ELECTRICAL CHANGES IK MUSCLE 



229 



from x to a. When the change reaches a this becomes the seat of an 

 electromotive force of such a direction that the current would pass in 

 the outer circuit from b to a. We may say therefore that a is negative to b. 

 A fraction of a second later the excitatory change has passed on to b and has 

 died away at a. Now b is negative to a,* and the current therefore passes 

 in the opposite direction. Between a and b therefore, there is a diphasic 

 current, the first phase representing negativity of a to b, and the second phase 

 A B 







FIG. 86. Diphasi9 response of uninjured sartorius (obtained by analysis of curves such as 

 Fig. 85). A, .at 8C. ; B, at 18C. (KEITH LUCAS.) 



representing negativity of b to a. A diphasic change is thus also a sign of a 

 propagated change. Every excitation of a normal muscle gives rise to a 



* The statement that the excited portion of the muscle becomes ' negative,' though 

 sanctioned by long usage, is not very exact and may give rise to misconception. When 

 we lead off the terminals of a copper- zinc couple or cell to a galvanometer, a current 

 flows outside the cell from copper to zinc and inside the cell from zinc to copper. In 

 this case the zinc is said to be electropositive to the copper, and in the same way we 

 must assume that the excited portion of a muscle is electropositive to the unexcited 

 portions. When therefore we speak of any part of a tissue being negative, we are 

 using a conventional expression to indicate the direction of the current in the outer 

 circuit, and not the electrical condition of the tissue itself. In order to avoid the con- 

 fusion which might result from an attempt to replace the loose expression ' negative ' 

 by the more correct expression ' electropositive,' Waller has suggested the employ- 

 ment of the term * zincative ' to indicate the electrical condition accompanying excita- 

 tion. This term would also serve to emphasise the fact that the excited portion, like 

 the zinc in a zinc-copper cell, is the chief seat of chemical change. 



