1224 PHYSIOLOGY 



may therefore be considerably prolonged, if the onset of oxygen lack be 

 postponed, by increasing the tension of this gas in the alveoli at 

 the commencement of the apnceic period. By forcibly breathing for a 

 period of two minutes in an atmosphere of oxygen, men have suc- 

 ceeded in holding their breath for as long a period as eight minutes 

 (Vernon). 



' Cheyne-Stokes ' breathing is almost invariably observed as one of the effects 

 of exposure to high altitudes, and is then especially marked during sleep. It is 

 often present when the activity of the respiratory centre is depressed, as in cases of 

 ursemia or pernicious anaemia. Under these circumstances it may be temporarily 

 removed by administering either oxygen or carbon dioxide (in small percentage) 

 to the patient. The oxygen improves the condition of the centre ; the carbon 

 dioxide acts as an added stimulus and rouses its activity. 



