PHYSIOLOGY OF NERVE 2G7 



and therefore that non-medullated nerve not possessing this store 

 should prove much more easily fatiguable. Miss Sowton ( 21 ), using 

 the olfactory nerve of the pike, stated that this was the case ; 

 but she omitted to exclude the possibility of the so-called " stimu- 

 lation fatigue," that is, the local injury at the spot where the 

 exciting electrodes rested. Garten ( 22 ), in his classic researches on 

 the same object (of which only very brief and incidental mention 

 can be made here), excluded this possibility, and found that true 

 fatigue of the conducted negative variation actually occurred, re- 

 covery taking place after a suitable interval, even in excised nerve. 



The objection might be made that the olfactory nerve of the 

 pike is not comparable to mammalian nerve, and certainly in the 

 latter " stimulation fatigue " is more easily observable than that 

 of the propagated change ( 23 ), but it is probable that the difference 

 that exists is one of degree rather than of kind. It would be 

 interesting to try the experiments quoted in the next section on 

 non-medullated mammalian nerve ; the probability is that fatigue 

 would be demonstrated even more readily than in the medullated 

 variety. It is not unlikely, therefore, from these and other reasons 

 to be presently referred to, that the structures known as sheath 

 (including under this heading medullary sheath and neurilemma) 

 act as stores of food material, and this does not exclude the idea 

 of Boruttau, that the perifibrillar substance may also function in 

 this capacity. 



There is, however, a more indirect way of approaching the 

 question that has led to very interesting results. If there is any 

 expenditure of energy in transmitting a nerve impulse, a second 

 impulse rapidly following the first should show some alteration. 

 This is found to be the case. 



Gotch and Burch ( 16 ) examined the electrical response in the 

 sciatic of the frog, when this was excited by induction shocks. 

 If these succeeded each other at longer interval than ^^ of a 

 second (taking round numbers), two negative variations were 

 observed ; if the interval were less than this, only the response 

 due to the initial stimulus appeared, and the time short enough 

 to abolish the second response was termed by these authors the 

 " critical interval." They found that this varied with the intensity 

 of the exciting stimulus and with the temperature of the nerve, 

 becoming longer as the temperature grew less. Stated differently, 

 this " critical interval " is the " refractory period " of the nerve. 



