PHYSIOLOGICAL EFFECTS OF LABOUR 209 



arched chest, upright stature, larger chest expansion than the 

 average, and prominent muscles. 



In exercise or work involving great speed, fatigue is due to the 

 frequency of the nervous excitation, to the effort needed to 

 attain the necessary speed. The exact nature of this fatigue is 

 not known, but it is suspected that it takes its rise in the termina- 

 tion of the motor nerves. This kind of work draws on the 

 nervous energy ; for example, stenographers and typists, etc., 

 when working at great pressure, may make mistakes and 

 miss out words. Men addicted to working at high speed 

 do not develop muscularly, but have slim, rather thin, bodies, 

 such as runners, dancers and fencers. They have generally less 

 appetite than other men. Unfortunately, no precise rule can be 

 formulated to estimate the degree of nervous fatigue. 



147. Causes of Fatigue. ( J ) Two phenomena, the one physical 

 and the other chemical, appear to be the determining causes of 

 fatigue. There is, on the one hand, an alteration of the elastic 

 properties and of the cohesion of the muscular fibres. Their 

 elastic force is greatly decreased, and they become pliable. ( 2 ) 

 On the other hand, the matter of the muscles changes its chemical 

 properties. It is clearly acid, and in its alcoholic extraction are 

 found several toxic bodies capable of giving fatigue, if injected, 

 into the bodies of animals. ( 3 ) The ponogeneous, toxic products 

 (from TTOVSO I force myself) diminish the irritability so that the 

 muscles no longer react except to stronger nervous excitations, 

 and there follows, therefore, an excessive expenditure in getting 

 into action, the origin of a new fatigue. ( 4 ) It has nearly always 

 been stated that nerves, in contrast to the muscles, are indefati- 

 gable, even such important nervous centres as the brain. It 

 would seem that this is not absolutely true, although ( 5 ) the 

 nervous fibres are much less easily tired Jthan the cells, and, as 

 has been shown, all fatigue, in whichever organ it is localised, 

 corresponds to the formation of toxic substances in the organism. 

 Further, these latter increase, owing to the fact that the albu- 

 minoid reserves are drawn upon when the respiration is impeded 

 (dypsnoea), as in the case of a tired man. And it has been found 



(*) See an interesting article by Lee, " The Nature of Muscle Fatigue " 

 (Amer. Journ. of Physiol., vol. ii., p. 11, 1905). 



( 2 ) Boudet De I' Elasticity Musculaire, These, Paris, 1880 ; A. Mosso 

 (Arch. Ital. Biol., vol. xxv., p. 371, 1896) ; loteyko (article y Fatigue," 

 in Dictionnaire de Physiol., of Ch. Richet, 1904). 



( 3 ) A. Mosso, " La Fatigue," translated, by Langlois, Pans, 1. >4. 

 () Treves Arch. Hal. BioL, vol. xxx., p. 1; 1898. 



() J Carvallo (C. R. Sc., vol. cxxvii., p. 774, 1900) ; FroehUdqZ^. AUg. 

 Phys., vol. iii., p. 468, 1904). 



