VELOCITY OF NERVE TRANSMISSION. 505 



is brought about by a molecular change in the axis cylinder, we 

 are at once struck with the rapidity with which impressions are 

 transmitted from one part of the body to another. This surprising 

 velocity is, however, only relatively great. When we compare it 

 with the velocity of the electric current or of light, we at once see 

 how incomparably slower the, rate of nerve impulse is, and that 

 it may, with more advantage, be compared with rates of motion 

 commonly under our observation. To take every-day examples : 

 viz., nine metres per second is about as fast as the quickest runner 

 can accomplish his 100 yards ; race-horses can gallop about 15 

 metres a second for a mile or so ; a mail train at full speed travels 

 at about 30 metres a second, and the velocity of nerve force has 

 been estimated to be in cold-blooded animals 27 metres per second ; 

 and in man about 33 metres per second. So that the intercom- 

 munications between man's brain and the various parts of his 

 body only travel about the same rate as an express train, and 

 about twice as fast as the quickest horse can gallop. 



In order to measure the rate of transmission of nerve force, 

 different methods may be employed ; the simplest of which is to 

 make a muscle draw two curves, one over the other, with a good 

 myograph, such as described in Chapter XVI, in one of which the 

 stimulation is applied to the nerve close to the muscle, and in the 

 other, as far as possible away from the muscle. The difference in 

 length of the latent period, as estimated by the tuning-fork tracing, 

 corresponds to the time the impulse has taken to travel along the 

 part of the nerve between the two points of stimulation. 



Utilizing the fact that the extent of deflection of the needle of 

 a galvanometer is in proportion to the duration of a current of 

 known strength passing through it for a short time, an accurate 

 measurement of the difference in time, of remote and near or 

 direct stimulation of a nerve, may be made. By a special 

 mechanism the time-measuring current is sent through the gal- 

 vanometer at the same moment that the stimulating current goes 

 through the nerve, and the instant the muscle begins to contract 

 it breaks the current passing through the galvanometer, so that 

 this time-measuring current lasts only from the moment when 

 the nerve is stimulated until the muscle begins to contract. 

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