INFLUENCE OF VARIOUS CONDITIONS ON RESPIRATION. 705 



muscular work, and this initial effect at least has been explained 

 in the same way for both mechanism. It is suggested,* for ex- 

 ample, that the voluntary nerve impulses coming down from the 

 brain to the muscles irradiate into the medullary centers and 

 quicken the heart rate by inhibiting the cardio-inhibitory center, 

 and at the same time stimulate the respiratory center or increase 

 its irritability so that it responds more readily to its intrinsic 

 stimulus. When the muscles begin contracting they need more 

 oxygen, and they produce more carbon dioxid to be removed. 

 The quicker heart rate supplies more arterial blood, and the quicker 

 respirations provide for the removal of the carbon dioxid and an 

 adequate supply of oxygen. The more slowly developing change 

 in the reaction of the blood, owing to the formation of acid products 

 in the muscles, may be regarded, perhaps, as a supplementary 

 provision in continued muscular activity for also raising the 

 irritability of the respiratory center and thus ensuring deeper and 

 more rapid breathing movements. 



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, f 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- 



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



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



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