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 
