712 PHYSIOLOGY OF RESPIRATION. 



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 chemical explanation can 

 be offered at present for this toxic action of oxygen at high tensions. 

 The matter is one of practical importance in connection with caisson 

 and submarine work and the therapeutical use of oxygen. 



Decreased Percentages of Oxygen. Numerous observers (Bert, 

 Zuntz, et al.) have shown that a fall in oxygen pressure has no 

 perceptibly injurious result until it reaches about 10 per cent. At 

 or somewhat below this pressure the hemoglobin is unable to take 

 up its full amount of oxygen, and the body consequently suffers 

 from a real deficiency in its oxygen supply, a condition designated 

 as anoxemia. According to Bert's experimental results, death with 

 convulsions quickly follows a fall of atmospheric pressure to 250 

 mms. (oxygen pressure, 50 mms. or 6 to 7 per cent.). Animals 

 supplied with an atmosphere containing a deficient amount of 

 oxygen show dyspneic respirations when the deprivation of oxygen 

 is made with some suddenness. When the oxygen is decreased 

 slowly, as in the case of a person rebreathing a confined portion of 

 air, and with provision made to absorb the CO 2 as it is expired, 

 it is said that the individual experiences so little discomfort that he 

 may voluntarily continue the respirations, in spite of a noticeable 

 cyanosis, until he falls unconscious. It would seem that the nor- 

 mal supply of oxygen, 20 per cent., offers a large factor of safety 

 and makes provision for the great increase in oxygen which is nec- 

 essary during muscular activity. When the supply of oxygen is 

 reduced to about 10 per cent, this factor of safety is removed. The 

 individual may suffer no inconvenience while resting, but active 

 muscular exercise may produce at once respiratory discomfort and 

 labored breathing. 



Increased Percentages of Carbon Dioxid. It was pointed out 

 by the researches of Friedlander and Herter* and later observers 

 that death from increased percentages of C02 is accompanied by 

 symptoms quite different from those due to lack of oxygen. As the 

 CO2 is increased a noticeable hyperpnea may be observed (Zuntz) at 

 a concentration of about 2 per cent. When the concentration of 

 CC>2 reaches 8 per cent, to 10 or 15 per cent, there is distinct dysp- 

 nea; but beyond this point further concentration, instead of aug- 

 menting the respirations, decreases them, and the animal dies, at 

 concentrations of 40 to 50 per cent., without convulsions, but with 

 the appearance, rather, of a fatal narcosis. It is probable that in 

 these concentrations the C0 2 exercises a direct toxic action on the 

 nerve cells. 



* Friedlander and Herter, "Zeitschrift f. physiol. Chemie," 2, 99, 1878, 

 and 3, 19, 1879; Haldane, loc. tit. 



