NERVE FIBRES.X 37 



increased by a rise of temperature, and diminished by cooling. With 

 currents of longer duration the converse effect is obtained, within limits, 

 the excitability being increased by cooling the nerve because by the 

 fall of temperature the subsidence of the excitatory process is delayed, 

 although at the same time the initiation of a propagated disturbance is 

 hindered. 



The effect of volatile drugs on nerve is tested by enclosing a length 

 of the nerve to a muscle in a glass tube which is made air-tight, and 

 rilling the tube by a side connection with the vapour of the drug to be 

 tested. Carbonic acid gas and ether abolish both the excitability and 

 conductivity of nerve, the excitability being the earlier to disappear. 

 If the gas or vapour be replaced by pure air, the nerve returns to its 

 normal condition, conductivity returning before excitability. Chloro- 

 form acts on nerve in the same way as ether but more powerfully, 

 recovery being incomplete or not occurring at all when the drug is 

 replaced by air. 



The effect of the passage of a constant current on the excitability 

 of nerve has already been described. The effect on conductivity is 

 fairly parallel with that on excitability. 



The refractory period which follows the stimulation of a nerve is a 

 result of the passage of the propagated disturbance, and not merely a 

 local condition of depressed excitability following the excitatory process. 

 This is proved by sending the second stimulus into the nerve at a 

 different point from the first, in which case the refractory condition is 

 found to be the same as when the second stimulus is applied at the same 

 spot as the first. The refractory state does not end abruptly, but 

 passes off gradually, so that there is a gradual fall in the strength of 

 the stimulus required to produce the second response. 



Electrotonic Currents in Nerve. Reference has already been made 

 to the fact that the electromotivity of nerve is altered by the passage of a 

 constant current. Uninjured nerve is isoelectric. If, however, a nerve 

 is injured, as, for example, by division of its fibres, and is connected 

 with a galvanometer by means of non-polarisable electrodes, one of 

 which is applied to the injured area, and the other to the uninjured 

 surface, a current, the so-called demarcation current or current of 

 injury, will flow through the galvanometer from the uninjured surface 

 to the injured part; that is, within the nerve itself the injured 

 part is electro-positive to the uninjured. If a constant current 

 ("polarising current") be passed through a part of the nerve, non- 

 polarisable electrodes being used, the demarcation current will be 

 increased at the anode, where the polarising current enters the nerve, 

 and diminished at the kathode. Electrotonic currents are set up in 



