192 PHYSIOLOGY 



The rheocord is used to modify the amount or strength of current flowing through 

 a preparation. One form of it is represented in Fig. 50. A constant source of current 

 at B causes a flow of electricity from a to b through a straight wire. As the resistance 

 of this wire is the same throughout its length, the fall of potential from a to b must 

 be constant. The nerve, or whatever preparation is used, is connected with the 

 straight wire at two points, at a and at c, by means of a sliding contact or rider. Sup- 

 posing that there is an electromotive difference of one volt between a and 6, it is evident 

 that if c is pushed close to b, the E.M.F. acting on the nerve will be also one volt. The 

 E.M.F., however, may be made as small as we like by sliding c nearer to a. Thus if 



ab is one metre, and there is a difference 

 of one volt between the two ends, then 

 if c be one centimetre from a, the E.M.F. 

 acting on the nerve will be -j-J^ volt. 

 Thus we alter the current passing through 

 the nerve by altering the E.M.F. which 

 drives the current. 



If a weak current from a Daniell's 

 cell (or any other form of battery) 

 be passed through a muscle or any 

 FIG. 50. part of its nerve, at the make of the 



current the muscle gives a single 



sharp contraction a muscle-twitch. In this contraction the whole of 

 the muscle fibres may be involved. During the passage of the current 

 no effect is apparently produced and the muscle seems to be quiescent, 

 though on careful observation we may see that there is a state of con- 

 tinued contraction limited to the immediate neighbourhood of the cathode, 

 which lasts as long as the current is passing through the muscle, and 

 is not propagated to the rest of the muscle. If the current be now 

 broken, the muscle may remain quiescent. If however the current is above 

 a certain strength, the muscle responds to the break of the current with 

 another single rapid contraction. With a current of moderate strength we 

 may get a contraction both at make and break of the current, but the make- 

 contraction may be stronger than the break-contraction. Thus stimulation 

 is caused by the make and break of a constant current, the make- stimulus 

 beingjmore effective than the break- stimulus. If the duration of the passage 

 of the current is sufficiently short, no contraction is produced 'at the break 

 of the current, however strong this may be. The same phenomenon of a 

 single twitch may be evoked by the passage of an induction shock. This 

 is the current of momentary duration produced in the second circuit of 

 an induction-coil by the make or break of a constant current in the primary. 

 Using this mode of stimulus, it is found that the contraction on break of 

 the primary current is much stronger than that on make. It must not 

 be imagined, however, that there is any contradiction between this and the 

 fact that the make of a constant current is a stronger stimulus than the break. 

 When we put a muscle in the secondary circuit and make a current in the 

 primary, there is a current of momentary duration induced in the secondary, 

 so that there is a current made and broken through the muscle ; and the 

 same thing takes place again when the primary circuit is broken. It has 

 been shown that, when we use currents of such short duration, the break 



