ADVANCED EXPERIMENTAL PHYSIOLOGY 151 



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- 

 pncea, 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 con- 

 tinued until " second wind " has been established, the alveolar air 

 will show less carbon dioxide and more oxygen. This accommoda- 

 tion varies in different subjects, but the following example may be 

 given. 



PERCENTAGE COMPOSITION OF ALVEOLAR AIR. 



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

 respiration and circulation to the demands of muscular work. 



CHAPTER XXIX 

 CHEYNE-STOKES RESPIRATION 



In certain cases of heart disease a well-marked alternation of 

 apncea and hyperpncea 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 apncea (Fig. 142). 



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 mouth and connected at the far end with a piece of tubing 

 about 260 cm. long and of about 2 cm. bore. The subject thus 

 rebreathes his own expired air after it has been deprived of carbon 

 dioxide by the soda lime. The percentage of oxygen necessarily 

 falls, the respiratory centre becomes excited and hyperpncea begins. 

 Some fresh air from outside the tube will be taken in with each 

 deep breath and the percentage of oxygen will rise. The hyper- 

 pncea, however, has washed out a quantity of carbon dioxide from 

 the blood and air in the lungs, and apnoea results owing to lack 

 of sufficient carbon dioxide to excite the respiratory centre. Thus 



