CHANGES ACCOMPANYING MUSCULAR EXERCISE 437 



entirely due to an increase in C0 2 , it should not have been more than 

 three times as much. Evidently therefore, some other factor than C0 2 -ten- 

 sion must have been responsible for the increased respiratory activity. This 

 conclusion is further confirmed by examination of the alveolar C0 2 

 during very strenuous muscular effort, when a relative decrease in the 

 C0 2 percentage becomes apparent. 



If it is true that the exciting agency has been only partly dependent on 

 an increase in the C0 2 -tension of the blood, we should expect that imme- 

 diately after discontinuing the muscular exercise the C0 2 -tension of the 

 alveolar air would fall to a level distinctly below normal, that it would 

 only slowly recover thereafter, and that further exercise before the re- 

 covery had occurred would produce a less marked increase in alveolar 

 C0 2 . These results we should expect because the store of C0 2 in the body 

 must have been depleted by the hyperpnea. By actual experiment these 

 suppositions have been found to be correct, as is shown in the following 

 table. 



TIME AFTER DISCONTINUING ALVEOLAR C0 2 TENSION 



A BRIEF PERIOD OF IN MM. HG 



MUSCULAR EXERCISE 



(Douglas.) 



In this table the figures of Period 1 represent the alveolar C0 2 

 tension in mm. Hg. immediately following a period of strenuous work. 

 The figures in Period 2 are for the same individual again performing 

 the same amount of work with, however, only a short period of rest in- 

 tervening, and the figures of the third period are a repetition of the same 

 conditions. It will be observed that the muscular exercise at first raised 

 the alveolar tension of C0 2 from the normal of 39 mm. to 49.2 mm., but 

 that in three minutes after the work had been discontinued the tension 

 was considerably below the normal. During the second period of mus- 

 cular exercise the C0 2 in the alveolar air collected immediately after the 

 effort did not increase above the normal level, and in the third period 

 the increase was still less. 



The other factors besides increase in C0 2 -tension may be appearance of 



