which an 

 apparent opening excitation is in reality the closure of an oppositely 

 directed current ; hence the excitation in these instances is, as in the 

 closing one, based upon the sudden development of catelectrotonus, 

 which, owing to the altered direction of the after effects, occurs at what 

 was the anode of the polarising current. 



This view is further supported by the fact that such opening excita- 

 tions, with their resultant muscular contractions, have the same time 

 relations as the closing ones, the period of delay being identical in the 

 two cases; whilst Tigerstedt found that such opening excitation is 

 favoured by all agents which increase the actual amount of the polarisa- 

 tion current, as estimated by appropriate galvanometric methods. 1 



It thus appears that, since the sudden establishment of catelectro- 

 tonus produces a closing single excitation, the same sudden establishment 

 of catelectrotonus through polarisation currents, etc., if it occurs on open- 

 ing at the anode, must produce an opening excitation. There is, however, 

 ample proof that this is not the only factor concerned in the opening 

 excitation. The matter has been already partly referred to; further 

 evidence will now be adduced to show that we are still bound to employ 

 Pfliiger's expression, and assume that the disappearance of the state of 

 anelectrotonus determines an excitation as well as the appearance of 

 that of catelectrotonus. 



Excitation in consequence of the disappearance of anelectrotonus is 



to be regarded as a rebound of the tissue from a greater state of 



stability (inexcitability) towards one of molecular instability. Bieder- 



mann has brought forward demonstrative evidence that the excitation 



1 Tigerstedt, "Biological Memoirs," Oxford, 1887, vol. i. pp. 49-74. 



