CHAPTER XI 



Conditions of Excitation of Nerve and Muscle by the 

 Galvanic Current 



THE passage of a galvanic current through a nerve or muscle produces 

 secondary polarisation of these tissues, caused by the accumulation of 

 positive and negative ions at or near the poles of the constant current. 

 This polarisation is accompanied by certain physiological changes, the 

 tissue being more excitable in the neighbourhood of the negative pole 

 or kathode, and less excitable in the neighbourhood of the positive 

 pole or anode. These effects both physical and physiological spread 

 for some distance beyond the actual poles. And not only is the tissue 

 rendered more excitable by the kathode, but this itself sets up 

 excitation, which, in the case of a muscle, may cause its contraction, 

 not only at the moment of closure, but during the whole time of 

 passage of the current. On breaking the circuit the part of the nerve 

 which was more excitable during the passage of the current becomes 

 instantaneously less so than the rest (physiological rebound). On the 

 other hand, the presence at the anode of a constant current not only 

 renders the tissue less excitable whilst the current is passing, but on 

 breaking the circuit there is again a rebound ; the part which was the 

 less excitable becoming the more excitable ; this passage from less to 

 greater excitability again acts as a stimulus. Hence, when a constant 

 current is sent through a nerve or muscle, there is excitation at the 

 kathode on making and at the anode on breaking the circuit. But the 

 latter furnishes a rather weaker excitation than the former. 



Polar excitation of muscle. 1. Engelmann's sartorius experiment. 

 A curarised sartorius is connected with a pair of non-polarisable 

 electrodes which are joined up through a mercury key with a battery 

 (Fig. 45). It will be observed that the twitch begins at the kathode 

 when the circuit is closed; indeed, the muscle may remain more or 

 less contracted at that end during the whole time of the passage of a 

 strong current. On the other hand, on opening the circuit the twitch 

 begins near the anode, and may again be followed by a prolonged con- 

 traction. These prolonged contractions show that the excitation is 



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