REGULATION OF THE RESPIRATORY MOVEMENTS 1147 



resulting on forced breathing, the great diminution of oxygen tension in 

 the alveoli is shown by the fact that the subject of the experiment becomes 

 blue, and may indeed lose consciousness.. There are at the same time 

 rhythmic changes in the blood pressure, which rises towards the ends of 

 the periods of the apncea, falling during the periods of respiration. The first 

 respiration after forced breathing is due to oxygen lack. The period of 

 apnoea 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 succeeded 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 uraemia or per- 

 nicious 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 activitv. 



