260 PHYSIOLOGY 



1J to 2 seconds. During this time the whole heart is in an excited 

 condition. Both base and apex are equally excited, and there can 

 be no difference of potential between them. The excitatory condition 

 then passes off, first at the base and then at the apex. There is thus 

 a small period of time in which the apex is still contracted or excited 

 while the base is relaxed, and the apex is therefore negative to the 

 base. This terminal negativity of the apex is shown on the photo- 

 graph by the excursion of the column of mercury away from the 

 point of the capillary. If one terminal, e.g. the apex, be injured, we 

 obtain quite a different variation, which is shown in Fig. 88 B. It 

 is evident from this figure that the electrical sign lasts practically 

 as long as the mechanical sign of the excited state, and that we are 

 not justified in regarding the first spike of the diphasic variation as 

 indicative of an excitatory wave attended by an electrical change 

 which is independent of the succeeding mechanical change. 



FIG. 89. Superimposed photographs of the electrical variation of the 

 sartorius in response to a single stimulus. (BunDON SANDERSON.) 



The only difference between the electrical changes in this case 

 and in that of voluntary muscle is that in the latter all processes 

 are very much quicker, so that as a rule the point a (Fig. 85) 

 has ceased to be negative before the negativity of b has attained 

 its full height, and there is thus no prolonged equipotential stage. 



Although in the case of the slowly contracting ventricle of the tortoise, 

 the record obtained of the electrical changes accompanying its contraction by 

 means of the capillary electrometer shows with great clearness the diphasic 

 nature of the variation, and therefore the wave character of the electrical 

 change, considerable difficulty is experienced sometimes in recognising that the 

 ' spike ' record of the electrical change in voluntary muscle or in nerve is also 

 due to a diphasic variation. In this case the electrical change at any spot lasts 

 only about -g-jjij second, and there is not a prolonged equipotential period, 

 as in the case of the heart. The nature of the variation is, however, obvious, if 

 we compare the electrometer record of an intact and therefore currentless muscle 

 with that of a muscle in which one of the leading-off points has been injured, so 

 as to give rise to a demarcation current. The two curves are given in Fig. 89, 

 the upper shadowy tracing being that obtained from the injured muscle. It will 

 be seen that the distinguishing character of an electrometer record of a diphasic 

 variation in the rapidly contracting striated muscle consists in the fact that the 

 downstroke of the image of the meniscus is as rapid as the upstroke, whereas the 

 monophasic variation of the injured muscle presents a slow fall produced by the 



