304 ELECTRO-PHYSIOLOGY. 



Our consideration will first be directed to the different electro-physiological 

 effect of the discharge or electric current, according to the direction in which it 

 traverses the nerves. By the direction of the current, we know, is meant the 

 direction in which the positive fluid developed by the chemical action under- 

 gone by the zinc, and which is diffused in the liquid, moves in the iuterpolar 

 arc. In a Leyden jar, of which the internal armature gives a discharge of posi- 

 tive electricity and the external of negative, the discharge is also said to be 

 directed from the internal armature to the external in the interpolar arc. In 

 speaking of the current transmitted in the nerves, it will be understood that 

 this may be propagated either from the nervous centres to the extremities, in 

 which case it is called direct, descending, or centrifugal, or it may be propar 

 gated from the extremities of the nerves to the nervous centres, being then 

 called inverse, ascending, or centripetal. 



This being premised, our first proposition is as follows: "In the mixed 

 nerves, the first and sole effect obtained is the contraction produced at the 

 moment 'N^hen the direct or descending current, rendered as little intense as 

 possible or propagated with the greatest slowness, begins to pass. On in- 

 creasing the intensity of the current or the velocity of the discharge, the second 

 electro-physiological effect which arises is the contraction excited, at the opening 

 of the circuit, by the inverse or ascending current; on still increasing the in- 

 tensity of the current the contractions occur at two other instants, namely, when 

 the direct current ceases and when the inverse begins to act. . These different 

 phenomena embrace to a certain point the known electro-physiological periods 

 of Kitter and Nobili, which chiefly consist in obtaining at first and while the 

 excitability of the nerve is yet Ytxy great, contraction as well at the opening 

 as at the closing of the circuit, whatever may be tjie direction of the current in 

 the nerve; and, during a succeeding period of less excitability, contraction at the 

 closing of the circuit only with the direct current, and contraction at the opening 

 only with the inverse current." 



1 shall not describe all the different means which have been employed to 

 establish this proposition experimentally, but shall limit myself to an account 

 of that which has been practiced with most success, briefly indicating, however, 

 in passing, the various causes of error which occur in other modes of operating. 



Our experiment consists in preparing the frog after tlie manner of Galvani, 

 in removing the muscles and bones of the pelvis, and cutting the symphysis of 

 this pelvis. In this way the frog is reduced to the two upper members, which 

 only remain united by means of the lumbar nerves connected with a portion of 

 the spine. It is readily perceived that by touching one of the extremities of 

 the frog or the corresponding nerve with a pole of the battery, and the other 

 member or its nerve with the other pole, we shall have at the same time and in 

 the same animal one of the nerves traversed by the direct, and in the other by 

 the inverse current. In this way we can test with more certainty and exact- 

 ness the effects developed by the current according to its direction in the nerves, 

 since it is the same current which passes at the same time in two directions in 

 two like nerves of the same animal. The nerve is not alternately subjected to 

 the current in opposite directions ; moreover, the direction in which the nerves 

 are traversed by the current is well known, which is not the case when the 

 poles of the battery are applied to a nerve laid bare in an animal and surrounded 

 on all sides by muscles. VVe know in effect that when the poles of a battery 

 are immersed in a liquid and much extended mass, or two points of an extended 

 metallic plate are touched by two poles, the electricity is distributed through all 

 the points of those conductors, forming, as it were, so many minute threads radi- 

 ating from the points touched by the poles. 



In the present case we must not forget that, by this diffusion of the current, 

 it may happen that while the nerve is traversed between the two poles by a 

 direct current from the positive to the negative pole, there may be in the same 



