496 MODIFYING CONDITIONS. 



The cause of fatigue is probably partly due to the accumulation of decomposition- 

 products " fatigue stuffs " in the muscular tissue, these products being formed 

 within the muscle itself during its activity. They are phosphoric acid, either free 

 or in the form of acid phosphates, acid potassium phosphate ( 294), glycerin- 

 phosphojic acid (?) and C0 2 . If these substances be removed from a muscle, by 

 passing through its blood-vessels an indifferent solution of common salt (0*6 per 

 cent.), or a weak solution of sodium carbonate [or a dilute solution of permanganate 

 of potash (Kronecker)], the muscle again becomes capable of energising (J. Ranke, 

 1863). The using up of O by an active muscle favours fatigue (v. Pettenkofer and 

 v. Voit). The transfusion of arterial blood (not of venous Bicliat) removes the 

 fatigue (Ranke, Kronecker), probably by replacing the substances that have been 

 used up in the muscle. Conversely, an actively energising muscle may be rapidly 

 fatigued by injecting into its blood-vessels a dilute solution of phosphoric acid, of 

 acid potassium phosphate, or dissolved extract of meat (Kemmerich). A muscle 

 fatigued in this way absorbs less O, and when so fatigued, it evolves only a small 

 amount of acids and C0 2 . The conditions which lead up to fatigue are connected 

 with considerable metabolism in the muscular tissue. 



[Zabludowski fouud that if a frog's muscles be systematically stimulated by maximum in- 

 duction shocks until they cease to contract, massage or kneading them rapidly restored their 

 excitability, while simple rest had little effect. Massage acts on the nerves, but chiefly by 

 favouriug the blood- and lymph-streams which wash out the waste products from the muscle. 

 A similar result obtains in man, so that the ancient Roman practice of "rubbing" after a bath 

 and after exercise was one conducive to restoration of the power of the muscles.] 



Modifying Conditions. In order to obtain the same amount of work from a 

 fatigued muscle, a much more powerful stimulus must be applied to it than to a 

 fresh one. A fatigued muscle is incapable of lifting a considerable load, so that 

 its absolute muscular force is diminished. If, during the course of an experiment, 

 an excised muscle be loaded with the same weight, and if the muscle be stimulated 

 at regular intervals with maximal stimuli (strong induction shocks), contraction 

 after contraction gradually and regularly diminishes in height, the decrease being a 

 constant fraction of the total shortening. Thus the fatigue-curve is represented by a 

 straight line [i.e., a straight line will touch the apices of all the contractions]. The 

 more rapidly the contractions succeed each other, the greater is the fall in the height 

 of the contraction [i.e., if the interval between the contractions be short, the fatigue- 

 curve falls rapidly towards the abscissa], and conversely. After a certain number 

 of contractions, an excised muscle becomes exhausted. 



This result occurs whether the stimuli are applied at short or long intervals 

 (Kronecker), and a similar result is obtained with sub-maximal stimuli (Tiegel). 

 A fatigued muscle contracts more slowly than a fresh one, while the latent period 

 is also longer during fatigue (p. 480). The fatigued muscle is said to be more 

 extensible (Bonders and van Mansvelt). If a muscle be so loaded that, when it 

 contracts, it cannot lift the load, fatigue occurs even to a greater extent than when 

 the load is such that the muscle can lift it (Leber). The metabolism and the forma- 

 tion of acid are greater in a contracted muscle kept on the stretch, than in a con- 

 tracted muscle allowed to shorten (Heidenhain). If a muscle contract, but be not 

 required to lift any load, it becomes fatigued only very gradually. If a muscle be 

 loaded only during contraction, and not during relaxation, it is fatigued more 

 slowly than when it is loaded during both phases; and the same is true when 

 a muscle has to lift its load only during the course of its contraction, instead of at 

 the beginning of the contraction. Loads may be suspended to perfectly passive 

 muscles without fatiguing them (Harless, Leber). 



[Signs of Fatigue (fig. 343). In the record of the series of contractions; (1) 

 the contractions become more prolonged; (2) they decrease in height; (3) the 

 latent period becomes longer ; (4) if maximal shocks be used, the beginning of the 



