ADVANCED EXPERIMENTAL PHYSIOLOGY 129 



The effect of the constant current upon the conductivity of the 

 nerve is determined upon the same preparation. The stimulating 

 electrodes are placed upon the central part of the nerve ; a minimal 

 stimulus is found, and its effect is recorded upon the stationary 

 drum. The polarising circuit is now closed through the nerve in 

 either the ascending or descending direction, and then the minimal 

 stimulus is again applied. It is no longer effective owing to the 

 decrease in the conductivity of the nerve. This change in the 

 conductivity of nerve is also shown in the experiment upon the 

 absence of fatigue in a stimulated nerve (Chapter XVII). 



CHAPTER XVII 

 THE ABSENCE OF FATIGUE IN A STIMULATED NERVE 



Nerves are not subject to obvious fatigue, if they be repeatedly 

 stimulated for long periods of time. The following experiment not 

 only demonstrates 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 

 (p. 124). 



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

 unpolarisable 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, 



