CHAP, ii.] THE CONTRACTILE TISSUES. 75 



by the curve is at first slow, but soon becomes more rapid, and 

 then slackens again until it reaches a maximum at c ; the whole 

 shortening occupying rather more than -^ sec. 



4. Arrived at the maximum of shortening, the muscle at once 

 begins to relax, the lever descending at first slowly, then more 

 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 rather more than T ^ sec. 



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

 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 



FIG. 12. CURVES ILLUSTRATING THE MEASUREMENT OF THE VELOCITY OF 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 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 b and ft', which may 

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

 fork corresponds to T f^ or -0083 sec. 



