IRRITABILITY AND CONDUCTIVITY 173 



a silk thread to the extreme end of the neive, 

 and fasten the thread to the end of the seeker 

 by a drop of wax cement. With the aid of the 

 seeker, pass the thread through the holes, and 

 draw the nerve after, so that the nerve lies on 

 the electrodes. The nerve should be drawn 

 through until the muscle is close to the gas 

 chamber. Stop the holes through which the 

 nerve passes with normal saline clay. Bring the 

 outer pair of electrodes against the central end 

 of the nerve near its exit from the gas chamber. 

 Determine which position of the pole-changer 

 corresponds to each pair of electrodes. Stimulate 

 the nerve first within the chamber, and then on 

 the central end of the nerve, using a current just 

 sufficient to cause tetanus. In both cases tetanus 

 will result. Now pour 20 per cent hydrochloric 

 acid on the marble in the generator. After the 

 gas has passed through the chamber for a moment, 

 stimulate as before. 



Stimulation of the portion of the nerve exposed 

 to the carbon-dioxide is no longer effective, while 

 stimulation of the part central to the gas chamber 

 still produces tetanus. 



But the nerve impulses created by stimulation 

 of the nerve central to the gas chamber cannot 

 reach the muscle except by passing along the 

 nerve and through the carbon-dioxide. The con- 



