452 NERVE. 



The excitatory state may be successively assumed by other portions 

 of the excited tissue, and is thus propagated from the point of stimula- 

 tion to more remote structures ; the changes evoked in such structures 

 are the objective sign of the possession by the stimulated nerve of the 

 property of " conductivity." 



The special structure of nerve fibres is the undoubted basis for the 

 high development in these of this attribute of " conductivity," the 

 natural and fundamental function of a living nerve, inasmuch as pro- 

 pagation is the purpose which the structure is especially framed to 

 subserve. 



It is evidently closely associated with that of excitability, but 

 although the relationship is an intimate one, the experimental investi- 

 gations of recent years seem to show that the two attributes, "con- 

 ductivity " and " excitability," must be regarded as more or less distinct, 

 since conditions may modify the manifestations of the one without 

 affecting those of the other. 



There is no doubt not only that an axis cylinder transmits in both 

 directions the excitatory state evoked by the stimulus, but that it 

 transmits this state along all its branches. 



The isolation of the excitatory nerve change just referred to is an 

 expression for the familiar fact that, when a particular group of nerve 

 fibres is excited, the excitatory state thus evoked is propagated along 

 the excited fibres and their branches, and along these only. The 

 definite character of the experimental results observed in the 

 excitation of efferent nerves, whether medullated or non-medullated, 

 is due to this definite restriction of the path along which the 

 propagation of the change occurs. This localisation, although so 

 familiar as to be generally accepted without criticism, is by no means 

 easily explained. 



It has been suggested that the sheaths, and in particular the 

 medullated sheath, may play an important part in securing adequate 

 insulation, but although it is not improbable that some such role 

 may be attributed to them, the fact of the insulation being present 

 where such sheaths fail (in non-medullated nerves) forbids the 

 assumption that their presence is essential for its attainment. It 

 must be regarded as the property of the axis cylinder itself, and 

 the sheaths, if they play any such part, can only afford additional 

 security. 



Although it appears that axis cylinder continuity is a sufficient 

 guarantee that the change, which is successively assumed by its various 

 portions, shall be localised in the nerve fibre in which it started, it is 

 evident that where such continuity ceases the isolation of the propaga- 

 tion also comes to an end, and that conduction will now become diffuse. 

 This is the case at the distal and central terminal branches. In the 

 case of the distal terminations, if these are motor, the specific character 

 of the ending, and its restriction to special muscle fibres, renders the 

 subsequent events still precise in their locality ; but, as regards the 

 central terminations, spread must take place, and the subsequent 

 propagation appears to be determined very largely by the extent of 

 such spread and the proximity of other terminal branches. Thus the 

 excitatory state must be assumed to leave its axis cylinder path where 

 this ends, and, crossing an intermediate region, to enter another path, 

 preferably that in more immediate contiguity. This passage from one 



