188 THE CIRCULATION OF THE BLOOD 



trie currents set up by the contact of metal with the saline constituents 

 of the muscle juices. 



If we connect a galvanometer by means of nonpolarizable electrodes 

 with two parts of a denervated muscle (the curarized sartorius of the 

 frog), it will be found that a current is set up whenever a wave of con- 

 traction passes over the muscle from one end to the other. The part 

 which first contracts becomes electrically negative to the rest of the muscle, 

 but as the wave of contraction passes along it, the " negativity" de- 

 creases at the end at which the wave started until, when the wave has 

 reached the middle of the strip, neither end of the muscle shows any 

 difference in potential, so that the string comes back to a position of 

 rest. However, as the contraction wave reaches the farther end of the 

 muscle, this lead in turn becomes negative, and the string swings in the 



^ Fig. 53. Diagram to illustrate the development and spread of the wave of negativity in a 

 strip of muscle (curarized sartorius) when stimulated at the end (P). The shaded portions show 

 the position of the negativity. The portion of the curve drawn by the deflections of the galvanom- 

 eter at each stage are shown at the right (a, b, c, and d). (After Lewis.) 



opposite direction (Fig. 53). From this comparatively simple experiment 

 it can be seen that a muscular contraction wave arises at the electrode which 

 is negative first, and that the movement of the string of the galvanometer is 

 most marked that is, the deflection is greatest when the two electrodes 

 are applied at the extreme ends of the muscle. When they are brought 

 closer together, the initial deflection becomes much less marked ; in other 

 words, the amplitude of the negative wave is greatest when the time 

 interval between the receipt of the excitation at the two contacts is 

 greatest. 



The application of these facts to the study of the initiation of the beat 

 in the auricle requires that we should consider another proposition: 

 namely, if a pair of contacts are arranged in the center of a circular 

 sheet of muscle and the edge of this sheet is stimulated at different 



