298 ELECTRO-PHYSIOLOGY. 



a sliort time, about tlie tentli of a second, after the body has left that position ; 

 hence it is clear that, if the rotation is accomplished in less than ^\ of a second, 

 our perception will be that of a ring or circle, because the body will return tn 

 the point from which it stalled before the first impression on the eye will have 

 ceased. But this appearance is no longer realized if the body and our eye are 

 illuminated by a light which lasts for an interval of time much shorter than 

 that of the rotation of the body. This is precisely the case with the electric 

 spark, whose duration has been measured upon principles already indicated, 

 and has been found to be less than -jtoo^o ^^ ^ second. A cord which seems 

 enlarged when it vibrates, the insect which seems of greater size when its wings 

 move rapidly, the liquid vein which appears continuous, are all illusions which 

 cease when these bodies are illuminated with the electric spark. 



The spark which endures for so short a time, and which is due to a very small 

 quantity of electricity, at least in comparison with that produced in a battery, 

 gives very strong shocks, or, in other words, violent pains and contractions. I 

 take a prepared frog, and submitting it to the action of a small Leyden jar, I 

 discharge the latter two, three, or four times in succession with a metallic arc, 

 until not only do I no longer obtain sparks, but on testing the jar with a delicate 

 electroscope I perceive no sign of electricity ; still, as you see, the frog repeat- 

 edly contracts when I discharge the jar through it. It is not, then, I repeat, on 

 the absolute quantity of electricity that the electro-physiological effect depends, 

 but rather on the duration of the variable electric state of the circuit in which 

 the nerves of the frog are included. This duration is, with an equal quantity 

 of electricity, smaller in the case of the discharge of the jar than with the voltaic 

 current. It is thus that the intense physiological effects of the discharge of the 

 jar or of the inducted currents are to be explained. 



These considerations have led some physicists to suppose a certain analogy 

 between the electro-physiological effects and the action of the curreut in develop- 

 ing inductive currents when h begins and ceases to act. To represent to our- 

 selves mechanically, and yet perhaps not inaccurately, how the excitation of a 

 nerve arises under electricity, we may suppose that there then occurs in that 

 nerve what occurs in a mass of soft iron or even in certain transparent bodies 

 under the action of a strong electric current : there is a new molecular equi- 

 librium which in the one case accompanies the new magnetic state, and which 

 in the other gives to the body the property of causing the rotation of the polar- 

 ized ray. The excitation of the nerve — that is, the faculty of causing the muscle 

 to contract, would consist in, or at least be accompanied by a new molecular 

 state, and the excitation of the nerve and the physiological effect would depend 

 on the passage from one molecular state of the nerve to another excited by elec- 

 tricity and by the greater or less rapidity ■srith which it is accomplished. 



I deem it important to dwell upon thesencousiderations, and shall employ for 

 exciting the muscular contractions, not the discharge of the jar, of which it is 

 impossible to know the quantity of electricity, but the electric current of a 

 battery, made to pass for a very short time into the nerves of a prepared frog. 

 In this experiment we can know the duration of the current, and hence the 

 quantity of zinc which is oxidized in the battery in that time, as well as the 

 quantity of muscular labor which the electric excitation of the nerve produces. 

 This investigation will conduct us to important consequences regarding the 

 nature of the electric excitation of the nerves. The experiment is quickly 

 arranged : I take a battery formed of very small elements of platina and zinc 

 amalgamated, because oxidation does not take place except when the circuit is 

 closed. In the circuit of this battery I place a galvanometer and a prepared 

 frog attached to the dynamometer. 



This circuit is interrupted at a point where the two extremities formed by two 

 springs of steel or brass are established very near, but uot so as to touch one 

 another. If we take a strip of brass and with it touch the two springs at the 



