106 TEXT-BOOK OF PHYSIOLOGY 



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 

 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 pass- 

 age of an excitatory impulse through the nerve-fiber. The so-called negative 

 variation of the resting nerve current the demarcation 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 demarca- 

 tion 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 negative 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 demarcation 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 under- 

 going 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 negative variation is about 28 meters a 

 second; that it is at first feeble, soon rises to a maximum, and then declines; 

 that is requires 0.0006 to 0.0008 of a second to pass a given point. From these 

 data it is evident that the negative variation or action current has a space 

 value of 0.0006 of 28 meters or about 18 mm. Transferring these state- 



