MOTION IN ANIMALS AND PLANTS. 



337 



in every excitatory response we have to investigate, provided that, as 

 I mentioned just now, the changes under one contact only are recorded. 

 To insure this the exploring contacts must be so arranged and the 

 nuiscle itself so prepared as to enable us to separate the part of the sur- 

 face 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-ofl' 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 etlect is due to changes at 

 the one or at the other contact. It is therefore essential for the cor- 

 rect observation of an electrical change at one of them that the other 

 should be protected from disturbing inlluences. 



THE FIRST 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 illus- 

 trated by diagram 3, in 

 which the band of parallel 

 libers represents the sar- 

 torius muscle. It is excited 

 (instantaneously) at /•. A change occurs there which is propagated 

 first to the proximal contact p, and then onward to the distal con- 

 tact d^ at a rate which in our preparation may be 150 centimeters per 

 second. This change is essentially a vital one, but it is attended by a 

 mechanical change represented by the muscle curve and an electrical 

 change, which we record photographically. Diagram 4 will serve to 

 explain what (as will be immediately seen) actually happens at the 

 moment the wave passes under p. - It means that a current sud- 

 denly appears there, of which the direction is from p> to d. When 

 the wave reaches d^ a second efl'ect of the same kind (/>') occurs, 

 of which the direction is opposed to the first. What the galvano- 

 scopic effect of this must be is easily understood from diagram 4, 

 in which the two curves P' and I)' are placed in a relative posi- 

 tion to each other which expresses their time relation. The two 

 effects sum together. In the diagram the curve S' expresses the 

 result of that summation, i. e., the actual variations of difference of 

 potential between the contacts which occurred while the wave was 

 SM 99 22 



t 



Dia^rAtn 2. 



