120 TEXT-BOOK OF PHYSIOLOGY. 



these facts it is evident that all points on the longitudinal surface are 

 electrically positive to the transverse surface and that the point of 

 greatest positive tension is situated near the equator (Fig. 50). 



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, 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 phenomena disappear. 



Negative Variation of the Nerve Current. During the pas- 

 sage of the nerve impulse the resting nerve current, or the demarca- 

 tion current, diminishes more or less completely in intensity, undergoes 

 a negative variation, as shown by the return of the galvanometer 

 needle, due to a change in its electromotive condition or to a diminu- 

 tion of the difference in potential between the positive longitudinal 

 and negative transverse sections. This negative variation of the de- 

 marcation current is observed equally well from either the central or 

 peripheral end of the nerve. If the two ends of the nerve are con- 

 nected with galvanometers and the nerve stimulated in the middle, 

 the demarcation currents simultaneously undergo a negative variation. 

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

 itself equally well in both directions. The negative variation is inti- 

 mately 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 per- 

 mitted 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 physiologic rheoscope, as was the case with 

 the muscle, there can be no doubt, both from experimentation and 



