228 PHYSIOLOGY CHAP. 



effects. Fr. W. Frohlich (1904), who made a long series of 

 accurate observations on this question, saw that, at a certain 

 stage of narcosis or asphyxia of the nerve, phenomena of apparent 

 inhibition set in which are perfectly analogous to those described 

 by Wedensky, and which Hermann referred to fatigue of the end- 

 organs. This paradoxical state, in which very strong and frequent 

 stimuli are less effective than weaker and less frequent stimuli, 

 can only be interpreted in these experiments as fatigue of the 

 part of the nerve which is exposed to narcosis or asphyxia. Such 

 manifestations of fatigue do not appear in nerve under normal 

 conditions, because the consumption of living matter is minimal, 

 and recovery is extraordinarily rapid. They are manifested only 

 when the restitution processes are much retarded by toxic or 

 other pathological influences. 



Although under normal conditions nerve is practically inex- 

 haustible to prolonged artificial stimuli, so long as these do not 

 alter its substance, its specific activities (excitability and con- 



Fio. 144. Griinhagen's experiment on the effect of CO^ on a limited portion of a frog's 

 sciatic nerve. Explanation in text. 



ductivity) may progressively diminish and eventually disappear 

 when it is deprived of the essential conditions of its existence. 



Since nerve in atmospheric air shows no signs of fatigue even 

 after protracted activity, the question naturally arose as to how 

 far its functions depend upon the supply of oxygen, and how 

 much they are altered when indifferent or toxic gases are sub- 

 stituted for atmospheric air. The earlier investigations of Kanke 

 and of Ewald (1867-69) are inconclusive ; they were incomplete 

 and yielded little result. 



Eanke stated that a nerve (frog's nerve-muscle preparation) 

 suffers no injury in an atmosphere of carbonic acid, and that it 

 keeps its excitability longer in an atmosphere of hydrogen than 

 in one of oxygen. Ewald was unable to discover any difference 

 in the period of declining excitability, whether the nerve was 

 immersed in oxygen or hydrogen, or was in vacuo. He concluded 

 that its vitality is independent of its oxygen supply. 



The experiment in which Giiinhagen allowed carbonic acid to 

 act not upon the entire nerve-muscle preparation of the frog, but 

 only upon a limited portion of the nerve, is more important. For 

 this purpose he introduced the nerve of the frog's leg into a glass 



