ELECTRICAL BREAK-EXCITATION. 105 



polarisation, yet this resulting- polarisation is alone of importance 

 here, since we only used weak currents (usually of less than one 

 Daniell) in our excitation-experiments, and closed the exciting 

 current for a period of a second on an average. We therefore 

 consider it safe to assume that in these experiments positive 

 polarisation has never occurred, or, to put it otherwise, has always 

 been more than compensated by far stronger negative polarisation. 



However important, then, this positive polarisation may be in 

 itself, we may treat it as if it had been absent in our experiments. 



The further part of our task, namely to show that the cause of 

 the break-excitation is to be sought, not in the disappearance of a 

 current, but in the appearance of the (negative) polarisation-current, 

 is now very easy. We shall bring together all the cases in which 

 break-excitations have been observed, and endeavour to determine 

 for each case whether there can be a polarisation-current in the 

 nerve without external connection. 



W^e shall first consider the law of contraction as it occurs when 

 the unpolarisable electrodes have been applied to a nerve at in- 

 different points. In this case the effect, in so far as shown in the 

 muscle, of closure of descending currents (which closure elicits 

 a response with currents of a certain strength and upwards), 

 increases with increase of the strength of current, but soon 

 reaches a maximum at which it remains. Break-excitations with 

 ascending currents follow exactly the same law, except that they 

 require stronger currents for their first production. 



On the other hand, it is known that make-contractions with 

 ascending currents increase with the strength of current up to 

 a certain current-strength, but decrease with greater current- 

 strengths until at last they disappear altogether. Break-con- 

 tractions with descending currents follow the same law, except 

 that they also require stronger currents for their first produc- 

 tion. 



The fact that the phenomena group themselves into these two 

 series is readibly intelligible on our hypothesis ; for the opening of 

 descending currents is equivalent to the closure of weaker ascending 

 ones, while the opening of ascending currents is equivalent to the 

 closure of descending ones. 



Excitation of a muscle or nerve, as Pfliiger, von Bezold, Engel- 

 mann, and Biedermann have shown, occurs at a pole on closure at 

 the cathode, and on opening at the anode. The latter excitation is 

 also the weaker. We should say, however, that the excitatory 



