292 PHYSIOLOGY 



fatigue comes on. Since both nerves have been excited throughout, it 

 is evident that the fatigue does not affect the nerve-trunk. We have 

 already seen that a muscle will respond well to direct stimulation when 

 stimulation of its nerve is without effect, and must therefore conclude 

 that the first seat of fatigue is the junction of nerve and muscle, i.e. the 

 end-plates. 



In the normal intact animal the break in the nemo-muscular 

 chain which is the expression of fatigue occurs still higher up, i.e. in 

 the central nervous system, and is probably due to some reflex inhibi- 

 tion of the central motor apparatus from the muscle itself. Thus after 



FIG. 109. 



complete fatigue has been produced in a muscle so far as regards 

 voluntary efforts, direct stimulation of the muscle itself or its nerve 

 will produce a contraction as great as would have been the case at the 

 beginning of the experiment. 



THE INFLUENCE OF DRUGS. The most important drugs with an 

 influence on nerve fibres are those belonging to the class of anaes- 

 thetics. Of these we may mention carbon dioxide, ether, chloroform, 

 and alcohol. 



The action of any of these substances on the excitability and conductivity 

 of a nerve may be studied by means of the simple apparatus represented in 

 Fig. 109. The nerve of a nerve-muscle preparation is passed through a glass tube 

 which is made air-tight by 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 electrodes which are stimu- 

 lated alternately, the pair within the tube serving to test the action of the drug 

 on the excitability, while the pair outside the tube shows 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 disappeared, before in its turn being also abolished. 



