Chap. VII. INTERRUPTED CURRENTS- 65 



the muscle preparation, and adjust it in the telegraph 

 as described. Take a Daniell's element, and cany 

 one wire from the positive pole to one side of the 

 friction key (Fig. 29) ; from the same side take 

 another wire directly to the screw s of the forceps of 

 the telegraph. Connect the negative pole of the 

 Daniell with the opposite side of the key, and from 

 that side take a wire to the hook in the ten do 

 Achilles. When the key is closed the current is 

 short-circuited ; when open, it passes through the 

 muscle. It will now be noticed that on opening and 

 closing the key, that is, on sending the current through 

 the muscle and on interrupting it, varying effects are 

 observed. Frequently there is only a feeble con- 

 traction of the muscle, shown by slight movement of 

 the telegraph signal ; the contraction is generally 

 more marked on interrupting the current ; but 

 while the current flows steadily through the muscle 

 no effect is apparent. Doubtless during the passage 

 of the current chemical changes are occasioned, and, 

 as will be seen in chap, viii., the excitability of the 

 nerve is altered; but 'no contraction occurs. In 

 other words, (1) a continuous current does not 

 stimulate to contraction, tohile (2) an interrupted 

 current does. Next, connect one pole of the 

 Daniell to the screw s" of the induction coil 

 (Fig. 23), and the other pole to one side of a simple 

 key, a wire from the other side of the key passing 

 to s'". By closing the key the current is sent round 

 the primary coil, and a single induction current or 

 shock is obtained, and the same on opening. Now 

 carry the wires from the secondary coil, one to the 

 forceps, and the other to the hook. On opening the 

 key a single vigorous contraction of the muscle occurs, 

 and the same on closing. Thus, (3) induced currents 

 of electricity are more stimulating than primary 

 currents. The arrangements for this experiment 

 P 7 



