THE PHYSIOLOGY OF MUSCULAR WORK 255 



are necessary to show the relationship of this substance to 

 muscular fatigue. 



Undue stress is frequently laid upon the capacity of the heart 

 to perform work. This is not peculiar to the heart, a similar 

 condition is seen in the respiratory muscles. Their activity is 

 rhythmic, work followed by rest ; they have been progressively 

 trained from birth and work efficiently and economically. By 

 constant practice a corresponding capacity for work can be 

 developed in the ordinary muscles of the body. When an 

 unusual amount of work is thrown upon the heart and the 

 respiratory muscles, signs of fatigue are observed in them similar 

 to those shown by other muscles. 



The absence of fatigue in nerves need not be discussed here, 

 for in the living body the nerve is a part of the nerve cell, and the 

 nerve cells are without doubt subject to fatigue. How largely 

 the nervous system is involved in fatigue is shown by the fact 

 that a much greater amount of work can be performed, without 

 the immediate experience of fatigue, when the subject is spurred 

 on by nervous excitement. The fatigue, however, in these cases 

 is only postponed, and the reaction is greater. The nervous ex- 

 citement inhibits the sensations of fatigue, and under such con- 

 ditions the body can be readily overtaxed. 



Nervous impressions may diminish the sensations of fatigue ; 

 soldiers at the end of a long march step out more briskly when 

 the band strikes up a lively tune. The interaction between the 

 nervous and muscular systems is of the closest nature. Good 

 mental work cannot be done after a hard day's sport, and study 

 for a serious examination is incompatible with training for a race. 



Fatigue is a protective sensation, a warning that the body needs 

 rest. There is nothing to be gained by resisting it by the use of 

 drugs. Work performed under such conditions is extravagant, and 

 the price is paid later in a greater reaction and depression. The 

 decrease in the general sensibility produced by fatigue renders a 

 man unmindful of discomforts which under ordinary conditions 

 would be intolerable ; he can sleep profoundly and thus restore 

 his energy. 



The rational method is to prevent as far as possible the con- 

 ditions which are favourable to fatigue. Progressive training 

 enables the body to work more economically, to adjust the supply 

 of blood to the needs of the various parts, and to strengthen the 



