46 THE MUSCLE CURVE. [BOOK i. 



rapidly, and at last more slowly again, until at d the muscle has 

 regained its natural length ; the whole return from the maximum 

 of contraction to the natural length occupying yj^, i.e. about -fa sec. 

 Thus a simple muscular contraction, a simple spasm or twitch as 

 it is sometimes called, produced by a momentary stimulus, such as 

 a single induction-shock, consists of three main phases : 



1. A phase antecedent to any visible alteration in the muscle. 

 This phase, during which invisible preparatory changes are taking 

 place in the nerve and muscle, is called the ' latent period '. 



2. A phase of shortening or, in the more strict meaning of the 

 word, contraction. 



3. A phase of relaxation or return to the original length. 



In the case we are considering, the electrodes are supposed to be 

 applied to the nerve at some distance from the muscle. Consequently 

 the latent period of the curve comprises not only the preparatory 

 actions going on in the muscle itself, but also the changes necessary 

 to conduct the immediate effect of the induction-shock from the 

 part of the nerve between the electrodes, along a considerable 

 length of nerve down to the muscle. It is obvious that these latter 

 changes might be eliminated by placing the electrodes on the 

 muscle itself or on the nerve close to the muscle. If this were 

 done, the muscle and lever being exactly as before, and care were 

 taken that the induction-shock entered into the nerve at the new 

 spot, at the moment when the point of the lever had reached 



a T>1> 



FIG. 5. CURVES ILLUSTRATING THE MEASUREMENT OP THE VELOCITY or A NERVOUS 

 IMPULSE. (Diagrammatic.) To be read from left to right. 



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 by 

 the pendulum myograph exactly in the same way. 



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

 traction, shewn by the dotted line, begins at V ; the whole latent period therefore is 

 indicated by the distance from a to &'. 



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

 shewn by the unbroken line, begins at b ; the latent period therefore is indicated by 

 the distance between a and 6. 



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 & and &', which may 

 be measured by the tuning-fork curve below. N.B. No value is given in the figure 

 for the vibrations of the tuning-fork, since the figure is diagrammatic, the distance 

 between the two curves, as compared with the length of either, having been purposely 

 exaggerated for the sake of simplicity. 



