198 



PRACTICAL PHYSIOLOGY 



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 hyperpnoea begins. Some fresh air from 

 outside the tube will be taken in with each deep breath and the 

 percentage of oxygen will rise. The hyperpnoea, however, has washed 



FIG. 191. Cheyue-Stokes respiration. (Pembrey and Allen). 



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 this alternation of breathing and apnoea 

 may continue for several minutes or, it may be, hours. Some healthy 

 men exhibit Cheyne-Stokes respiration readily when they perform this 

 experiment; others do not. 



Apnoea can be abolished by either (i) air containing 3 or 4 per 

 cent, of carbon dioxide, or (ii) pure oxygen, or (iii) air containing a 

 deficiency of oxygen, about 1 2 per cent, of oxygen. 



