CHAP. IX.] THE CONTRACTILE TISSUES. 405 



Measure of The progress of fatigue may be gauged by the effects 



Fatigue. Ex- produced in a muscle on applying at intervals a constant 

 periments of stimulus. When a muscle is tetanized the course of 

 H. Kronecker. f a tig ue is indicated by the gradual extension of the 

 tetanized muscle, which takes place at first with accelerating, but 

 afterwards with diminishing velocity, until the original length is almost 

 attained. When the muscle (of a frog), which is moderately weighted, 

 is stimulated at regular intervals of 2 12 seconds with a constant 

 (maximal) stimulus from an induction-machine, the heights through 

 which the weight is lifted diminish regularly in an arithmetical 

 series; i. e. the curve of fatigue forms a straight descending line 1 . 



Such a series of lifts may be called a fatigue-series, the members 

 of which exhibit a constant difference D. The value of D diminishes 

 as the intervals of stimulation increase ; but with constant intervals 

 it is independent of the load which the muscle is made to lift. That 

 is to say, the curves of fatigue with different loads are a parallel series 

 of descending lines so long as the intervals of stimulation remain the 

 same for all the loads. 



Causes of The causes of fatigue and exhaustion are very 



Fatigue and obscure ; but since the fatigue of muscles in which the 

 circulation has ceased may be readily removed by 

 renewing the current of blood or even by washing out the blood- 

 vessels with indifferent salt solutions, especially such as contain per- 

 manganate of potash (*05 per cent.) 2 , we may suppose them to be due 

 either to the accumulation of the products of contraction, or to the 

 defect of constituents, such as oxygen, which the blood can supply, or 

 to both these causes combined. It is at least certain that carbon 

 dioxide has an injurious influence upon muscles 3 , which is shared by 

 the lactic acid arising during contraction 4 ; while it has been observed 

 that the addition of '05 to *1 p. c. of sodium carbonate to a '6 p. c. salt- 

 solution enhances its power of maintaining the activity of a frog's 

 heart fed with such a solution 5 : the beneficial effect of the addition 

 is gradually lost as the salt-solution continues to be used ; but it may 

 be regained by adding a fresh supply of the carbonate, or by shaking, 

 up the old solution in the air. Ranke found that all acids ha TJr e 

 a diminishing influence on irritability. It is said that lactic acid 

 diminishes also the electromotive force of muscle 6 . The accumu- 

 lation of these products, therefore, could not fail to promote ex- 

 haustion. It is also certain that the renewal of the blood current 

 through muscle is swiftly followed by the revival of fatigued muscles; 



1 H. Kronecker, "Ueber die Ermiidung u. Erholung der Muskeln." Ber. der math- 

 phys. Classe der k. sacks. Gesell. der Wissensch., 1871, p. 690. 



2 H. Kronecker, Loc. cit., p. 694. 



3 Georg Liebig, Hermann, Op. cit. 



4 J. Eanke, Tetanus. Leipzig, 1865, p. 350. 



5 Sti^non, "Die Betheiligung der einzelnen Stoffe des Serums an der Erzeugung des 

 Herzschlages." Arch. f. (Anat. u.) PhysioL, 1878, p. 263. 



6 Eanke, Op. cit. Eoeber, Arch. f. Anat. u. Physiol., 1870, p. 615. (Original not 

 seen.) 



