THEORY OF THE. BREAK- CONTRACTION. 67 



as yet too weak to prevent the excitation being propagated to the 

 muscle, just as with an ascending polarising current of weak or 

 moderate strength the closing excitation at the negative pole is not 

 prevented from reaching the muscle. 



Strong currents are characterised by this, that the blocking 

 which takes place at the positive pole overpowers the excitation 

 proceeding from the negative pole. When the current is descending 

 this is of no consequence ; but if, on the other hand, the current is 

 ascending, the blocking at the positive pole is sufficient to stop the 

 excitation occurring at the negative pole. 



If the polarising current is ascending, the polarisation -current is 

 descending. However strong it may be made by strengthening 

 the polarising current, it must always evoke a contraction, since 

 there is nothing to hinder the excitation occurring at its negative 

 pole from being propagated to the muscle. 



It is otherwise if the polarising current is descending. In that 

 case the polarisation-current is ascending, and the excitation has 

 to pass the positive pole. The stronger the polarising current is, 

 the stronger also do the polarisation-current and the block at its 

 positive pole become. Yet, in order completely to prevent the 

 excitation from reaching the muscle, the strength of polarisation 

 must be considerable. Experience, moreover, teaches that the 

 descending break-contraction is far from disappearing with a 

 strength of current which is sufficient completely to do away with 

 the ascending make-contraction. It may even happen, as Pfliiger 

 expressly stated in his original scheme of the law of contraction, 

 that the descending break-contraction does not disappear at all. 

 The reason, of course, is that the polarisation-current is not 

 sufficiently strong. 



The polarisation remaining over from previous stimulations has 

 also in all likelihood much to do with the disappearance of the 

 descending break-contraction. In experiments on Pfliiger's law of 

 contraction, as is well-known, we soon find that the ascending make- 

 contraction disappears. It only remains then to bring about the 

 disappearance of the descending break-contraction to demonstrate 

 the whole law. If we now gradually increase the strength of the 

 polarising current, we need no longer keep changing its direction, 

 but may content ourselves with testing its action when descending, 

 since we have already become acquainted with the whole law of 

 contraction so far as concerns an ascending current. Every such 

 stimulation, however, with a descending current leaves behind it a 



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