CONDITIONS AFFECTING A NERVOUS IMPULSE 261 



of a nerve-muscle preparation is passed through a glass tube which is made air-tight bv 

 plugs of normal saline clay surrounding the nerve at the two ends of the tube. By means 

 of two lateral tubulures a current of CO 2 , or air charged with vapour of ether or other 

 narcotic, can be passed through the tube. The nerve is armed with two pairs of elec- 

 trodes which are stimulated alternately, the pair within the tube serving to test the 

 action of the drug on the excitability, while the pair outside the tube show the 

 presence or absence of any effect on the conducting power of the nerve below it. 



Of the gases and vapours mentioned above, C0 2 and ether both diminish 

 and finally abolish the excitability and conductivity of the nerve fibres. 

 The conductivity, however, persists after all trace of excitability has dis- 

 appeared, before in its turn being also abolished. On removing the gas 



FIG. 109. Tracing to show the effect of ether on excitability and conductivity of nerve. 

 Nerve excited by single induction shocks alternately within and above ether, chambar. The 

 vertical lines indicate contractions of the muscle (gastrocnemius.) The lower line indicates 

 the periods during which the nerve was exposed to the action of ether. 



A, disappearance of excitability ; B, reappearance of excitability ; c, disappearance of 

 conductivity ; D, reappearance of conductivity. (From a tracing kindly lent by PROF. GOTCH.) 



or vapour by blowing air over the nerve, the conductivity and excitability 

 gradually return in the reverse order to their disappearance (Fig. 109). 



Alcohol is said to increase the excitability or leave it unaffected, while 

 diminishing the conductivity of the nerve. 



Chloroform rapidly abolishes both excitability and conductivity. It 

 is a much more severe poison than the drugs just mentioned, so that in many 

 cases its effects are permanent, and no, or only a very partial, recovery of the 

 nerve is obtained on removal of the chloroform vapour from the apparatus. 



