NERVE 687 



excitability. In the neighbourhood of the kathode it is easier 

 to cause excitation than in the normal nerve (increased excita- 

 bility), but it is less easy for an excitation set up elsewhere to 

 pass through (diminished conductivity). Change of tempera- 

 ture also, for certain kinds of stimuli, at any rate, acts in the 

 opposite way on these two properties of nerve. The excita- 

 bility of frog's nerve is increased by cooling (from 35 C. to 

 2 C.) for mechanical and chemical stimulation, and for stimula- 

 tion by the opening or closure of a voltaic current, unless of very 

 short duration (p. 681), but cooling diminishes and heat increases 

 the conductivity. Carbon dioxide and monoxide depress the 

 excitability without affecting the conductivity. Alcohol vapour 

 rapidly impairs the conductivity without for a time affecting the 

 excitability. On ceasing to apply the vapour the conductivity is 

 restored much sooner than the excitability (Gad and Sawyer, 

 Piotrowski). Munk found that in a dying sciatic nerve certain 

 points may be quite inexcitable to the strongest stimuli, while 

 weak stimulation of points lying nearer the central end may cause 

 muscular contraction. These facts indicate that the process by 

 which the nerve-impulse is propagated may not be the same as that 

 by which it is originated, and therefore is not merely an excitation 

 of each nerve-element by the one next it, as some have 

 supposed. 



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). 

 Pressure also abolishes the conductivity of sensory fibres sooner than 

 that of motor fibres. 



Double Conduction. When a nerve (or muscle) is stimu- 

 lated artificially, the excitation runs along it in both directions 

 from the point of stimulation ; so that nerve-fibres which in the 

 intact body are afferent can conduct impulses towards the 

 periphery, and efferent fibres can conduct impulses away from 

 the periphery. In the normal state, however, double conduc- 

 tion must seldom occur, for efferent fibres are connected centrally, 

 and afferent fibres peripherally, with the structures in which 

 their natural stimuli arise. In general, too, an impulse, if it 

 did pass centrifugally along an afferent fibre, would not give 

 any token of its existence, for the peripheral organ would not 

 be able to respond to it ; and we have no reason to believe that 

 the central mechanisms connected with afferent fibres are better 

 fitted to answer such foreign and unaccustomed calls as impulses 

 reaching them along normally efferent nerves. There is good 



