CHAP, ii.] THE CONTRACTILE TISSUES. 77 



travel very rapidly. It is obvious therefore that by far the greater 

 part of the latent period is taken up by changes in the muscle 



FIG. 13. CURVES ILLUSTRATING THE MEASUREMENT OF THE VELOCITY or A NERVOUS 



IMPULSE. 



The same muscle-nerve preparation is stimulated (1) as far as possible from the 

 muscle, (2) as near as possible to the muscle ; both contractions are registered in 

 exactly the same way. 



In (1) the stimulus enters the nerve at the time indicated by the line a, the con- 

 traction begins at b' ; the whole latent period therefore is indicated by the distance 

 from a to b'. 



In (2) the stimulus enters the nerve at exactly the same time a ; the contraction 

 begins at b ; the latent period therefore is indicated by the distance between a and b. 



The time taken up by the nervous impulse in passing along the length of nerve 

 between 1 and 2 is therefore indicated by the distance between b and b', which may 

 be measured by the tuning-fork curve below ; each double vibration of the tuning- 

 fork corresponds to ^ or -0083 sec. 



itself, changes antecedent to the shortening becoming actually 

 visible. Of course, even when the electrodes are placed close to 

 the muscle, 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 be found that the latent period remained about the 

 same, that is to say, that in all cases the latent period is chiefly 

 taken up by changes in the muscular as distinguished from the 

 nervous elements. 



2. Such difference as does exist between the two curves in 

 the figure 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 have already spoken 

 of as constituting a nervous impulse ; and the above experiment 

 shews that it takes a small but yet distinctly 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 b', 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 



