NER VE 689 



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 con- 

 duction). 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. 697.) It has, 

 however, been shown that bifurcation of nerve-fibres may occur 

 in the spinal cord (Sherrington) . The axis-cylinder of a peri- 

 pheral nerve-fibre only begins to branch where complete isolation 

 of function is no longer required, as within a muscle. The 

 experiment of Kiihne on double conduction, mentioned 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. 713). For 

 motor fibres the simplest method is to stimulate a nerve suc- 

 cessively 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 (pendulum or spring myograph) (p. 643) . 

 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 contrac- 

 tion 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 considerable influence (p. 679). 



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 tempera- 

 ture, 50 metres being about the normal rate. This is greater than 

 the speed of the fastest train in the world. According to Piper's 

 recent measurements the velocity in human medullated nerve is even 

 greater than Helmholtz concluded, about 120 metres a second under 

 ordinary conditions. The rate is independent of the intensity of the 

 excitation. 



The velocity with which the negative variation is propagated 

 (p. 723) is the same as that of the nerve-impulse. 



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