330 PRACTICAL PHYSIOLOGY 



CHAPTER XVII. 

 THE ABSENCE OF FATIGUE IN A STIMULATED NERVE. 



NERVES are not subject to fatigue, even if they be repeatedly stimulated 

 for long periods of time. The following experiment not only demon- 

 strates this fact, but at the same time shows that the passage of a 

 constant electrical current through a portion of a nerve blocks the 

 transmission of the excitatory state which is produced in the nerve by 

 a stimulus applied above the polarising electrodes (page 324). 



An induction coil is arranged for faradic shocks, and a pair of un- 

 polarisable electrodes are connected by a Du Bois key with a Daniell 

 cell. The two sciatic nerves of a pithed frog are dissected up to their 

 points of exit from the vertebral column, which is then cut across above 

 the nerves. The thighs are cut away above the knee, and the two legs 

 with their nerves are placed in a moist chamber, and are fixed by pins 

 pushed through the lower extremities of the femora. The stimulating 

 electrodes, which are connected with the secondary coil by means of a 

 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 XVIII. 

 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. 



