192 PHYSIOLOGY 



thus, 1, 3, 5 are connected to one pole, while 2, 4, 6 are connected to the other. 

 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 6 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. Supposing 

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

 if c is pushed close to 6, the B.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 T ^- 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 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 continued 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 

 being more 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 



