580 FATIGUE OF MUSCLE. 



to lift the weight. The metabolism and the formation of acid are, thus, 

 greater in a stimulated muscle maintained in an extended position 

 than in one that contracts when stimulated. If a muscle loaded with 

 no weight is made to contract by stimulation, it becomes fatigued but 

 gradually. If the muscle is weighted only during the contraction, but 

 not during the extension, it tires more slowly than if it were continuously 

 weighted; as it does also if it is required to lift its weight only in the 

 course of its contraction, instead of raising it at once at the beginning of 

 the contraction. The suspension of weights from a muscle that is con- 

 tinually at rest does not cause fatigue. 



If the arteries of a warm-blooded animal are ligated, complete fatigue will 

 result after from 120 to 240 contractions, in from two to four minutes, on irritation 

 of the nerve. Direct irritation of the muscle, however, will still be able to excite 

 an additional series of contractions. The fatigue-tracings in both cases are straight 

 lines. 



If the circulation in the muscles of a warm-blooded animal be uninterrupted, 

 the contractions first increase in height, and then diminish, to pursue a straight 

 line on stimulation of the nerve. Accordingly, it is found in persons that have 

 used their muscles to the point of fatigue that the muscles and their nerves 

 respond more actively to galvanic and faradic stimulation in the beginning, but 

 to a steadily diminishing degree in the further course of the work. 



Novi has demonstrated with greater detail the course of the contraction to 

 the point of fatigue. According to him, the isolated muscle stimulated to the 

 point of fatigue exhibits several phases in its action. The first phase exhibits a 

 period in which the contractions occur rapidly and increase in size an indication 

 that the repetition of the stimulus causes an increase in the irritability of the 

 muscle. In the second phase, of longer duration, the rapidity of the contractions 

 is maintained, but their height diminishes a sign that the irritability of the 

 muscle is now decreasing. The third phase, again shorter, embraces contractions 

 of slower course, the height remaining unchanged. In a fourth phase the con- 

 tractions become still slower, but again increase in height. Finally the fifth phase 

 exhibits uniform diminution in the height of the contractions and increase in 

 their duration, until exhaustion occurs. Only this last phase corresponds to 

 Kronecker's law. 



According to v. Kries a fatigued muscle tetanized in maximum degree behaves 

 like a fresh muscle tetanized in submaximum degree. Both exhibit an incom- 

 plete transition from the passive to the active state. 



Recovery from the condition of fatigue may be brought about by the 

 passage of a constant galvanic current through the entire length of the 

 muscle, likewise by the injection of fresh arterial blood into its vessels, 

 or of small doses of veratrin. 



Relatively small amounts of sugar (30 grams) increase the muscular energy. 

 Cocoa, coffee, tea, and other substances exert a stimulating influence on muscular 

 activity. 



^ Among the poisons, curare and the putrefaction-toxins (ptomains) cause the 

 fatigue-curve to pursue an irregular course. 



A. Mosso and Maggiora made observations on living persons, by having a weight 

 lifted by the flexors of the middle finger, with the arm in a fixed position. Mosso 

 found that the muscle tires sooner when stimulated directly than when excited 

 indirectly through its nerve. Only for medium weights is the fatigue-tracing a 

 straight line; for smaller weights it is S~shaped, and for larger ones hyperbolic. 

 If a tetanizing, electrical stimulus be continued until the muscular power is ex- 

 hausted, there will still be left in the muscle a residue of energy that can be utilized 

 by the will; and, conversely, a muscle finally exhausted by voluntary contractions 

 can still perform some work when impelled by an electrical stimulus. If both 

 forms of excitation be employed in immediate succession, they will exhaust the 

 muscle completely. Mental exertion diminishes the muscular energy in a marked 

 degree, as do likewise hunger and high temperature, especially in conjunction with 

 marked humidity and diminution of atmospheric pressure; also local artificial 

 elevation or diminution of the muscle-temperature. The strongest muscular con- 



