150 THERMAL AND CHEMICAL CHANGES IN MUSCLE [CH. XIII. 



Fatigue. 



If the nerve of a nerve-muscle preparation is continually stimu- 

 lated, the muscular contractions become more prolonged (see p. 117), 

 smaller in extent, and finally cease altogether. 



The muscle is said to be fatigued : this is due to the consump- 

 tion of the substances available for the supply of energy in the 

 muscle, but more particularly to the accumulation of waste products 

 of contraction ; of these, sarco-lactic acid is probably an important 

 one. Fatigue may be artificially induced in a muscle by feeding it on 

 a weak solution of lactic acid, and then removed by washing out the 

 muscle with salt solution containing a minute trace of an alkali. If 

 the muscle is left to itself in the body, the blood-stream washes away 

 the accumulation of acid products, and fatigue passes off. 



The question next presents itself, where is the seat of fatigue ? 

 Is it in the nerve, the muscle, or the end-plates ? If, after fatigue has 

 ensued and excitation of the nerve of the preparation produces no 

 more contractions, the muscle is itself stimulated, it contracts ; this 

 shows it is still irritable, and, therefore, not to any great extent the 

 seat of fatigue. 



If an animal is poisoned with curare, and it is kept alive by arti- 

 ficial respiration, excitation of a motor nerve produces no contraction 

 of the muscles it supplies. If one goes on stimulating the nerve for 

 many hours, until the effect of the curare has disappeared, the block 

 at the end-plates* is removed and the muscles contract : the seat of 

 exhaustion is therefore not in the nerves. 



By a process of exclusion it has thus been localised in the nerve- 

 endiiigs. 



When the muscle is fatigued in the intact body, there is, however, 

 another factor to be considered beyond the mere local poisoning of 

 the end-plates. This is the effect of the products of contraction 

 passing into the circulation and poisoning the central nervous system. 

 It is a matter of common experience that one's mental state influ- 

 ences markedly the onset of fatigue and the amount of muscular 

 work one can do. This aspect of the question has been specially 

 studied by Waller and by Mosso. Mosso devised an instrument 

 called the ergograph, which is a modification of Waller's dynamograph 

 invented many years previously. The arm, hand, and all the fingers 

 but one are fixed in a suitable holder ; the free finger repeatedly lifts 

 a weight over a pulley, and the height to which it is raised is regis- 

 tered by a marker on a blackened surface. 



By the use of this and similar instruments it has been shown 



* Another convenient block which is sometimes used is to throw a constant 

 current into the nerve between the point of excitation and the muscles. This pre- 

 vents the nerve impulses from reaching the muscles. 



