7 8 TEXT-BOOK OF PHYSIOLOGY. 



axis of the transverse surface have an electromotive force in the frog muscle 

 of from 0.037 to 0.075 of a Daniell cell. 



The electric currents in the muscle ^ are intimately associated with the 

 chemic changes underlying its nutrition,' and hence their intensity rises and 

 falls with all the conditions which maintain or impair muscle nutrition and 

 irritability. The currents observed in the injured muscle during the inactive 

 state have been termed currents of rest. Du Bois-Reymond regarded them 

 as pre-existent, intimately connected with the living condition of the muscle, 

 and essential to the performance of its functions, and to be explained by the 

 view that the entire muscle is composed of molecules each of which exhibits 

 the same difference of potential on its longitudinal and transverse surfaces 

 as the muscle prism itself. Hermann denies the existence of currents in 

 normal resting muscle and attributes them to injuries of the surface, due to 

 methods of preparation, in consequence of which the tissue dies and becomes 

 electronegative to the uninjured area, which remains electropositive. These 

 currents Hermann terms "demarcation currents." 



Negative Variation of the Muscle Current. If a muscle exhibiting a 

 current of injury be excited to activity by tetanizing induced currents 



applied to the opposite end of the 

 muscle, it will be observed that as 

 the contraction wave passes over 

 the muscle there is a movement of 

 the galvanometer needle toward 

 the zero point, indicating a dimin- 

 ution of the potential on the longi- 

 tudinal surface. To this dimin- 

 ution in the strength of the cur- 

 rent the term negative variation 

 was given. On the withdrawal of 

 FIG. 36. THE NEGATIVE VARIATION OF THE the stimulus the needle again re- 

 DEMARCATION CURRENT. A. The contraction turns i n a s hort time to its former 

 wave, which as it passes beneath the electrode at B 



causes a diminution of potential. position. The diminution of po- 



tential on the longitudinal surface 



of the muscle is now attributed to the passage of the excitation and contrac- 

 tion processes, to a temporary disintegration of the muscle substance (Fig. 

 36). With their disappearance and the subsequent restoration of the 

 nutrition of the muscle, the former electric condition returns. 



The primary deflection of the galvanometer needle is due to the demarca- 

 tion current which arises as a result of the difference in electric potential 

 produced by the destructive chemic changes taking place at the cut end^of 

 the muscle. The negative variation is caused by the fact that the activity 

 of the muscle, with its attendant chemic changes, will always be greater in 

 the uninjured equatorial region, and hence will always tend to counterbalance 

 the original source of difference in electric potential. 



Electric Currents from Non-injured Muscles. Though perfectly 

 normal resting muscle, according to Hermann, is isoelectric, nevertheless 

 electric currents are developed during activity to which he has given the term 

 action currents, and which are attributed to the propagation of the contrac- 

 tion wave. 



