CHANGES OF HEIGHTS OF TWITCH IN FATIGUE 69 



Next study the effect of fatigue in producing alterations in the 

 height of contraction, i.e. in the amount of work it is able to perform 

 with constant load, by the following method : — 



Experiment 2. — Drive the cut-out key, fig. 19, from the shafting em- 

 ployed to drive the drum, so that it gives two shocks per second. Connect 

 it with wires to give make shocks only. Fit up a chronograph to give a 

 seconds time tracing. Dissect out the gastrocnemius or hyoglossus, and 

 having fitted it to the myograph lever attach the electrode wires to stimulate 

 directly, and then bring the secondary coil up to the primary until the con- 

 tractions produced on each make of the primary circuit are maximal. Gear 

 down the drum until it rotates at about the rate of 10 cm. per minute. Bring 

 the writing point to the recording surface, start the drum, and then open the 

 short-circuiting key in the secondary circuit and record the contractions for 

 about five minutes. 



In this manner the tracing of fig. 1, pi. 1, was obtained. The 

 tracing was given by a hyoglossus preparation which was loaded by 

 the weight of the lever only. The heights of twitch only are recorded. 

 It is seen that the first ten twitches give a typical ' staircase ' effect, 

 and from that point onwards, the heights gradually diminish to the end 

 of the tracing, at first rapidly and then more slowly. The second point 

 of interest is with regard to the line joining the basal points of the 

 twitches. This is seen to gradually leave the zero abscissa line, as 

 fatigue sets in, which is to be explained by the fact we have already 

 studied in the previous experiment, viz. that elongation becomes 

 greatly prolonged, and hence a new stimulus reaches the muscle 

 before it has had time to completely relax. The time at which the 

 succeeding stimulus reaches the muscles falls progressively at earlier 

 stages of the relaxation. There is produced therefore a condition of 

 contractur. On stopping the stimulations the muscle is seen to 

 slowly relax, but had not regained its initial length when the traeing 

 was stopped. 



The muscle should now be allowed to rest for about ten minutes, 

 and the experiment once more repeated, when a result similar to that 

 of fig. 2, pi. 1, will be obtained. It gives as it were an epitome of the 

 changes observed in the previous tracing. The changes in the 

 twitches are brought about very rapidly instead of slowly, as in the 

 first tracing. If this tracing be carried on for a very long time it will 

 be found that gradually almost complete fusion is obtained, and then 

 again the undulations become much more marked. This last change 

 occurs when the late stage is reached in which the relaxation process 

 begins to shorten again. 



Experiment 3. — Study the effect of fatigue upon the tetanus curve by 

 arranging the rate of the vibrating reed to just give complete fusion and 

 record a tetanus by means of this, keeping the stimulation up until complete 

 fatigue sets in. 



