and Plants to the Electrical Phenomena associated with it. 45 



meter varied according to the strong line, the movement of the head of the 

 mercury column would be expressed by the faint line. We shall see as 

 we proceed that one or other of the three forms of photographic curve, 

 which correspond to the three forms of electrical change, just desig- 

 nated as typical, presents itself in every excitatory response we have 

 to investigate, provided that, as I mentioned just now, the changes 

 under one contact only are recorded. 



To ensure this, the exploring contacts must be so arranged, and the 

 muscle itself so prepared, as to enable us to separate the part of the 

 surface we desire to investigate from the rest, so far as concerns its 

 effect on the instrument we are using as indicator. It is obvious that 

 when we apply our leading-off electrodes to two parts of the surface, 

 both of which are at the same time undergoing change, there must 

 always be a difficulty in determining how far the effect is due to 

 changes at the one or at the other contact. It is therefore essential 

 for the correct observation of an electrical change at one of them, that 

 the other should be protected from disturbing influences. 



The Fir.4 Fundamental Experiment. 



An experiment will show how this may be accomplished. It will 

 also bring us face to face with a phenomenon which is, perhaps, the 

 most fundamental of those which at present concern us, the phenomenon 

 of the wave of excitation, or, to use the designation given to it by its 

 discoverer, the Reizwelle. The nature of the experiment is illustrated 

 by Diagram 3, in which the band of parallel fibres represents the 

 sartorius muscle. It is excited (instantaneously) at r. A change 



Diagram 3. 



occurs there which is propagated first to the proximal contact p, and 

 then onwards to the distal contact d, at a rate which in our preparation 

 may be 150 cm. per second; This change is essentially a vital one, 

 but it is attended by a mechanical change represented by the muscle 



