IX.] PEOPEETIES OF CONTEACTILE TISSUE. 93 



4. Interrupted Current. 



Introduce into the secondaiy circuit a key to 

 short circuit the current, and remove the wires 

 of the Daniell's cell from the screws at the top 

 of the primary coil to those at the base\ The 

 machine should work in such a way that im- 

 mediately the key in the primary circuit is put 

 down the hammer is at once thrown into oscilla- 

 tions (as indicated by its noise). The key in the 

 secondary circuit being open, shut the primary 

 key. Immediately that the sound of the hammer 

 is heard, the leg is thrust out straight by the 

 tetanic contractions of the muscle, and remains 

 so as long as the current continues to be thus 

 made and broken. Open the primary key; at 

 once the limb becomes flaccid and quiet. The 

 'interrupted current' should not be applied for 

 more than a few seconds. 



5. Chemical Stimulation. 



Expose the heart (cp. Less. II. A. § 1), and bleed 

 the frog by cutting across the aorta. Out through 

 the tissue above the sciatic nerve up to a quarter 

 of an inch beyond the end of the urostyle, and 

 there cut it across, with scissors and seeker isolate 

 the nerve up to the cut. Let the extreme end 

 of the nerve dip into a saturated solution of 

 sodium chloride ; watch the leg. 

 In a variable time twitchings of the toes will be 

 seen, and after a while these will increase in 



' Cp. Appendix. 



