THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 765 



the muscle, since it has a shorter stretch of narcotized nerve to 

 traverse. This experiment, then, in reality affords no proof that 

 excitability and conductivity can vary independently. Facts are 

 also known, to which allusion need not be made here, but which 

 greatly modify the ordinary interpretation of the experimental 

 results obtained in the first stage of narcosis, and upon the whole 

 it may be said that these direct methods of determining the question 

 have failed to yield a satisfactory answer. Indirect evidence 

 exists, however, that the local process initiated by stimulation is 

 not quite the same as the process involved in the propagation of 

 the disturbance (Lucas). Thus, a brief current too weak to set up a 

 propagated disturbance, nevertheless causes so me change at the point 

 of stimulation, since a second current, also too weak to be effective 

 by itself, will, when thrown in a short time after the first, cause 

 a disturbance which is propagated along the nerve. There is good 

 reason to believe that the change produced by the first current is 

 not the same in kind as that produced by the second, only weaker, 

 but that it is inherently different in quality. Above all, it is a 

 local change incapable of being itself propagated, but constituting 

 the necessary prelude to the starting of the propagated disturbance. 

 Lucas has called this preliminary local effect the ' local excitatory 

 process.' 



There are many facts which indicate that the capacity of different 

 functional or anatomical groups of nerve-fibres for responding to stimu- 

 lation and for conducting the nerve-impulse can be differently affected 

 by one and the same influence. For example, pressure abolishes the 

 conductivity of sensory fibres sooner than that of motor fibres. 



Cocaine locally applied to a nerve diminishes or abolishes its con- 

 ductivity, according to, the dose. It exercises a selective action as 

 regards nerve-fibres of different kinds, picking out and paralyzing 

 sensory fibres before motor; vagus fibres conducting upwards before 

 those conducting downwards, vaso-constrictors before vaso-dilators, 

 and broncho-constrictors before broncho -dilators (Dixon). 



The conduction or propagation of a definite disturbance or impulse is 

 a phenomenon not confined to nervous tissue. It is also characteristic- 

 ally seen in muscle, although there the mechanical effect which con- 

 stitutes the normal response to the arrival of the propagated disturbance 

 obtrudes itself and tends to divert attention from the latter. It is 

 unlikely that the conduction process in muscle should be essentially 

 different from that in nerve, and in muscle, as in nerve, there is 

 evidence that it is associated with only a small, perhaps not even a 

 detectable, liberation of heat. The main heat- product ion in muscle 

 is essentially a feature not of conduction, but of contraction. Con- 

 duction in muscle can be completely dissociated from the contraction 

 process in various ways. For example, if a portion of a muscle is 

 immersed for a time in distilled water, so-called water rigor ensues, and 

 the altered muscle has lost the power of contraction. It will never- 

 theless conduct the impulse which on reaching the unaltered part of the 

 muscle causes it to contract normally. 



Double Conduction. When a nerve (or muscle) is stimulated 

 artificially, the excitation runs along it in both directions from the 



