NERVOUS CONDUCTION AND EXCITATION 135 



distinct events : (i) the initiation of some local change at the 

 point of application of the stimulus constituting nervous 

 excitation in the restricted sense employed below ; (2) the pro- 

 pagation of a disturbance of some kind along the nerve-fibre, 

 or as it is commonly called the conduction of the nervous 

 impulse ; (3) the production of some change at the junction 

 between nerve and muscle. We shall consider each aspect 

 of the process separately. 



{a) Excitation. — Nerve, like muscle, may be stimulated 

 to activity by means of thermal, electrical, mechanical, and 

 chemical stimuli. Of these only electrical stimuli are appro- 

 priate for the manipulative requirements of quantitative experi- 

 ment. In both cases electrical stimulation may be brought 

 about either by induced or direct currents. The latter on 

 the whole yield more instructive results. Up to the present 

 excitation phenomena have been studied pre-eminently in 

 amphibian motor nerve. 



For the purpose of investigating the nature of excitation, 

 the excitability of nerve is usually defined in terms of the 

 minimal intensity of stimulus required to evoke an impulse, 

 other conditions (duration, etc.) being constant. The first 

 thing to note is that the local change which constitutes excitation 

 is a reversible one. When two induced electrical stimuli are 

 sent into a nerve successively, it is found that the effect of 

 the second depends on the time which elapses between it 

 and its predecessor. The receipt of the first stimulus is 

 followed at first by a brief interval during which the nerve is 

 incapable of being excited by any strength of stimulus ; this 

 interval, the absolute refractory period , is followed under certain 

 conditions by a restoration of excitability which increases 

 beyond its original value, so that for a further brief interval, 

 the supernormal phasCy the nerve is capable of being excited 

 by a stimulus appreciably less than that necessary to evoke 

 response when presented singly. Thus when two stimuli 

 which exceed the threshold intensity for a single shock are 

 applied successively with the lapse of an interval less than the 

 absolute refractory period, the second is completely ineffective ; 

 when, on the other hand, two stimuli are applied successively, 



