viii CONDUCTIVITY AND EXCITABILITY OF NERVE 89 



protected from drying, and other injurious circumstances) is very 

 striking, and emphasises the radical physiological differences 

 between the two. Excitability and conductivity will, in a nerve- 

 trunk which is isolated for a considerable length, and connected 

 only at one end with its terminal organ (in which the circulation 

 is wholly abolished), persist for hours, even in warm-blooded 

 animals. 



Non-medullated nerves, on the other hand, are far more 

 perishable. The nerves of crayfish and lobster, at least, do not 

 maintain their excitability for even approximately so long a time 

 as frog's nerves, when isolated and stimulated. Piotrowsky found 

 the experiment quite impossible in summer, while excitability 

 disappeared in winter after 8-10 min. Klihne also found the 

 non-medullated olfactory nerve of the pike to be excitable for a 

 very short time only. 



It is evident that we can estimate the true duration of survival 

 in an excised nerve only when the nerve alone, and not the 

 terminal organ (which is the sole indicator of activity in the 

 former), is withdrawn from normal conditions of nutrition. No 

 certain conclusion as to survival in an excised warm or cold- 

 blooded nerve can therefore, as a rule, be formulated from 

 observations on a perfectly isolated nerve-muscle preparation : the 

 muscle obviously becomes inexcitable long before the nerve. 



The fact that nerves, when removed from their natural situa- 

 tion and laid across the electrodes, remain excitable for hours even 

 in warm-blooded animals, points to an enormous power of resistance, 

 and it follows that the medullary sheath is an effective means of 

 protection. 



If any excitation, best an exactly measurable electrical stimulus, 

 be employed to test the excitability of a nerve which has been 

 separated from its centre, by observing the gradual alterations in 

 the reaction of the terminal organ (e.g. muscle), the alterations of 

 excitability at any one point of the nerve, as well as at different 

 points along its course, will be of a very involved character. The 

 simplest hypothesis, as regards the former, is that the excita- 

 bility of every particle of the nerve sinks in time by regular 

 gradations to zero. Certain observations of Eosenthal (38) 

 appear to contradict this theory, as showing that a considerable 

 increase of excitability precedes its diminution, at every point. 

 Biedermann, however, like Mommsen and more recently Werigo 



