70 THE CONTRACTILE TISSUES. 



after the shortening, takes an appreciable time. In the figure, the whole 

 curve from a to d takes up about the same time as eleven double vibrations 

 of the tuning-fork. Since each double vibration here represents T ^ 7 second, 

 the duration of the whole curve is rather more than y 1 ^ second. 



2. In the first portion of this period, from a to 6, 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 T ^ second, 

 that the shortening begins. The shortening, as shown 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 

 rfo second. 



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 -Q second. 



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 elec- 

 trodes along a considerable length of nerve down to the muscle. It is obvi- 

 ous 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 exactly the same point of the travelling 

 surface as before, two curves would be gained having the relations shown in 

 Fig. 22. 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 f 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 short- 

 ened 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 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 itself, 

 changes preparatory to the actual visible shortening. 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 elec- 

 trodes and the muscular fibres. To eliminate this with a view of determin- 

 ing 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 



