88 PRACTICAL PHYSIOLOGY 



Du Bois key, are placed under both sciatic nerves ; the unpolarisable 

 electrodes are placed under one sciatic nerve midway between the 

 muscle and the stimulating electrodes. The induction shocks are now 

 allowed to pass through both nerves for a few seconds ; the muscles of 

 both legs are thrown into tetanus. The stimulation is stopped and 

 the polarising current is passed through the one sciatic nerve. The 

 faradisation of both nerves is again commenced; the muscle in the 

 one case will be sent into tetanus and quickly fatigued, but the other 

 muscle shows no contraction, for the polarising current passing 

 through its nerve blocks the passage of the nervous impulses evoked 

 by the stimulating electrodes. When the first muscle is fatigued the 

 polarising current should be broken ; the block is removed from the 

 course of the sciatic nerve of the other muscle, which is at once 

 tetanised by the stimulation of its nerve. 



CHAPTER XXIII. (Advanced). 

 THE ELECTROMOTIVE PROPERTIES OF MUSCLE AND NERVE. 



THREE simple experiments upon the electromotive properties of 

 muscle have already been described (page 51). The following ex- 

 periments require more care and very excitable tissues. 



Secondary Twitch from the Heart. If a freshly prepared and very 

 excitable nerve be laid upon the heart of a frog, 1 so that the cut end 

 of the nerve is on the base and the longitudinal surface upon the 

 apex of the ventricle, a twitch of the muscle connected with the nerve 

 is observed at each contraction of the ventricle. Each time the 

 muscle-fibres of the ventricle contract, a "current of action" is pro- 

 duced and stimulates the nerve 



A fine glass rod should be placed under the middle portion of the 



*For these preparations the frogs should have been kept cold for some time 

 before the experiment. 



