6lz ELECTRO-PHYSIOLOGY. 



phenomenon of voltaic alternadves. It was Volta who first observed that on 

 causing an electric cnrrent to pass for a long time in a frog until contraction had 

 ceased, this phenomenon was realized anew when the current was made to pass 

 in the opposite direction. 



The exact analysis of this phenomenon is as follows : We have here the frog 

 cleft, as usual, in the middle and traversed for a certain time by the current ; it 

 is thus reduced to the state in which therQ is no logger contraction, except in 

 the inverse member at the opening of the circuit. I reverse the position of the 

 electrodes or of the frog, previously staining with a drop of ink the member 

 which was traversed by the direct current, and which no longer contracted. 

 The current being allowed again to pass for a certain time, the limb which no 

 longer contracted, and which is now traversed by the inverse current, begins 

 anew to be contracted, while at the closing of the circuit you have observed the 

 other limb contract through the action of the direct current, which before did 

 not occur, and this from the excitability maintained in its nerve by the inverse 

 current. In a word, the voltaic alternatives proceed from the different action 

 which the current exerts on the nerves accorcling to its direction : in the inverse 

 nerve the excitability is preserved and augmented, so that when the direct cur- 

 rent begins to pass, the contraction is excited, which Avould not be the case if 

 the nerve had not undergone the effect of the inverse current or had been sub- 

 jected to the direct current ; in the direct nerve the transmission of the inverse 

 current restores the excitability, so that it again contracts at the opening of the 

 circuit. 



It is by means of the dynamometer that this important proposition has been 

 rigorously demonstrated. We easily contrive to have both halves of the same 

 frog with a portion of the spine so fixed in the dynamometer that we can insert 

 the lower hook now in one, and now in the other leg, and cause successively 

 the same direct current to pass in one of the nerves and the inverse in the other. 

 By then measuring the elevations obtained in the different experiments v/e arrive 

 at an exact determination of the proposition in question. Thus we see that, in 

 causing the direct current to pass at certain intervals of time by opening and 

 immediately closing the circuit, the contraction due to this current decreases 

 with great rapidity ; yet it may be restored and with greater strength, at least 

 within certain limits, either by abbreviating the passage of the current or by 

 repeating its action after having left the circuit open long enough for the effect 

 of the current to disappear. It is quite different with the inverse current. 

 With this we see the contraction remain constant in the other half of the frog 

 under a frequent repetition of the passage of the current. The effect of the 

 inverse current may also be distinctly seen by causing it to pass, first for a 

 fraction of a second, and then for three, four, or five seconds. In this Avay we 

 obtain a contraction at the opening of the circuit which increases within these 

 limits in proportion to the time of the passage of the current. 



This experiment is even more conclusive when the inverse current is made to 

 act upon a nerve whose excitability has been weakened by the passage of the 

 direct current ; to obtain the maximum of contraction it is necessary that the 

 passage of the inverse current should be prolonged for a space of time extending 

 from tw(inty-five to thirty seconds. When we operate on the animal alive or 

 recently killed, it is suflicient, in order to dispel the effects of the continuous 

 passage of the current direct or inverse, to leave the frog to itself, that is, out- 

 side tiie circuit. It will be found that the nerve which has lost its excitability 

 through the direct current becomes from repose again capable of contraction by 

 means of the same current. 



I cannot leave this subject without calling your attention anew to the fact 

 already noticed of the tetanic and prolonged contraction, by which the member 

 traversed by the inverse current is attacked after a passage of thirty or forty 

 minutes. This fact has in latter times furnished, through a known physical 



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