RESPIRATION 299 



vessel containing alkali. Exposure of the air to the 

 alkali will remove the carbon dioxid, but there is no 

 provision here for a restoration of the consumed oxygen. 



The subject of such an experiment as this will breathe 

 for a long time without any exaggerated efforts and 

 without feeling distressed. He may become quite blue 

 and will perhaps lose consciousness if the trial is not in- 

 terrupted. It appears that lack of oxygen does not 

 strongly excite the respiratory center even when it is 

 threatening to prove fatal. The complementary ex- 

 periment consists in giving a man a mixture of carbon 

 dioxid and oxygen to breathe. In this case labored 

 breathing will result, even though the mixture be 95 

 per cent, of oxygen to 5 per cent, of carbon dioxid. The 

 blood is perfectly arterialized so far as oxygen is concerned 

 but a very moderate increase in its carbon dioxid, due 

 to the difficulty of shaking off the waste under such 

 circumstances, is enough to act powerfully on the center. 



Forced Breathing. If a person sets himself at work 

 to breathe as fast and deeply as he can he will soon be 

 uncomfortable. He may be dizzy or faint. The in- 

 clination to stop forcing the breathing becomes hard to 

 resist. In fact it is almost as hard to overbreathe as 

 to underbreathe, if these expressions may be allowed. 

 The effects of breathing a greater volume of air than that 

 needed to meet current needs are attributed to a reduc- 

 tion of the carbon dioxid in the blood. The condition 

 in which the carbon dioxid of the arterial blood is below 

 the standard is known as acapnia. 



In view of the fact that carbon dioxid is the foremost 

 of animal waste-products it might be supposed that it 

 could not be too thoroughly removed for the advantage 

 of the organism. But our natural impression has proved 

 to be wrong. When the lungs are rapidly ventilated 

 the alveolar air comes to have an unusual composition. 

 It approximates to the fresh outside air. The increase 

 in oxygen, perhaps from 14 to 18 per cent., has little effect 

 on the blood. The simultaneous decrease in the alveolar 



