THE EXTRA-CURRENTS 19 



low resistance in parallel with the primary coil, the induced shocks are 

 rendered of nearly equal value. Test this on the nerve-muscle preparation. 

 On varying the position of the secondary coil as in the preceding experiment, it 

 will now be found that the strength of the break shock has become nearly 

 equal to that of the make shock, which has also been somewhat reduced. 



Thus in one experiment it was found that the farthest position of 

 the secondary coil from the primary at which a break shock caused 

 a twitch of the muscle was 26i cm. A make shock was first effective 

 when the coil was brought up to 1(H cm. With the deriving circuit 

 of low resistance as in experiment 2 the break shocks first produced 

 a twitch when the coil stood at 10 cm., and the make shock was 

 effective when the coil stood at 9^ cm. 



Experiment 3. — Demonstrate upon the nerve-muscle preparation the 

 existence of the break extra-current, arranging the apparatus as in Experi- 

 ment 2, p. 14 (fig. 21). 



Experiment 4. — Demonstrate the make extra-current, arranging the 

 apparatus as in fig. 25. A current is sent through the primary coil and elec- 

 trodes arranged in parallel and with a Du Eois key k 2 interposed so that both 

 may be short-circuited. Interpose a 

 friction key k 1 and a resistance-box r 

 in the main circuit. Also place a key 

 k :; in the electrode circuit. The cur- 

 rent on reaching the key k 2 divides, 

 and as the resistance of the piece of 

 nerve across the electrodes is very 

 high, most passes through the primary 

 coil, which therefore acts as a deriving ' 1 B^T 



circuit. Close the key k 1 and open _, „_ . . 



-, • , J i r . , Fig. 25. — Arrangement of Apparatus 



k-, and now interpose enough resist- „ „ „ , „ 



... 1 . i °i • ■? for Showing Make Extra-current. 



ance at R until opening and closing K A 



gives no contraction of the muscle. Next, with k 1 and k 3 closed, open and 

 close k-. Each time k 2 is opened the muscle contracts, stimulated by the 

 make extra- current in the primary coil, for the strength of constant current 

 at the same time sent through the nerve has, by increasing the resistance r, 

 been reduced until it no longer was able to stimulate on make. On closing 

 the key k 2 the currents through both primary and nerve are short-circuited, a 

 break extra-current is produced in the primary, which, however, is short- 

 circuited by the key k 2 . The break of the current through the nerve is not 

 sufficient to stimulate, and the muscle does not contract. 



If the key K 2 be kept open and k 1 closed and opened, a contraction occurs 

 both at make and break. This arrangement thus demonstrates both make 

 and break extra-currents. 



MAKE A GRACILIS AND SEMIMEMBRANOSUS 

 PREPARATION 



First study the relations of these muscles as given in figs. 22 and 23. 

 Pith a frog and dissect away the skin from the thigh, carefully cutting 

 through the fibres of the rectus interims minor, which are inserted into the 

 skin on the adductor surface of the thigh. The muscles can then be readily 

 made out on the inner side of the thigh separated from one another by the 

 rectus interims minor. The gracilis, or rectus interims major, a, is to be seen 

 from the front of the thigh, being in relation on its outer edge with the 



c2 



