478 NERVE. 



in the immediate neighbourhood ; as this spreads along the fibres, in 

 accordance with the known distribution of currents in moist conductors, 

 the nerve in the vicinity of the section is most subjected, and the 

 further portions less subjected to its influence. Wherever it leaves the 

 fibres, these are placed into a cathodic state of heightened excitability. 

 The method, therefore, of preparing a nerve with its attached muscle, 

 freeing it from all other connections and placing it in a moist chamber, 

 involves both the alterations due to arrest of the circulation and 

 those due to the section of the nerve at its central end, together with 

 other similar changes wherever a branch from the main trunk has 

 been cut. In order to dimmish the extent of these injury effects, the 

 sciatic nerve of the chloralised frog has been used. The removal of the 

 sensorium, the exposure of the heart, the bleeding of the animal, and 

 the slight dissection for the adequate exposure of the sciatic nerve in a 

 central and a peripheral region, are the only operative procedures 

 employed. In such a preparation the changes in excitability are 

 ascertained by determining the intensity of a single given localised 

 stimulus, which is adequate to evoke a minimal muscular twitch, i.e. 

 the minimal exciting value of a local stimulus. Some form of electrical 

 stimulus has been generally used in order that the physical factors may 

 remain unaltered, and the particular one chiefly employed has been the 

 break induced current. The minimal exciting value of such a stimulus 

 for the more central regions is at first attained with slightly lower 

 intensity than for the peripheral; subsequently, the conditions are 

 gradually altered, until finally the central end demands a more intense 

 stimulus than the peripheral one. Two sets of factors may contribute 

 to the production of this final condition. The excitability alone may 

 change, or the conductivity, or both functions ; the phenomena would 

 be equally present, whether failure in conductivity or in excitability 

 existed in different degrees throughout the nerve. The separation 

 between the properties of conductivity and excitability clearly admits 

 of these two possible interpretations of the experimental facts. 



Indeed, it has been suggested that the initial apparent increase in 

 excitability observed in the neighbourhood of a nerve section is not due 

 merely to the circumstance that the excitatory state at this point is 

 more easily produced, but also to the development of an avalanche 

 effect, occurring during the propagation of the state along the nerve to 

 the muscle, the excitatory state gathering volume as it progresses. 1 

 There is, however, little confirmatory evidence of such an increment. 

 It is certainly not present with all stimuli, for on using chemical stimu- 

 lation the peripheral part of the nerve appears to be more excitable 

 than the central, 2 whilst the experiments of Tigerstedt with mechanical 

 excitation show that in an uninjured nerve all parts are at first equally 

 excitable. 3 Even in the case of induced currents, experiments made 

 under the direction of the writer by Dear show that, with nerves kept 

 in appropriate saline solutions, the excitability after eight hours gradu- 

 ally fails in all parts of the nerve equally, thus contradicting the 

 Eitter-Valli law. 



Maintenance of continuity with nerve cells. Every nerve fibre, 

 whether medullated or non-medullated, is the specialised process of 



1 Pfliiger, "Untersuch. ueber Electrotonus," Berlin, 1859, S. 140. 



2 Griitzuer, Arch.f. d. ges. Physiol., Bonn, 1893, Bd. liii. S. 83. 



3 Tigerstedt, "Studien u. mechanisch. Nervenreizung," 1880. 



