CELLULAR RESPIRATION 135 



The modifications during exercise of other vertebrates have 

 not been studied in such detail. The speed of fish swimming is 

 most rapid for short distances and they are well able to set up 

 O2 debts. Over longer distances respiratory and cardiac factors 

 are more limiting. Lactic acid accumulation persists for a very 

 long time (12 hours) after a period of rapid swimming and the 

 increased O2 consumption lasts for much shorter times. In 

 animal studies it is difficult to be sure how 'hard the fish is 

 trying' and in man there is no doubt that behavioural factors 

 play an important part. As the first person to run a mile in under 

 four minutes has written: Though physiology may indicate 

 respiratory and circulatory limits to muscular exercise, psycho- 

 logical and other factors beyond the ken of physiology set the 

 razor's edge of defeat or victory and determine how closely an 

 athlete approaches the absolute limits of performance.' (R. G. 

 Bannister, Chapter IX, The Structure and Function of Muscle, 

 Vol. II. Academic Press, 1960.) 



