260 THE NERVOUS SYSTEM 



the stage of exalted excitability due to the first, the second 

 stimulus becomes effective. This is known as summation. 



3. The Passage of a Constant Current — Electrotonus. — • 

 Though there is no propagated disturbance while a con- 

 stant current is passing through a nerve there is a change 

 in excitability, known as electrotonus. This takes the form 

 of diminished excitabihty at the anode {anelect rot onus) and 

 increased excitabihty at the kathode {hatelectrotonus). 



4. Drugs. — Narcotics, such as alcohol and COg, depress 

 both conductivity and excitabihty. 



The Changes accompanying a Nervous Impulse 



1. Current of Actioyi. — When an impulse passes along a 

 nerve-fibre this shows a current of action resembhng that 

 found in muscle, the part of the nerve which is in a state 

 of excitation being negative to the rest of the nerve. On 

 this is based a method for determining whether a nerve is 

 active in situ, e. g. the depressor nerve. 



2. Evolution of Heat. — By means of the thermopile it has 

 been found that a very minute though indisputable rise 

 of temperature accompanies the passage of an impulse. 



3. Gaseous Metabolism. — When a nerve-fibre is deprived 

 of oxygen it loses its excitability more rapidly when it is 

 stimulated than when it is not. This shows not only that 

 oxygen is necessary for the maintenance of the fibre in a 

 healthy condition, but also that oxygen is used up in the 

 passage of a nervous impulse. 



Similarly it has been shown that the CO2 output of a 

 nerve is increased 2*5 times when it is stimulated. These 

 facts therefore point to an unmistakable gaseous interchange 

 accompanying a nervous impulse. 



The Nature of the Nervous Impulse 



From the fact that a nervous impulse is generated on 

 make of a constant current at the kathode, and on break 

 at the anode, and that it depends upon, the rate of 

 change of current rather than upon the current itself, the 



