THE RESPIRATORY SYSTEM. 287 



the reasons why the pulse rate remains rapid for some time after the 

 exercise has come to an end. 



Second Wind. It is a matter of common knowledge that, in a 

 trained person, the respiratory discomfort which occurs soon after the 

 beginning of exercise usually passes off in a few minutes, and the 

 exercise can then be continued for a long period without further incon- 

 venience, the individual being said to have gained his second wind. 

 Its causation has been much discussed, and is not fully understood, 

 though it is probably due in part to the fact that a trained person 

 uses his muscles more economically than an untrained person, and 

 therefore produces less carbonic acid. The observation that the 

 tension of carbonic acid in the alveolar air may fall with the onset of 

 second wind suggests that such is the case. If the muscles are used 

 more economically after a brief period of exercise, the decrease in the 

 production of carbonic acid will lessen the stimulus to the respiratory 

 centre, and will account for the absence of panting and respiratory 

 distress in second wind. It is probable that adjustment also takes 

 place in the circulation, since the process of training consists essenti- 

 ally in the better adaptation of the circulatory system to withstand 

 extra strain. 





