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PRINCIPLES OF GENERAL PHYSIOLOGY 



hours and a half, before signs of fatigue appear (Fletcher, 1902, p. 491). The 

 contractile power continues longer in oxygen, as we shall see presently We may 

 note here that the onset of fatigue shows that something has been used up, and 

 that it has not been replaced. Supply of oxygen to a muscle, fatigued in nitrogen, 

 even in the case of an excised muscle, to whose interior the access of oxygen is 

 difficult, brings about recovery to a very considerable extent (see Fig. 135, which 

 shows also that spontaneous rigor is hindered by the presence of oxygen). 



In order to discover what chemical change occurs in the act of contraction 

 itself, we must exclude the influence of oxygen. We know that contraction can 

 take place in its absence, so that the contractile process itself does not make use of 

 it. An important part of the problem to be solved is, in fact, the part played by 

 oxygen. That muscles stimulated in the absence of blood supply become acid, and 

 that lactic acid is produced by muscle as it dies and enters into rigor mortis, are old 

 observations, but the production of lactic acid in the normal contraction was first 

 elucidated by the work of Fletcher and Hopkins (1907). Surviving excised 



