CHAP, ii.] THE CONTRACTILE TISSUES. 47 



exactly the same point of the travelling surface as before, a curve 

 like that shewn by the plain line in Fig. 5 would be gained. It 

 resembles the first curve (indicated in the figure by a dotted line) 

 in all points, except that the latent period is shortened : the con- 

 traction begins rather earlier. From this we learn two facts : 



The greater part of the latent period is taken up by changes 

 in tlie Nmuscle itself, preparatory to the actual visible shortening, 

 for the two latent periods do not differ much. Of course, even in 

 the second case, the latent period includes the changes going on in 

 the short piece of nerve still lying between the electrodes and the 

 muscular fibres. To eliminate this with a view of determining the 

 latent period in the muscle itself, the electrodes might be placed 

 directly on the muscle poisoned with urari. If this were done, it 

 would still be found that the latent period was chiefly taken up 

 by changes in the muscular as distinguished from the nervous 

 elements. 



2. Such difference as does exist indicates the time taken up by 

 the propagation, along the piece of nerve, of the changes set up at 

 the far end of the nerve by the induction-shock. These changes 

 we shall hereafter speak of as constituting a nervous impulse ; and 

 the above experiment shews that it takes some appreciable time 

 for a nervous impulse to travel along a nerve. In the figure the 

 difference between the two latent periods, the distance between b 

 and 6', seems almost too small to measure accurately ; but if a long 

 piece of nerve be used for the experiment, and the recording 

 surface -be made to travel very fast, the difference between the 

 duration of the latent period when the induction-shock is sent in 

 at a point close to the muscle, and that when it is sent in at a 

 point as far away as possible from the muscle, may be satisfactorily 

 measured in fractions of a second. If the length of nerve between 

 the two points be accurately measured, the rate at which a nervous 

 impulse travels along the nerve to a muscle can thus be easily 

 calculated. This has been found to be in the frog about 28, and in 

 man about 33 metres per second. 



Thus when a momentary stimulus, such as a single induction- 

 shock, is sent into a nerve connected with a muscle, the following 

 events take place : 



1. The generation at the spot stimulated of a nervous impulse, 

 and the propagation of the impulse along the nerve to the muscle. 

 The time taken up by this varies according to the length of the 

 nerve but is always very short. 



2. The setting up of certain molecular changes in the muscle, 

 unaccompanied by any visible alteration in its form, constituting 

 the latent period, and occupying on an average about y^th sec. 



3. The shortening of the muscle up to a maximum, occupying 

 about sec. 



