72 MUSCLE-CUKVE. [BOOK i. 



2. In the first portion of this period, from a to b, there is no 

 visible change, no raising of the lever, no shortening of the muscle. 



3. It is not until b .that is to say, after the lapse of about 

 jj^ sec. that the shortening begins. The shortening as shewn 

 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 T |^ 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 



TO o 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 which may be 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 elimi- 

 nated 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 exactly the same 

 point of the travelling surface as before, two curves would be 

 gained having the relations shewn in Fig. 12. The two curves 

 resemble each other in almost all points, except that in the curve 

 taken with the shorter piece of nerve, the latent period, the 

 distance a to b as compared with the distance a to b' is shortened : 

 the contraction begins rather earlier. A study of the two curves 

 teaches us the following two facts : 



1. Shifting the electrodes from a point of the nerve at some 

 distance from the muscle to a point of the nerve close to the 

 muscle, has only shortened the latent period a very little. Even 

 when a very long piece of nerve is taken, the difference in the two 

 curves is very small, and, indeed, in order that it may be clearly 

 recognized or measured, the travelling surface must be made to 



