MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 751 



tractions, say, of one of the flexor muscles of a finger, in raising a 

 weight (isotonic method) or in deforming a spring (isometric method) 

 is taken on a drum. When the contractions are repeated every 

 second, or every half-second, distinct evidence of fatigue is seen on 

 the tracing after a longer or shorter 

 period, according to the conditions.* 

 What is the cause of muscular 

 fatigue ? An exact answer is not 

 possible in the present state of our 

 knowledge, but we may fairly con- 

 clude that in an isolated preparation 

 it is twofold : (i) Waste products, 

 among which some are so directly 

 related to the onset of fatigue as to 

 deserve the name of ' fatigue sub- 

 stances,' are formed by the active 

 muscle faster than they can be re- 

 moved, oxidized or otherwise decom- 

 posed. (2) The material necessary for 

 contraction is used up more quickly 

 than it can be reproduced or brought 

 to the place where it is required. That the accumulation of fatigue 

 products has something to do with the exhaustion is shown by the 

 fact that the muscles of a frog, exhausted in spite of the continuance 

 of the circulation, can be restored by bleeding the animal, or washing 

 out the vessels with physiological salt solution, while injection of a 

 watery extract of exhausted muscle into the bloodvessels of a 



Fig. 260. ' Staircase ' in Skeletal 

 Muscle : Frog. Stimulation by an 

 automatic arrangement. 



Fig. 26?. 'Staircase' in Cardiac Muscle. Contractions recorded on a much more 

 4iuilfely moving drum than in Fig. 260. The contractions were caused by stimu- 

 'lati^j a heart reduced to standstill by the first Stannius' ligature (p. 199). The 

 ..contractions gradually increase in height. 



curarized muscle renders it less excitable (Ranke). This observer 

 supposed that it was specially the removal of the acid products of 

 contraction which restored the muscle. Such acid products as 

 carbon dioxide and lactic acid, or the lactates which it may form 

 with bases in the blood, lymph or tissues, when they act on muscle 



* Recent observations (by Ryan and Agnew) with improved methods have 

 emphasized the necessity for care in the interpretation of ergographic records. 

 This caution is timely, inasmuch as in modern war the question of the rela- 

 tion of fatigue to industrial practice acquires first-class importance. 



