ADVANCED EXPERIMENTAL PHYSIOLOGY 87 



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 XXII ) 



CHAPTER XXII. (Advanced). 

 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 86). 



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 



