INFLUENCE OF VARIOUS CONDITIONS ON RESPIRATION. 711 



heart-rate that accompanies even slight muscular activity, and it 

 seems probable that a similar effect is exercised upon the respira- 

 tory center.* When muscular activity is maintained for some 

 time the lactic acid formed in the muscle escapes to some extent in 

 the blood f and tends to lower its alkalinity. This factor also 

 should augment the activity of the respiratory center. In long- 

 continued muscular exercise, in fact, the fixed acid thus added to 

 the blood may be sufficient to cause a lowering of CO 2 pressure as 

 indicated by a fall in the concentration of CO2 in the alveolar air. 



The Effect of Variations in the Composition of the Air 

 Breathed. Variations in the amount of nitrogen in the inspired 

 air have no distinct physiological effect. The important elements 

 to consider are the oxygen and the carbon dioxid. 



Increased Percentages of Oxygen. The normal pressure of oxygen 

 in the air is 20 per cent, or 152 mms. We may increase this pres- 

 sure either by changing the volume per cent, of the gas or by raising 

 the barometric pressure by compression. The somewhat natural 

 supposition that breathing pure oxygen that is, oxygen at a pres- 

 sure of 760 mm. should have a beneficial effect on the oxidations 

 of the body has found no support in physiological experiments. 

 Atmospheric air supplies us with an excess of oxygen over the needs 

 of the body; a still further increase of this excess has no positive 

 advantage. This is true at least for ordinary conditions of rest or 

 moderate activity. In excessive and prolonged muscular exertion 

 the supply may be inadequate, and under these or similar condi- 

 tions an increase in the percentage of oxygen in the respired air 

 would naturally be advantageous. Paul Bert, in his interesting 

 work on barometric pressures, J has called attention to the fact that 

 at a certain pressure oxygen is not only not beneficial, but, on the 

 contrary, is markedly toxic. From experiments made upon a great 

 variety of animals and plants he concluded that all living things are 

 killed when the oxygen pressure is sufficiently high, say, 300 to 400 

 per cent. Warm-blooded animals die with convulsions when sub- 

 mitted to 3 atmospheres of pure oxygen or 15 atmospheres of air. 

 At these high pressures the blood contains about 28 volumes 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 



* Krogh and Lindhard, "Journal of Physiology," 47, 112, 1914. 



t Ryffel, "Journal of Physiology," 39, 1909. 



I "La pression barometrique," p. 764, Paris, 1878. 



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



