494 EFFECTS OF DIMINUTION OF PRESSURE. [Book ii. 



bear a scanty supply of oxygen for a much longer time than 

 usual before dyspnoea sets in, as is seen in the case of clivers. 



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. 



§ 307. The composition of the atmosphere, the pressure re- 

 maining the same, may be modified by the introduction of foreign 

 gases. To some of these the respiratory mechanism is indiffer- 

 ent ; 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 pro- 

 cesses 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 deficiency of oxygen. Sulphuretted hydrogen inter- 

 feres with the oxygenation 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 effect 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. 



§ 308. The Effects of Changes in Atmospheric Pressure. 

 Diminution of pressure. The partial pressure of the oxygen in 

 the inspired air may be changed, not only by altering the com- 

 position of the air entering at the ordinary atmospheric pressure, 

 but also by altering the total pressure of the atmosphere without 

 changing its composition. The results of the latter are however 

 complicated ; we have then to deal not merely with the effects 

 on the interchange of gases in the lungs but with the effects on 

 the whole organism. All the complicated machinery of the body 

 is adapted and arranged to work under what we may call ordi- 

 nary atmospheric pressure, that is to say, within the limits of 7 GO 

 mm. mercury at the sea level and about 500 mm., correspond- 

 ing to an altitude of 6000 feet, this being the range of ordinary 

 human dwellings. Any great increase or decrease of pressure 

 beyond these limits will affect not only the exit of carbonic acid 

 from and the entrance of oxygen into the blood, but, in varying 

 degree, all the physical and chemical processes of the body. A 

 gross instance of this is seen when an animal is suddenly sub- 



