VELOCITY OF NERVE TRANSMISSION. 503 



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, dif- 

 ferent methods may be employed ; the simplest of which is to make 

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

 graph 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 

 galvouometer is in proportion to the duration of a current of known 

 strength passing through it for a short time, an accurate measure- 

 ment of the difference in time, of remote, and near or direct stimu- 

 lation of a nerve, may be made. By a special mechanism the 

 time-measuring current is sent through the galvanometer 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. 



