ELECTRICAL CHANGES IX MfsrLE 



also passes down the muscle at the same rate as the mechanical 

 change which it precedes. If we are leading off from x and a, the 

 electrical change ensues immediately upon the stimulus, i.e. there 

 is no latent period to the electrical change. On leading oS from 

 a and b there is a latent period between the stimulus and the first 

 change, representing the time taken for the change to travel from 

 ./: 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 6 to a. We may say. therefore, that a is 

 negative to b. A fraction of a second later the excitatorv change has 



o * 



passed on to b and has died away at a. Xow b is negative to a,* 

 and the current therefore passes in the opposite direction. Between 

 './ and b. therefore, there is a diphasic current, the first phase repre- 

 senting negativity of a to b, and the second phase 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 diphasic variation of such a direction that the point stimu- 

 lated first becomes negative to all other points of the muscle, and 

 this 'negativity,' to use a loose but convenient expression, pa- 

 as a wave down the muscle, preceding the wave of contraction and 

 travelling at the same rate. 



If one leading-off point be injured, e.g. at b. the change accompany- 

 ing excitation is absent at that point. A single stimulus applied at x 

 will in this case sive only a monophasic variation in which a is 



relativelv negative to b. 



- , 



When we study the time relation of the electrical variation ensuing 

 on a single stimulus, we find that the electrical change under the 

 electrodes begins at the moment that the stimulus is applied. It 

 takes about '0025 sec. to attain its cumiinating-point. At this 

 point the mechanical change or contraction of the muscle begins. 



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 mis- 

 conception. 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 electro- 

 positive to the copper, and in the same way we must assume that the excited 

 portion of a muscle is really electropositive to the unexcited portions. When, 

 therefore, we speak of any part of a tissue being negative, we are using a con- 

 ventional 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 confusion which might result from an attempt to replace the loose 

 expression 'negative ' by the more correct expression Electropositive,' Waller 

 has suggested the employment of the term " zincative " to indicate the electrical 

 condition accompanying excitation. This term also serves to emphasise the 

 fact that the excited portion, like th^ zinc in a zinc-copper cell, is the chief 

 of chemical change. 



17 



