THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 793 



return till some months later, when the nerve-fibres, after degenerating, 

 would have been replaced by new sensory fibres growing down from 

 the dorsal nerves (Ranvier). For a similar reason the so-called union 

 of the peripheral end of the cut hypoglossal nerve (motor) with the 

 central end of the cut lingual (sensory) proves nothing as to double 

 conduction, nor as to the possibility of motor nerves taking on a sensory 

 function. For while sensation is after a time restored in the affected 

 portion of the tongue, this is due to the growth of sensory fibres from the 

 central stump of the lingual down through the degenerated hypoglossal, 

 and not to the conduction upwards of sensory impulses by the motor 

 fibres of the latter. 



Every fibre of a nerve is physiologically isolated from the rest, 

 so that an impulse set up in a fibre runs its course within it, and 

 does not pass laterally into others (law of isolated conduction). In 

 connection with this physiological fact there is the anatomical fact 

 that nerve- fibres do not normally branch in the trunk of a peripheral 

 nerve. (But see p. 802.) It has, however, been shown that bifurca- 

 tion of nerve-fibres may occur in the spinal cord (Sherrington). 

 The axis-cylinder of a peripheral nerve-fibre only begins to branch 

 where complete isolation of function is no longer required, as within 

 a muscle. The experiment of Kuhne on double conduction, men- 

 tioned above, shows that an excitation set up in one twig or one 

 fibril of an axis-cylinder which has branched can spread to the rest. 



Velocity of the Nerve- Impulse. We have said that the nerve- 

 impulse travels with a measurable velocity. It is now time to 

 describe how this has been ascertained (p, 818). For motor fibres 

 the simplest method is to stimulate a nerve successively at two 

 points, one near its muscle, the other as far away from it as possible, 

 and to record the contractions on a rapidly-moving surface (pendu- 

 lum or spring myograph) (p. 746). The apparent latent period of 

 the curve corresponding to the nearer point will be less than that 

 of the curve corresponding to the point which is more remote, by 

 the time which the impulse takes to pass between the two points. 

 The distance between these points being measured, the velocity is 

 known. Helmholtz found the velocity for frog's nerves at the 

 ordinary temperature of the air to be a little under, and for human 

 nerves, cooled so as to approximate to the ordinary temperature, 

 a little over 30 metres per second. For observations on man the 

 contraction curves of the flexors of one of the fingers or of the 

 thumb may be recorded, first with stimulation of the brachial plexus 

 at the axilla, and then with stimulation of the median or ulnar 

 nerve at the elbow. Probably at the same temperature there is 

 little difference in the rate of transmission in the nerves of warm- 

 blooded and cold-blooded animals, but temperature has a con- 

 siderable influence (p. 782). 



By cooling a frog's nerve Helmholtz reduced the rate to ^ of its value 

 at the ordinary temperature. In the human arm he found a variation 

 from 30 to 90 metres per second, according to the temperature, 50 metres 



