GENERAL PHYSIOLOGY OF NERVE-TISSUE. 103 



The electromotive force of the nerve current varies in strength with the 

 length and thickness of the nerve. The strongest current obtained from 

 the nerve of the frog is equal to the 0.002 of a Daniell cell; that obtained 

 from the nerve of the rabbit, 0.026 of a Daniell. The existence of the nerve 

 current, its strength, duration, etc., depend largely on the maintenance of 

 physiologic conditions. All influences which impair the nutrition of the 

 nerve diminish the current. With the death of the nerve all electric phenom- 

 ena disappear. 



Negative Variation of the Nerve Current. During the passage of 

 the nerve impulse the resting nerve current, or the demarcation current, 

 diminishes more or less completely in intensity, undergoes a negative varia- 

 tion, as shown by the return of the galvanometer needle, due to a change in 

 its electromotive condition or to a diminution of the difference in potential 

 between the positive longitudinal and negative transverse sections. This 

 negative variation of the demarcation current is observed equally well from 

 either the central or peripheral end of the nerve. If the two ends of the 

 nerve are connected with galvanometers and the nerve stimulated in the 

 middle, the demarcation currents simultaneously undergo a negative varia- 

 tion. This may be taken as a proof that the excitation process propagates 

 itself equally well in both directions. The negative variation is intimately 

 connected with changes in the molecular condition of the nerve and is not 

 due to any extraneous electric or other influence. And du Bois-Reymond 

 was also enabled to obtain a negative variation of the current in the nerves 

 of a living frog which were yet in connection with the spinal cord. In this 

 experiment the sciatic nerve was divided at the knee and freed from its 

 connections up to the spinal column; the transverse and longitudinal surfaces 

 were then placed in connection with the electrodes of the galvanometer wires 

 and the current permitted to influence the needle. The animal was then 

 subjected to the action of strychnin. Upon the appearance of the muscle 

 spasms the needle was observed to swing backward toward the zero point to 

 the extent of from i to 4 degrees, and upon the cessation of the spasms to 

 return to its previous position. In an experiment of this nature it is obvious 

 that the negative variation was the result of a physiologic stimulation of the 

 nerve arising within the spinal cord. 



The question also here arises as to whether the negative variation is due 

 to a steady, continuous decrease of the natural current, or whether it is due 

 to successive and rapidly following variations in its intensity, similar to that 

 observed in muscles. Though this cannot be demonstrated with the physio- 

 logic rheoscope, as was the case with the muscle, there can be no doubt, both 

 from experimentation and analogy, that the latter supposition is the correct 

 one. It has been shown that when non-polarizable electrodes connected 

 with Siemen's telephone are placed in connection with the longitudinal and 

 transverse sections of a nerve, low, sonorous vibrations are perceived during 

 tetanic stimulation a proof that the active state of the nerve is connected 

 with the production of discontinuous electric currents. The oscillations 

 of the mercurial column of the capillary electrometer also reveal similar 

 electric changes. It was also demonstrated by Bernstein with a specially 

 devised apparatus, the repeating rheotome, that the negative variation is 

 composed of a large number of single variations which succeed each 



