310 ELECTRO-PHYSIOLOGY. , 



water in which are plunged the two electrodes of a battery, or by directly ap- 

 plying these electrodes to the tAVO nerves. 



It is known in physics that when the cuiTent traverses a liquid by means of 

 two electrodes of platina, the products of the electrization — that is, the oxygen 

 and the acids which go to the positive pole, the hydrogen and oxides which go 

 to the negative pole — become tenaciously fixed on the metals of the electrodes, 

 so as to give place, by closing the circuit betAveen these- alone, to a current which 

 we call secondary, and which has a contrary direction to the current of the bat- 

 tery ; because, as is known in electro-chemistry, the current developed between 

 hydrogen and oxygen, between an oxide and an acid, passes in the liquid from 

 the first to the second. And that current, in effect, would be obtained in the> 

 same manner if those plates of platina which form the electrodes had been for 

 a certain time immersed, the one in the hydrogen gas or the potassa, the other 

 in the oxygen gas or the acid. 



When the current is made to pass by means of the electrodes of platina placed 

 on the nerves of the prepared frog, then, too, the secondary polarity is produced, 

 and hence at the opening of the circuit of the battery and by closing the circuit 

 of the electrodes alone we have the secondary current, Avhich is capable of ex- 

 citing the muscular contraction. If we wish, in some very delicate experiments, 

 to avoid this secondary polarity, we use for electrodes, instead of wires of platina, 

 wires of zinc amalgamated, and between these and the nerves is interposed a 

 card moistened with a saturated solution of sulphate of zinc. Under these con- 

 ditions, as I shall show in the sequel, the secondary polarity disappears entirely 

 or nearly so, and hence there is a method, the great utility of which we shall 

 presently see, of excluding from the circuits into which animal electro-motors ' 

 enter, such as the muscles or organs of electrical fish, the secondary current, 

 which, as being opposed to that of these electro-motors, fails not to diminish and 

 even entirely destroy it. 



Let lis turn to the experiment with which I propose to show the action ex- 

 erted by the continuous current on the excitability of the nerves, according to 

 the direction in which it traverses them. In order that there may be no uncer- 

 tainty in the results, and that you may have before you different proofs of the 

 same truth, I have arranged four similar experiments, viz: four frogs prepared 

 alike and all traversed from one extremity to the other by the electric current. 



If the experiment is newly prepared wc already know what occurs. When 

 I open the circuit the member traversed by the inverse current alone contracts ; 

 and Avhcn I again cause the current to pass, that member contracts through 

 which flows the direct current. This first experiment never fails when we use 

 the frog recently prepared and possessing much vitality or a very strong cur- 

 rent, for at first the two members contract as well at the opening as the closing 

 of the circuit; but after a few moments the phenomenon appears, Avhich may be 

 distinguished as normal, namely, contraction in the member, which for brevity 

 wc will call direct, at the closing of the circuit, and contraction in the inverse 

 member when the circuit is opened. 



Before proceeding further, I must notice an important observation of Maria- 

 nini: that the contraction corresponding to the opening of the circuit is mani- 

 fested even when there has been no contraction at the closing. To show this, 

 instead of caiv«ing the current to pass by immersing the electrodes of the battery 

 in the water, I close the circuit with a conductor moist but not thoroughly wet. 

 such as would be suiiplied by our fingers or a piece of flannel or cotton dipped 

 in the water of one of the goblets. In this way the water penetrating layer by 

 layer into the material or the hand, the current also enters and increases from 

 degree to degree in the circuit, and hence the contraction is absent; not so, how- 

 ever, when the circuit is interrupted — then the passage of the current ceases of 

 a sudden and the contraction is obtained. 



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