DOCKERAY : PHYSICAL FATIGUE AND MENTAL EFFICIENCY. 203 



for long times, or in larger amounts for shorter times, Lee 

 produced an increase in the height of the work curve. Still 

 larger amounts produced a decrease at once. Burridge's re- 

 sults lead him to believe that "a quick recovery is to be as- 

 sociated with potassium salts alone or in excess while a slow 

 and prolonged one has only been obtained as an effect of lactic 

 acid. It has also been shown that the effect of an exhausting 

 tetanus is almost identically the same as that found to occur 

 when a high concentration of lactic acid has been perfused 

 through a muscle. There is thus reason to believe that the re- 

 sults noted in the early stages of fatigue are mainly referable 

 to potassium and those of the later stages to lactic acid." 

 (p. 303.) The results of these investigations, particularly 

 Lee's, conform to what is generally observed in the fatigue 

 curve of an isolated muscle, or the curve from voluntary con- 

 traction. Hough (16) and Lee show that the fatigue sub- 

 stances also affect the nerve fibers as well as the muscles. 



The relation of the fatigue products to mental fatigue has 

 been little studied. Lehmann (22) measured the amount of 

 carbon dioxide exhaled during mental work. He found that 

 mental work of a determined kind and amount has, with the 

 same individual, a constant increase in the amount of carbon 

 dioxide and corresponds to a definite energy measure. 



To summarize, we find the relation of physical and mental 

 fatigue much in dispute. Mosso finds fatigue general; Lom- 

 bard considers it special, but of central origin ; while Joteyko's 

 work would indicate that it is partially of central origin and 

 more or less general in character, through the fatigue of the 

 voluntary center. Many of the other tests have been too care- 

 lessly performed, or by their nature have been inadequate, to 

 throw much light on the subject. The only experiments which 

 in any way correspond to the experiments to be reported are 

 those of Bettmann and of Bolton. The former, unfortunately, 

 tested only one subject. The latter, though his work was 

 thorough, aimed to test methods that he considered unreliable, 

 and consequently his work throws little light upon the relation 

 of physical fatigue to mental efficiency. 



2 — Univ. Sci. Bull., Vol. IX, No. 17. 



