132 THERMAL AND CHEMICAL CHANGES IN MUSCLE [CH. XII, 



nitrogen is negligible, and only occurs when the muscles do not 

 receive a due share of non-nitrogenous food. (See more fully chapter 

 on Metabolism.) 



Fatigue. 



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

 lated, the muscular contractions become more prolonged (see p. 97), 

 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 pro- 

 ducts of contraction ; of these, sarcolactic acid is 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 the peripheral end 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-endings. 



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. 

 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 registered by 

 a marker on a blackened surface. 



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

 that the state of the brain and central nervous system generally is a 



* 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. 



