260 



PHYSIOLOGY 



subsides as 

 Since both 



secondary currents pass through the upper ends of both nerves in series (Fig. 

 107). At the same time a constant cell is connected with two non-polarisable 

 electrodes (np, np) placed on the nerve of B, so that a current runs in the 

 nerve in an ascending direction. The effect of passing a constant current 

 through a nerve is to block the passage of impulses through the part traversed 



by the current. When the con- 

 stant polarising current is made, 

 _ x the muscle may give a single 

 Liiiiiuiniiiip _:i twitch, and then remains quies- 

 s_^_~/-' cent. The exciting current is 

 then sent through both nerves by 

 the electrodes e t and e z . The 

 muscle A enters into tetanus, 

 which gradually subsides owing 

 to " fatigue." When A no longer 

 responds to the stimulation, the 

 constant current through the nerve 

 of B is broken. B at once enters 

 into tetanus, which lasts as long 

 as the contraction did in the case 



1*0.107. ArrangcmL of experiment for demon- f A ' and 8* 

 strating the absence of fatigue in medullated fatigue comes on. 



nerve fi res. nerves have been excited through- 



EC, exciting circuit ; CP, polarising circuit. 



out, it is evident that the fatigue 



does not affect the nerve-trunk. We have already seen that a muscle 

 will respond 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 neuro-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 inhibition of the central motor 

 apparatus from the muscle itself. Thus 

 after 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 contrac- 

 tion 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 Fm - 108 - 



class of anaesthetics. 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. 108. The nerve 



