GENERAL PHYSIOLOGY OF NERVE-TISSUE. 121 



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 trans- 

 verse 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 electrom- 

 eter also reveal similar electric changes. It was also demonstrated 

 by Bernstein with a specially devised apparatus, the repeating rheo- 

 tome, that the negative variation is composed of a large number of 

 single variations which succeed each other in rapid succession and 

 summarize themselves in their effect on the needle. 



Electric Currents from Uninjured Nerves. The pre-existence 

 of electric currents in living and wholly uninjured nerves while at 

 rest has also been denied by Hermann, who regards all portions of 

 the nerve as isoelectric, any difference of potential being the result of 

 some injury to its surface. 



Action Currents. For reasons to be stated below, it is very diffi- 

 cult to determine the presence of diphasic action currents during the 

 passage of an excitatory impulse through the nerve-fiber. The so- 

 called negative variation of the resting nerve current, the demarca- 

 tion current, which is occasioned by tetanic stimulation, Hermann 

 regards as the expression of an action current which flows in the nerve 

 in a direction opposite to the demarcation current. The origin of this 

 action current is to be sought for in the continuous negativity of that 

 portion of the longitudinal surface of the nerve in contact with the 

 diverting electrode, while the dying substance of the transverse surface 

 takes no part in the excitation. This tetanic action current, or nega- 

 tive variation, was discovered by du Bois-Reymond, and Bernstein 

 later succeeded in obtaining this action current during the passage 

 of a single excitation process. That the return of the galvanometer 

 needle toward the zero point is not due to an annulment of the demar- 

 cation current itself, but to the appearance of an action current, is 

 shown by the fact that if the former be compensated by a battery 

 current until the needle rests on the zero point the appearance of the 

 latter current will cause the needle to swing in a direction the opposite 

 of that caused by the demarcation current. The negative variation 

 and action current may therefore be regarded as one and the same 

 thing. It is the expression of the change the nerve is undergoing 

 during the passage of the nerve impulse. The rapidity with which 

 the negative variation or action current travels, the variation in its 

 intensity from moment to moment, the time required for it to pass 

 a given point, would express the change in the nerve to which the 

 term nerve impulse is given. From experiments made with the 

 differential rheotome, Bernstein calculated that the speed of the 



