150 PRACTICAL PHYSIOLOGY 



centre. In order to check any effects of change in resistance or 

 of suggestion the gas bag should, unknown to the subject, be filled 

 with pure air and the experiment repeated. Air containing 8 or 



9 per cent, of carbon dioxide will produce intolerable discomfort 

 or distress. 



Influence of Breathing Different Percentages of Oxygen. After 

 breathing air for some time, the subject breathes pure oxygen from 

 a bag : the rate and volume breathed generally show no change, 

 if precautions have been taken to avoid the effects of suggestion. 

 If the oxygen be moistened with water most men cannot distinguish 

 it from air taken from a similar bag. 



Air containing about 15 per cent, of oxygen can be collected 

 free from carbon dioxide by breathing slowly through a flask or 

 tin of soda lime into a gas bag. Experiments with this gas will 

 show no change in the rate or volume of the air breathed. A fall 

 of 5 or 6 per cent, in the amount of oxygen in the air is not detected. 

 When the oxygen is only 10 per cent, effects are produced ; these 

 will be studied in later experiments. 



Influence of Holding the Breath. - Hold the breath to the " break- 

 ing point " and then collect a sample of alveolar air. The carbon 

 dioxide will rise to 7 or 8 per cent. ; the oxygen will fall to about 



10 per cent. 



Repeat the experiment after breathing oxygen for two or three 

 minutes. The " breaking point " will not occur so soon, but the 

 rise in the carbon dioxide will be the determining factor, for the 

 oxygen in the alveolar air may be above 20 per cent, at the end of 

 the experiment. The carbon dioxide may rise to 10 per cent. 



Influence of Forced Breathing. Take a series of rapid and deep 

 breaths for about half a minute, recording the movements by the 

 stethograph. 1 Stop breathing when a sensation of giddiness is 

 experienced. There will be no inclination to breathe for about a 

 minute. The condition is one of apnoea, due to the washing out 

 of carbon dioxide from the lungs and blood. The composition of 

 the alveolar air will indicate the changes which occurred, as shown 

 by the following example. The subject breathed rapidly and 

 deeply, seventeen times in eighteen seconds. A sample of alveolar 

 air from the last expiration yielded on analysis 2-50 vols. per cent, 

 of carbon dioxide and 19-23 of oxygen. Apnoea followed. The 

 sample of the first expiration, when a desire to breathe was felt, 

 had the following composition : carbon dioxide 5-59 vols. per cent., 

 oxygen 12-59 per cent. 



The experiment should then be repeated, with this difference : 

 oxygen instead of air should be breathed. The period of apncea 

 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 pro- 

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



i See p. 134. 



