54 EXPERIMENTAL PHYSIOLOGY 



the base of the ventricle. If this electrode is the anode, on closing the 

 key it will be observed that the part of the ventricle underneath it 

 does not participate in the contractions, but remains quiescent, and, if 

 the heart be filled with blood, even bulges during general systole : on 

 opening the key this part passes into systole, even during general 

 diastole (physiological rebound). If the current be reversed and 

 the cotton- wool be made the . kathode, the reversed effects are 

 obtained. 



Polar excitation of nerve. Take a muscle-nerve preparation with 

 as long a nerve as possible and arrange it on the myograph. Non- 

 polarisable electrodes, connected with a constant battery through a 

 mercury key, are placed the anode in contact with the uppermost 

 end of the nerve, the kathode in contact with the lowermost end, i.e., 

 close to the muscle. Insert an electro-magnetic signal into the circuit 

 and cause it to mark on the drum just below the myograph lever. 

 Record two contractions, one produced by closing the mercury key, 

 the other (on a different abscissa) by opening it. Make a time 

 tracing below, and measure exactly the period of latency in each case, 

 i.e., the time elapsing between the current of the electro-magnetic 

 signal and the commencement of rise of the curve. ^Notice that it is 

 slightly greater as the result of breaking the circuit than 011 making 

 (by the time taken for the nerve impulse to traverse the length of 

 nerve), since the excitation at breaking is at the anode, i.e., at a point 

 of the nerve furthest from the muscle, whereas on making the excita- 

 tion was at the kathode, i.e., close to the muscle. 



If, as represented in Fig. 47, an ascending current is used instead 

 of a descending one, the result is complicated by the blocking effect of 

 the constant current on conduction (see p. 51). Thus, on making such 

 an ascending current, if it were a strong one, the excitation being at 

 the kathode, i.e., at the uppermost end of the nerve, and the inter- 

 mediate part of the nerve being at the same moment traversed by the 

 current, this would block the passage of the nerve impulse generated 

 at the kathode, and no contraction would result. Therefore, instead 

 of obtaining a contraction at both make and break, only the break 

 would produce a visible effect under these circumstances. On the 

 other hand, if the constant current is weak, its removal may not be 

 followed by contraction of the muscle, because the breaking of such a 

 current furnishes a smaller excitation than its making. 



The varying effects of opening and closing a galvanic current 

 (misnamed "the law of contraction") are illustrated by an experiment 

 devised by Pfliiger. The nerve 6f a nerve-muscle preparation is placed 

 on non-polarisable electrodes, which are connected with a battery of 

 at least three Daniell cells through a commutator and rheochord : a 



