THE ELECTRICAL RESPONSE. 415 



A bar of non-conducting material revolves on a vertical axis a, 5 to 10 

 times per second; p x and p 2 are isolated metal points which dip into the 

 mercury pools, Q Q ; when both of them touch the mercury, the galvanometer 

 circuit is closed ; this happens every time the bar revolves. The opposite end 

 of the bar carries a pin p which touches the wire d and so closes the primary 

 circuit just before ^> l and p 2 dip into the mercury. The time-interval between 

 the two contacts — at d and Q — can be determined by reference to the scale, 

 which is divided into hundredths of a revolution. At the same moment that 

 p touches d, the muscle is excited at r. As long as the interval between the 

 two contacts remains the same, each repetition of the stimulus as the bar goes 

 on revolving, produces the same electrical effect. Consequently the similar 

 effects rapidly following one another may be taken as one, which has been so 

 magnified by repetition as to be measurable. 



To make an observation, the instrument is brought into circuit by closing 

 the key, A' 4 (Fig. 226), the binding screws of which are connected with Q and 

 Q. The contacts of ^j 1 and p 1 with Q Q may be so adjusted, that, supposing 

 the rate of revolution of the bar to be 10 per second, the duration of closure of 

 the galvanometer circuit shall be T §o of the period of revolution, i.e. 0"002 

 second. The rheotome is set in motion, and the key in the circuit of the 

 secondary coil, S, opened. If any deflection is observable, it is compensated, 

 and the key again closed. The contact p d is then so adjusted that d is struck 

 by p, say 0"01 second before the galvanometer circuit is closed at QQ; then, 

 the rheotome being still in motion, the same key is opened, for as short time as 

 is necessary to obtain a full effect, and the deflection noted. The time-interval 

 between the contacts is then reduced from 0*01 second to, say, 0'008 second, and 

 the observation repeated, and so on with further reduction of the time-interval 

 by the same amount each time until there is no longer any deflection, a result 

 which will be obtained when the interval has been reduced to - 002. This 

 having been accomplished, the first observation (time-interval 0"01 second) is 

 repeated, and a second series made in which the time-interval is lengthened 

 between each two observations, in the same way as that in which it was before 

 shortened. In this way a series of numbers, each representing a galvanometric 

 deflection, is obtained. The corresponding time-intervals between excitation 

 and closure of the galvanometric circuit vary from '002 second to 0'020 second. 

 If the circuit is arranged as in Fig. 223, the first set of deflections will 

 probably be southwards, the second set northwards. It is important to note 

 that between each observation and its successor, the muscle current (pre-existing 

 difference of potential) must be carefully compensated, for a muscle cannot be 

 subjected to repeated excitations without its electrical condition being altered. 



The result of such a series of observations is stated by Bernstein as 

 follows : — When a muscle is excited at r at a given moment, by an in- 

 duction shock, a certain time elapses before the excitatory variation begins 

 at I {i.e. at the proximal leading-off electrode). Here the effect culminates 

 very rapidly, and disappears so promptly, that it has already ceased by 

 the time that the " wave of contraction " has reached the same point. 

 The shading represents the " part of the fibre," the elements of which 

 participate in the process of negative change at the moment that I is 

 reached. This process lasts in each " element," according to Bernstein's 

 observations, about T i F to ^ho second. It advances along the muscular 

 fibre at a rate of 3 or 4 cms. in t £q second. He therefore designated 

 it the Eeizivelle or Excitation-wave. What he supposes actually to 

 happen in an uninjured muscle is illustrated by the diagram (Fig. 227). 

 Each excitation wave that starts from r r reaches first I 1 then I 2 . When 



it is at I 1 a current exists in the direction of the arrow >, when it 



is at P there is a current in the opposite direction. In such a muscle 



