CHAP. IL] RESPIRATION. 641 



The phenomena of slow asphyxia, where the supply of air is 

 gradually diminished, are fundamentally the same as those result- 

 ing from a sudden and total deprivation. The same stages are 

 seen, but their development takes place more slowly. 



377. Deficiency of air results not only in a diminution 

 of the oxygen but also in an increase of the carbonic acid of 

 the blood. We have seen however ( 372) that the phenomena 

 of asphyxia are in the main due to the former, and that the 

 accumulation of carbonic acid in the blood has subsidiary effects 

 only. 



If the percentage of oxygen in the inspired air be increased 

 instead of diminished, the total pressure of the atmosphere re- 

 maining the same, the partial pressure of the oxygen alone being 

 changed, no marked results follow. We have already seen ( 354) 

 that the percentage of oxygen in the ordinary atmosphere leaves 

 a wide margin of safety, and that ( 374) the phenomena of 

 apncea are in the main at least to be explained as the result not 

 of an increase in the oxygen of the blood but of nervous impulses 

 ascending the vagus nerves. We have no satisfactory evidence that, 

 provided the respiratory mechanism is in good working order, an 

 increase of oxygen in the inspired air even to a whole atmosphere 

 seriously modifies the respiratory act. 



378. The composition of the atmosphere, the pressure 

 remaining the same, may be modified by the introduction of 

 foreign gases. To some of these the respiratory mechanism is 

 indifferent ; for instance, hydrogen may be substituted for nitrogen 

 without any change in the respiration, provided of course that the 

 oxygen is not diminished. Other gases may produce poisonous 

 effects, either by interfering with some of the respiratory processes 

 or in other ways. Thus carbon monoxide, by combining with the 

 haemoglobin of the red corpuscles, and so preventing the corpuscles 

 from acting as oxygen carriers, produces asphyxia through de- 

 ficiency of oxygen. Sulphuretted hydrogen interferes with the 

 oxygenatioii of the blood by acting as a reducing agent. Some 

 gases while allowing the ordinary respiratory changes of the blood 

 to go on as usual produce toxic effects by acting on one or other of 

 the tissues. Thus, as we have seen, an excess of carbonic acid in 

 the blood seems to have a special effect on the central nervous 

 system and so acts as a narcotic poison. The peculiar effects of 

 nitrous oxide (laughing gas) are similarly due to the direct action 

 of the gas in the blood on the central nervous system. Some 

 gases are irrespirable and may interfere with respiration, even 

 causing suffocation, on account of their causing spasm of the 

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

 an atmosphere 'which is wholly or largely composed of carbonic 

 acid. 



379. The Effects of Changes in Atmospheric Pressure. Di- 

 minution of Pressure. The partial pressure of the oxygen in the 



