480 ELEMENTARY PHYSIOLOGY less. 



which the needle of the galvanometer begins to move as 

 the result of the passage of the impulse, started at jc, 

 under the terminal b at the point C. If now we measure 

 the length of the piece of nerve between x and b we can 

 at once calculate the I'ate at which the impulse travels 

 along the nerve. Thus if the distance from x to b is 25 

 millimetres (1 inch), about -00089 of a second elapses 

 before the impulse started at x makes itself obvious as an 

 electrical disturbance at b. That is to say the impulse 

 travels at the rate of about 28 metres or DO feet per 

 second in the nerve of a frog. 



The rate of transmission of an impulse along a (motor) 

 nerve may also be determined in the following way, using 

 a muscle nerve preparation such as is figured on page 298. 

 The muscle is suspended from a clamp, as shown in 

 Fig. 154 ; a light horizontal lever is attached by a hook to 

 the tendon at the lower end of the muscle, so that when 

 the muscle is made to contract the free end of the lever 

 moves upwards and thus indicates the moment at which 

 the contraction of the muscle commences. The sciatic 

 nerve is then arranged in such a way that it may be 

 stimulated either at a point .i; (Fig. 154) as close as possible 

 to its junction with the muscle, or at a point y as far 

 away as possible from the muscle. By the use of suitable 

 apparatus it is easy to measure the interval of time which 

 elapses between the moment of ajjplying the stimulus at x 

 and the moment at which the end of the lever begins to 

 move. This is found to be, in an ordinary experiment, 

 about Y5o*^^^ ^^ ^ second. If now the nerve is stimulated 

 at y, it is found that the end of the lever begins to move 

 slightly later than it did wlien the stimulus was applied 

 at X ; that is to say, the muscle begins to contract rather 

 later when its nerve is stimulated at y than at x. This 

 difference can only be due to the fact that ivhen the 

 impulse is started at y it tnlies longer to reacli the muscle than 

 when it is started at x. Since the length of the piece of. 

 nerve between y and x is known by direct measurement, 



