196 PRACTICAL PHYSIOLOGY 



ventilation of the lungs is determined by a meter. Then, unknown to 

 the subject, the air to be breathed is taken from a gas bag containing 

 air with 3 or 4 per cent, of carbon dioxide. The breathing is 

 increased. Carbon dioxide stimulates the respiratory 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 1 per cent, effects are produced ; these will be studied in later 

 experiments. 



Influence of holding the breath. Hold the breath to the " breaking 

 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. Stop breathing when a sensation of giddiness is ex- 

 perienced. 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, 17 times in 18 

 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. 



