CHEMICAL SIGNS OF IRRITABILITY 49 



anabolic processes considerably, thus prolonging the one 

 phase of the refractory period or increasing thus the 

 inefficiency of the nerve. From these considerations we 

 may conclude that the nerve may be fatigued by repeated 

 stimulations if we can prolong the time interval of either 

 the excitatory or the repair state. 



The general conclusion to which this leads us is that 

 what we call fatigue in a tissue of any kind is due to a 

 failure of the tissue to recover completely its normal 

 state after it is excited. In some tissues this state of 

 fatigue is very easily demonstrated, but in medullated 

 nerves the mechanism of recovery is so perfect that 

 ordinarily the restoration of the nervous substance to its 

 original state after the passage of the impulse takes a 

 very short time -a fraction of a thousandth of a second. 

 Nevertheless, by the conditions stated, namely, by lack 

 of oxygen, by partial anesthetization, by the action of 

 drugs like yohimbin and protoveratrin, the recovery is 

 delayed, and in these cases the nerve exhibits phenomena 

 which may properly be called fatigue. The failure of a 

 nerve to show fatigue under ordinary circumstances 

 should not, therefore, cause us to conclude on this 

 account that there had been no destruction of nerve 

 substance by its excitation, but rather that the nerve 

 had in its medullary sheath an especial supply of a food 

 particularly formed to serve as a speedy pabulum for 

 the fibers, and that the means of reconstituting the 

 nerve tissue after excitation had been so perfected that 

 the result was accomplished in a very brief time. 



Heat formation. Another evidence which has been 

 often cited as showing that there was no chemical change 

 accompanying the nerve excitation is the fact that there 



