FATIGUE OF MUSCLE 67 



twitches will have been recorded, and unless the original contractions were 

 great, and the writing point very fine, the lines will be found to have 

 in many places largely obliterated one another. To obviate this it is best to 

 only record each tenth or twentieth contraction, moving the writing point 

 free from the surface during the intermediate twitches. In bringing it once 

 more in position it is necessary that it should be brought back exactly to its 

 first position, so that the point of stimulation is correctly placed. To carry 

 this out with complete accuracy requires some mechanical means of adjust- 

 ing the writing point to the surface. 



Fig. 64 reproduces a series of curves obtained in this way. The 

 first six curves are numbered in the order in which they were taken. 

 No. 1 was the first twitch ; 2 the sixth ; 3 the eleventh ; 4 the twenty- 

 first ; 5 the thirty-first ; 6 the forty-first ; and the remainder at 

 intervals of ten twitches. Curve No. 1 gives a typical simple twitch. 

 Curve 2 is seen to differ from this in a few points : — (i.) It starts from 

 a rather higher level. This is not of great importance in this instance 

 for this difference is chiefly of instrumental origin, (ii.) Both the 

 period of contraction and that of relaxation in 2 are rather longer 

 than in 1. (hi.) On measurement the height of the twitch in 2 is 

 found to be slightly higher than in 1. On contrasting 3 with 2 these 

 differences come out even more clearly. 



After curve 3 the heights begin to gradually decrease, and the times 

 of the different periods to increase. Thus the latent period gradually 

 increases from '017 second to - 03 second. The contraction time 

 increases, though not'very greatly ; but the chief change is in the period 

 of relaxation, which is seen to become very greatly extended, so much 

 so that the drum has revolved and a fresh stimulus been received 

 before relaxation is complete, as seen by the fact that the next con- 

 traction starts with the lever at a higher level than in the previous 

 curve. This condition of more or less permanent contraction is 

 spoken of as contractur. If the experiment be carried on for a longer 

 time it is found that the relaxation process to a certain degree 

 recovers itself. Eelaxation time slowly diminishes after about four 

 or five thousand twitches, but never attains the previous speed seen 

 when the muscle was fresh. 



The condition of fatigue, then, is chiefly characterised by a slowing 

 of the usual processes of a twitch, a diminution in height, i.e. in the 

 total work performed, but most prominently by the prolonged time 

 required for relaxation. The series of changes observed in fatigue 

 may be to a considerable extent modified by variations of the external 

 conditions. Thus, heat markedly accelerates the onset of fatigue. 

 Load also hastens the production of fatigue ; but as it aids the relaxa- 

 tion the condition of contractur is not so clearly seen. 



F 2 



