ELEMENTARY EXPERIMENTAL PHYSIOLOGY 197 



The experiment should then be repeated with this difference; 

 oxygen instead of air should be breathed. The period of apnoea will 

 be much longer, for the subject of the experiment will have more 

 oxygen in his lungs and more in his venous blood. 



Forced breathing interferes with the circulation and often produces 

 giddiness. An examination of the pulse will show that the systolic 

 pressure is diminished by each inspiration. If oxygen is taken in 

 during forced breathing there is less discomfort; the brain receives 

 more oxygen even if its circulation of blood is disturbed. 



Influence of Muscular Exercise. The subject of the experiment 

 should take vigorous muscular exercise sufficient to produce hyper- 

 pnoea, but not long enough for the production of " second wind." A 

 sample of alveolar air taken immediately after the exercise will show 

 in many cases a considerable rise in the percentage of carbon dioxide 

 and a small fall in that of oxygen. If the exercise be continued until 

 "second wind" has been established, the alveolar air will show less 

 carbon dioxide and more oxygen. This accommodation varies in 

 different subjects, but the following example may be given. 



PERCENTAGE COMPOSITION OF ALVEOLAR AIR. 



Carbon dioxide. Oxygen. COg 



Vols. Vols. ~07* 



5-27 14-32 079 At rest. 



7'36 14-03 1-06 After running i mile. 



5-91 14-62 0-93 After running mile more. 



' ' Second wind. " Sweating. 



"Second wind" appears to be a complex adjustment of the respira- 

 tion and circulation to the demands of muscular work. 



CHAPTER LIV. 

 CHEYNE-STOKES RESPIRATION. 



IN certain cases of heart-disease a well-marked alternation of apnoea 

 and hyperpnoea was observed and described by Cheyne and Stokes. 

 This phenomenon is characterised by a period of waxing and waning 

 respiration followed by a period of apnoea (Fig. 191). 



In some healthy men Haldane and Douglas have shown that this 

 type of periodic breathing can be produced in the following way. The 

 subject breathes through a small tin of soda lime provided with wire 

 gauze to prevent the suction of small pieces of soda lime into the 



