5°° 



NER VE. 



The closing anodic and opening cathodic blocks are evidenced in a 

 different form of experiment, namely, that which affords evidence for 

 the well-known third stage of Pffiiger's law of contraction. Here 

 the passage of an ascending current of considerable intensity excites, 

 on closure, at the upper or central pole (the cathode), the trans- 

 mission from this has to traverse the lower pole, the anode, and the 

 size of the transmitted effect may in consequence be so much reduced 

 as to be inadequate to evoke a muscular response. In a similar way the 

 feeble muscular response evoked on opening a strong descending current 

 is due to the inability of the opening excitation which occurs at the 

 upper anodic contact, to force the after block present at the cathode. 

 Both experiments show that the interference with conductivity must 

 be established at the instant of closure and opening respectively. The 

 terms block and after-block thus imply a condition of the tissue at the 

 anode on closure, at the cathode on opening, in which conductivity is 

 impaired. The degree of impairment, i.e. the extent of the block, 

 depends primarily upon the intensity of the polarising current causing 

 the change, but it also depends upon the duration of the closure ; for 

 although the anodic blocking commences at the instant of closure, it 

 resembles the anodic fall in excitability in being augmented during 

 the continued flow of the current. Thus a current of very short 

 duration does not impair anodic conductivity to the same extent as one 

 of long duration, and it may often happen that the stimulation of that 

 portion of a nerve, central to the polarising current, may be at first 

 adequate to force the block and evoke the muscle response, but may 

 cease to be adequate as the polarising current continues to flow, render- 

 ing the block far more complete. An experiment illustrating this is 

 the following. A series of galvanic currents of sufficient but uniform 

 intensity are closed at regular intervals in the ascending direction 

 through the sciatic nerve of the frog. The duration of closure is gradually 

 increased from a minimal time of about "001 sec. or less. As the duration 

 of each current is increased, the muscular response, which at first is not 

 present, commences and increases in magnitude ; it then diminishes and 

 can utterly fail ; finally, it reappears again, and may reattain its initial 

 maximal size. 1 The failure, termed by Fick, the gap, is obviously 

 due to an effective block at the lower anodic pole, and the resultant 

 interference with the propagation of the excitatory effect evoked on 

 closure at the upper cathodic pole. This block is only rendered adequate 

 when a certain duration of current flow has been reached ; beyond this 

 point the return of the contraction is due to the opening excitation at 

 the anodic lower pole. Increased duration thus acts like increased 

 intensity in augmenting the anodic block, and the variation of either 

 of these factors when the other is kept uniform can produce similar 

 results. Thus the passage of a series of very brief galvanic currents 

 of constant duration, but variable intensity, causes the same series of 

 phenomena ; the muscular responses appear when a certain intensity 

 is reached, increase in size, and then diminish until they fail, and 

 finally they reappear again and attain a definite maximum. The same 

 results can even be obtained by the use of ascending induced currents, 

 and their employment furnishes another proof of the truth of the 

 statement, that the anodic interference, although produced at the 



1 Fick, SUzungsb. d. k. Akad. d. Wissensch., Wien, 1862-63, Abth. 3, Bde. xlvi., xlvii., 

 xlviii. 



