212 



PHYSIOLOGY 



CHAP. 



factor that lowers functional activity ; on the other hand it is 

 effectively reinforced by all stimuli that promote activity. On 

 warming the nerve to 35-40 C. the duration of the negative 

 variation diminishes; it is prolonged by cooling the nerve to 

 5 C. Lowering the temperature also delays the propagation of 

 the negative variation. 



Waller studied the course of electromotive phenomena in 

 nerve by photographing the galvanometer deflections in a long 

 series of rhythmical tetanisations. These -records give valuable 

 indications in regard to the effect of anaesthetics, salt solutions, 

 alkaloids, gases, etc., when applied directly to a length of excised 

 nerve. He concluded as follows : 



Fio. 137. Photograph as before. (Waller.) The figure shows that after applying.chloroform to 

 the nerve (black line) the electrical reactions are permanently abolished. 



(a) Anaesthetics (ether and chloroform) temporarily abolish 

 the current of action and the excitability of the nerve. The 

 return of the action current after inhibition by ether is invariably 

 followed by a secondary augmentation : its suppression by chloro- 

 form is not only more prolonged, but may be permanent if the 

 dose is too strong (Figs. 136 and 137). 



(&) Oxygen, nitrogen, hydrogen, nitrous oxide, carbonic oxide, 

 have no appreciable effect upon the current of action ; on the 

 other hand, carbon dioxide in small quantities (e.g. 4 per cent, as 

 in expired air) increases it ; in larger percentages carbon dioxide 

 acts exactly like ether (Figs. 138 and 139). 



(c) Potassium salts have a decidedly depressing influence ; 

 sodium salts are less depressing. Calcium and strontium salts, 

 on the contrary, increase the current of action. 



