A TEXTBOOK OF PHYSIOLOGY 



two. An atmosphere containing under 70 per cent, of oxygen can be 

 breathed with impunity for any length of time. 



The Effects of Want of Oxygen. The symptoms produced vary 

 greatly according to the rate at which such want is produced. When 

 immediate, as on breathing into the lungs marsh-gas, nitrogen, or 

 hydrogen, there is rapid loss of consciousness, followed by convul- 

 sions (Fig. 170), with a slight rise of blood-pressure, followed by 

 cessation of respiration, broken only by occasional inspiratory gasps, 

 fall of blood-pressure, due to vagus inhibition, and death. 



FIG. 170. 



Up}>er tracing, respiration recorded by diaphragm slip; lower tracing, carotid blood- 

 pressure. Time=two seconds. During period indicated 5 per cent, oxygen in 

 nitrogen was inhaled. (F. H. Scott.) 



A match or candle will not burn when there is less than 17 per 

 cent, of oxygen in the atmosphere, but a man feels no inconvenience 

 until the oxygen percentage falls below 14 per cent. Then there 

 supervenes a hyperpnoea of gradual onset, with a slight rise of blood- 

 pressure, increased pulse-rate, and marked cyanosis; and when about 

 6 per cent, is reached, hns of consciousness quickly takes place. At 

 first, on breathing 14 to 10 per cent. O 2 , there is a slight exaltation. 

 The person has the greatest confidence in himself, and is quite con- 

 fident that he is " all there," whereas, in reality, his mental capacity 

 is greatly affected. The breathing is deepened, but no dyspnoea 

 is present. Then, at about 8 per cent., without warning, or possibly 



