CHAP. IT.] RESPIRATION. 381 



variations in amount of nitrogen have of themselves no effect, this 

 gas being eminently an indifferent gas as far as physiological pro- 

 cesses are concerned. 



Poisonous gases. Carbonic oxide produces the same effects as 

 deficiency of oxygen, inasmuch as it preoccupies the haemoglobin 

 and so prevents the blood from becoming properly oxygenated, see 

 p. 340. Sulphuretted hydrogen produces similar effects, but in a 

 different manner ; it acts as a reducing agent, see p. 337. Some 

 gases are irrespirable, on account of their causing spasm of the 

 glottis, and this is said to be, to a certain extent, the case with 

 carbonic acid. 



The Effects of changes in the Pressure of the Air breathed. 



Gradual Diminution of Pressure. The symptoms are those of 

 deficiency of oxygen ; the animals die of asphyxia. The blood con- 

 tains less and less oxygen as the pressure is reduced, the quantity 

 present in the arterial blood soon becoming less than that in 

 normal venous blood. The quantity of carbonic acid in the blood 

 is also diminished. The increasing dyspnoea is accompanied by 

 great general feebleness ; and convulsions though frequent are not 

 invariable. The occurrence of these seems to depend on the 

 suddenness with which the oxygen of the blood is diminished. 



Sudden Diminution. Death in these cases ensues from the 

 liberation of gases within the blood-vessels and the consequent 

 mechanical interference with the circulation. The gas which is 

 found in the blood-vessels on examination after death consists 

 chiefly of nitrogen. 



Increase of Pressure. Up to a pressure of several atmospheres 

 of air, the only symptoms which present themselves are those 

 somewhat resembling narcotic poisoning. At a pressure however 

 of 4 atmospheres of oxygen, corresponding to 20 atmospheres of air, 

 and upwards, a very remarkable phenomenon presents itself. The 

 animals die of asphyxia and convulsions, exactly in the same way 

 as wheu oxygen is deficient. Corresponding with this it is found 

 that the production of carbonic acid is diminished. That is to say, 

 when the pressure of the oxygen is increased beyond a certain 

 limit, the oxidations of the body are diminished, and with a still 

 further increase of the oxygen are arrested altogether. The oxida- 

 tion of phosphorus is quite analogous ; at a high pressure of oxygen 

 phosphorus will not burn. Not only animals but plants, bacteria, 

 and organised ferments, are similarly killed by a too great pressure 

 of oxygen. 



