702 PHYSIOLOGY OF RESPIRATION. 



At these high pressures the blood contains about 28 vohimes of oxy- 

 gen to each 100 c.c. of blood instead of the usual 20 volumes. The 

 additional 8 volumes are contained in solution. Fish also are killed 

 when the oxygen pressure is increased to such a point that the 

 water contains 10 volumes of dissolved oxygen to each 100 c.c. 

 In more recent experiments by Smith,* made upon mice, it was 

 found that oxygen at pressures of 100 per cent, to 130 per cent, 

 proves fatal in a few days, the animals showing inflammatory 

 changes in the lungs. Oxygen at 180 per cent, kills mice and 

 birds within twenty-four hours. Pressures of two atmospheres of 

 air (40 per cent. O) have no injurious effect. No adequate chem- 

 ical explanation can be offered at present for this toxic action of 

 oxygen at high tensions. The matter is one of practical impor- 

 tance in connection with caisson and submarine Avork and the 

 therapeutical use of oxygen. 



Decreased Percentages of Oxygen. — The older experiments upon 

 the effects of a decreased percentage of oxygen in the air were 

 made by placing men or animals in a so-called pneumatic cham- 

 ber in which the composition of the air could be maintained at 

 any desired percentage of oxygen. Modern observers have used 

 this same method, but have employed also the simpler and 

 more convenient method of rebreathing. In rebreathing the 

 individual is connected with a tank of air by means of a suitable 

 mouth-piece. Breathing through the nose is prevented by a cUp, 

 and the individual inspires air from the tank and expires air into 

 it through valves. The expired air passes through a drum or 

 cartridge containing an absorbent to remove the COo. By this 

 arrangement the oxygen in the fixed amount of air contained in 

 the tank is used up gradually, so that in a certain period of about 

 one-half hour the percentage falls from 20 to about 7 or 8 per 

 cent. The change in volume of the air caused by the absorption 

 of the carbon dioxid and the oxygen which would alter the pressure 

 relations is compensated by arranging the tank to work as a spirom- 

 eter. Experiments of this character were made in large numbers 

 upon aviators during the war to determine their reactions to low 

 pressures of oxygen, since it is believed that the physiological 

 effects of high altitudes are due mainly to the decreasing amounts 

 of oxygen, f Rebreathing a certain amount of air until the oxygen 

 falls from 20 to 7 per cent, is equivalent to ascending from sea- 

 level to an altitude of 26,000 to 30,000 feet. Lack of oxygen is 

 designated as a condition of anoxemia. Experiments show that 



* "Journal of Physiology," 24, 19, 1899. 



t Schneider, "Manual of Medical Research Laboratory War Depart- 

 ment, Air Service," Washington, 1918, and "Journal of the Amer. Med. 

 Assoc," October 26, 1918. • 



