GENERAL PHYSIOLOGY OF NERVE-TISSUE 



105 



gitudinal surface are electrically positive to the transverse surface and that 

 the point of greatest positive tension is situated near the equator (Fig. 51). 



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., depehd 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. ! j 



Negative Variation of the Nerve Cur- 

 rent. 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 variation, 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 nega- 

 tive 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 galvanom- 

 eters and the nerve stimulated in the middle, 

 the demarcation currents simultaneously un- 

 dergo a negative variation. This may be 

 taken as a proof that the excitation process 

 propagates itself equally well in both direc- 

 tions. The negative variation is intimately 

 connected with changes in the molecular con- 

 dition of the nerve and is not due to any ex- 

 traneous 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 longi- 

 tudinal 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 ap- 

 pearance 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 experi- 

 ment 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- 



FIG. 51, DIAGRAM TO ILLUS- 

 TRATE THE CURRENTS IN NERVES. 

 The arrowheads indicate the direc- 

 tion; the thickness of the lines in- > 

 dicates the strength of the currents. 

 (Landois and Stirling.) 



