420 THE PROPERTIES OF STRIPED MUSCLE. 



such curves of an uninjured parallel-fibred muscle are (1) The rate of 

 propagation of the excitatory wave ; (2) the time after excitation at 

 which the two contacts become equipotential ; and (3) the moments at 

 which the first and second phases respectively attain their maxima, and 

 the differences of potential which exist at these moments. The propaga- 

 tion-rate is determined by the comparison of the curves of two 

 observations in which the distance between the seat of excitation and 

 the contact nearest to it is different (the distance between the leading- 

 off electrodes remaining the same). The time-interval between the 

 moment of excitation and the moment corresponding to the summit 



FIG. 232. Copies of the diphasic (black) and monophasic (grey) curves 

 of the sartorius, as they are seen when the two photographic records 

 are placed one over the other, so that the beginnings coincide. Up 

 to the beginning of the second phase the curves are identical. Rate 

 of movement of the plate as in Fig. 231. 



of the curve in each case can thus be estimated and compared. 

 The moment of equipotentiality is that at which the positive 

 difference of potential, indicated by the downward inclination of the 

 curve, becomes equal to the negative, corresponding to the upward 

 displacement of the meniscus. This event is indicated in the diagram, 

 Fig. 233, by the vertical line, and coincides with the arrival of the 

 excitatory wave at the midpoint between the contacts. The rate of pro- 

 pagation and the moment of equipotentiality having thus been determined, 

 the times of culmination of each of the two phases of the excitatory 

 wave can be readily deduced from them. Thus, if the propagation-rate 

 be 2000 mm. per second, and the distance 

 between the electrodes 20 mm., it is obvious 

 that the excitatory change must arrive at the 



proximal electrode 0*005 seconds "before the 



FIG. 233. moment of equipotentiality, at the distal 



electrode 0'005 second after that event. It 



may be seen from the photographic curve reproduced in Fig. 232 that 

 the culmination of the first phase coincides with the middle of the 

 ascending limb of the curve, that of the second phase with the beginning 

 of the descending limb. The photographic record of a monophasic 

 response, as obtained from the same sartorius muscle when one end has 

 been devitalised by immersion in normal salt solution at the temperature 

 of over 60 C., is shown in Fig. 232, by the shaded area. It will be observed 

 that it differs from that yielded by the uninjured muscle chiefly by the 

 absence of the second phase. In both cases the arrangement of the 

 leading-off contacts was the same, the proximal contact being near the 



