MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 725 



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 Fi S; 253.-' Staircase' in Skeletal 



, . , , Muscle : Frog. Stimulation by 



contraction is used up more quickly atl aut0 matic arrangement, 

 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 con- 

 tinuance 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 blood- 



Fig. 254. ' Staircase ' in Cardiac Muscle. Contractions recorded on a much more 

 quickly moving drum than in Fig. 253. The contractions were caused by stimu- 

 lating a heart reduced to standstill by the first Stannius' ligature (p. 197). The 

 contractions gradually increase in height. 



vessels of a 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 in more than a certain concentration, produce the 



