258 



PHYSIOLOGY 



These time-relations vary with the temperature of the muscle. We 

 have already seen that the effect of lowering the temperature is to 

 increase the latent period of the contraction. In the same way it 

 slows the rise of the electrical change and the rate of propagation 

 of the wave of electrical change. This is shown in Fig. 87, in which 

 are given the diphasic response of the sartorius first at 8 C. and 

 secondly at 18 C. We are therefore justified in regarding the electrical 



sec 



FIG. 87. Diphasic response of uninjured sartorius (obtained by analysis of curves 

 such as Fig. 86). A, at 8 C. ; B, at 18 C. (KEITH LUCAS.) 



change as an index to the chemical changes evoked in the muscle 

 as the direct result of the stimulus. The flow of material, which is 

 responsible for the change in form of each contracting unit, is secondary 

 to these changes. As the result of stimulation, a chemical change is 

 aroused at the point of excitation and travels thence along the muscle 

 fibres at a rate of about three metres per second, i.e. the same rate 

 as that of the following wave of mechanical change, and, like this, 

 varying with the temperature. Under certain conditions an excita- 

 tory condition may be propagated without the presence of a visible 

 contraction, Thus, if tjie middle third of the sartorius be soaked 



