104 MUSCLE CURRENTS. [BOOK i. 



i.e. A will be negative relatively to B, and this will be shewn by 

 a deflection of the needle. A little later, B will be entering into 

 contraction, and will be becoming negative towards the rest of the 

 fibre, including the part under A, whose negativity by this time 

 is passing off; that is to say, B will now be negative towards A, 

 and this will be shewn by a deflection of the needle in a direction 

 opposite to that of the deflection which has just previously taken 

 place. Hence, between two electrodes placed along a fibre, a single 

 wave of contraction will give rise to two currents of different 

 phases, to a diphasic change ; and this, indeed, is found to be 

 the case. 



This being so, it is obvious that the electrical result of tetanizing 

 a muscle when wave after wave follows along each fibre, is a com- 

 plex matter ; but it is maintained that the apparent negative 

 variation of tetanus can be explained as the net result of a series of 

 currents of action, due to the individual contractions, the second 

 phase of the current in each contraction being less marked than 

 the first phase. We cannot, however, enter more fully here into a 

 discussion of this difficult subject. 



When we study, as we may do with the help of appropriate 

 apparatus, the rapidity with which the electrical change accompany- 

 ing a muscular contraction travels, we find it to be the same as 

 that of the contraction wave itself. The older observations seemed 

 to shew that the electrical change fell entirely within the latent 

 period, and might, therefore, be regarded as an outward token of 

 invisible molecular processes, occupying the latent period, and 

 sweeping along the muscular fibre ahead of and preparing for the 

 visible change of form. And, indeed, since we are led to regard 

 the change of form as the result of chemical processes taking place 

 in the muscular substance, we must suppose that the change of 

 form is preceded by molecular chemical changes. But, as we have 

 said, a latent period of measurable length does not appear to be 

 an essential feature of a muscular contraction ; we may, under 

 certain circumstances, fail to detect a latent period. And some 

 recent observations seem to shew that the electrical change and 

 the change of form may begin at the same time. Indeed, some 

 have maintained that the former is the result of the latter, and 

 not, as suggested above, of the forerunning molecular events. The 

 question however is one which cannot at present be regarded as 

 settled. 



The Changes in a Nerve during the passage of a Nervous 



Impulse. 



65. The change in the form of a muscle during its contrac- 

 tion is a thing which can be seen and felt ; but the changes in a 

 nerve during its activity are invisible and impalpable. We stimu- 

 late one end of a nerve going to a muscle, and we see this followed 



