482 ELEMENTARY PHYSIOLOGY less. 



applied to a nerve for starting an impulse along it, is, as 

 we have said (p. 477), an electrical stimulation. Further, 

 as we have seen, each point of the nerve undergoes an 

 electrical change, as the impulse reaches that point. 

 These two facts fre(}uently give rise to the entirely 

 erroneous idea that a nervous impulse is of the nature of an 

 electric current, similar to that wliich passes along a wire, 

 as used for telegraphy. But this is by no means the case, 

 since, without goinginto any othermore abstruse reasons, we 

 have shown that the rate at which an impulse travels along 

 a nerve is on an average about 100 metres, or 300 feet, per 

 second, whereas we know that electricity travels along a 

 wire at a rate such that the transmission of signals over 

 the wires of an ordinary hand-line is practically instan- 

 taneous. Even in one of the cables across the Atlantic 

 Ocean (2,500 miles in length) only two-tenths of a second 

 elapse after contact is made with the battery at one end 

 before the etfect can be first detected at the other end. 

 Now, if a nerve could be used for transmitting an impulse 

 as a signal frou), say. Land's End to John o'Groat's 

 (600 miles), the signal woidd take nearly three hours 

 (176 minutes) to reach its destination, travelling as it does 

 at the rate of 300 feet pev second. We have spoken of 

 a nervous impulse as a "molecular disturbance" pro- 

 pagated along a nerve. And if we may illustrate what 

 is meant by this expression, by likening the process of 

 the transmission of a nervous impulse to the transmission 

 of any other condition with which most people are familiar, 

 we might compare it with the passage of the explosion 

 along a train of gunpowder when a spark is applied to one 

 end of it. In this case the spark merely sets up a 

 molecular change or disturbance in the grains of powder 

 to which it is applied ; the change thus set up leads to a 

 similar change in the next neighbouring grains and so on 

 along the whole train of powder, so that ultimately the' 

 result of applying the spark at one end makes its 

 appearance as a similar result at the other end of the train. 



