EXCITATION OF NERVE AND MUSCLE BY GALVANIC CURRENT 51 



contraction. These prolonged contractions show that excitation is produced 

 not only at the make and break but also during the passage and for a short time 

 after the cessation of a strong constant current. 



2. An instructive variation of this experiment is to dissect out the rectus 

 abdominis muscles of a curarised frog, and place the non-polarisable electrodes 

 one in contact with the anterior, the other with the posterior end of the flat 

 muscular mass (iig. 45). The muscles are divided into several parts by tendinous 



FIG. 45. POLAR STIMULATION OP EECTUS ABDOMINIS. m, MUSCLE CURAKISED AND 



STRETCHED BETWEEN TWO PIECES OF CORK (AFTER VERWORN). 



septa, and it will be seen that during the passage of the constant current each 

 of these parts has the part directed towards the kathode in a condition of con- 

 traction, and the part directed towards the anode in a condition of relaxation. 



3. The effect of the poles of a constant current upon cardiac muscle can 

 be exhibited on the frog's heart. The frog is killed by destroying the brain, 

 and the heart is exposed in situ. Using non-polarisable electrodes and the 

 whole current of a Daniell cell with a mercury key and a commutator in 

 the circuit, place one electrode either in the mouth or on any part of the body 

 of the frog, and connect the other, by means of a short piece of cotton- wool 

 wetted with Ringer solution and drawn to a point, with the heart so as to touch 

 it near 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 partici- 

 pate 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 tuning-fork tracing 

 below, and measure exactly the period of latency in each case, i.e. the time 

 elapsing between the current of the electromagnetic signal and the commence- 

 ment of rise of the curve. Notice that it is slightly greater as the result of 

 breaking the circuit than on 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. 46, an ascending current is used instead of a 

 descending one, the result is complicated by the blocking effect of the con- 

 stant current on conduction (see pp. 43 and 48). 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 intermediate 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. 



