EFFECTS OF BREATHING CONDENSED AND RAREFIED AIR 291 



till the pressure was reduced to 410 mm. There are towns on the 

 high tablelands of the Andes, and in the Himalayas, where the 

 barometric pressure is not more than 16 to 20 inches, yet the in- 

 habitants feel no ill effects. And in the caissons of the Forth Bridge 

 the workmen were engaged in severe toil under a maximum pressure 

 of over three atmospheres, while in the caissons of the St. Louis 

 Bridge in America a maximum pressure of over four atmospheres 

 (i.e., more than three atmospheres in addition to the ordinary air- 

 pressure) was reached. 



Inside ths caissons the men sometimes suffer from pain and noise in 

 the ears, due to excessive pressure on the external surface of the tym- 

 panic membrane. If the pressure in the tympanum is raised by a 

 swallowing movement, which opens the Eustachian tube and permits 

 air to enter it, the symptoms generally disappear. The suddenness of 

 the change of pressure has much to do with its effects, and it is found 

 that the men are most liable to dangerous symptoms while passing 

 through the air-lock from the caissons to the external air. It may be 

 concluded, from experiments on animals, that some of the most serious 

 of these the localized paralysis usually affecting the legs (paraplegia) 

 and the circulatory disturbances are due to the formation of gaseous 

 emboli, by the liberation of nitrogen in the blood and other body- 

 fluids when the pressure is abruptly reduced. And, indeed, it is found 

 that the symptoms can often be caused to disappear, both in animals 

 and men, by promptly subjecting them again to compressed air. To 

 avoid gas embolism on decompression, the shift should be so short that 

 the body-fluids do not become fully saturated with nitrogen, and the 

 decompression should be slow. Even with a rate of decompression of 

 twenty minutes for each atmosphere of excess pressure, the equilibrium 

 between the dissolved and the atmospheric nitrogen is not entirely 

 established fifteen minutes after decompression. 



But that the action of air under a high pressure is not merely mechan- 

 ical follows from the singular fact that in pure oxygen at a pressure 

 of 4 to 5 atmospheres, which corresponds to air at 20 to 25 atmospheres, 

 convulsions are often produced in vertebrate animals, while exposure 

 to 6 to 25 atmospheres of oxygen causes dyspnoea and coma, usually 

 without convulsions. All animals, so far as investigated, are instantly 

 convulsed and killed under a pressure of 50 atmospheres of oxygen 

 (Hill and Macleod). Even seeds and vegetable organisms in general 

 are killed in a short time in oxygen at 3 to 5 atmospheres; and an 

 atmosphere of pure oxygen, equal to 5 atmospheres of air, hinders 

 the development of eggs. Lo train Smith has shown that in small birds 

 and mice exposure for many hours to a pressure of between i and 2 

 atmospheres of pure oxygen causes pneumonia. He confirms Bert's 

 observations on the acute toxic effects produced by higher pressures, 

 and supposes that in the production of caisson disease the special action 

 of the oxygen at high pressure may play a part as well as the rapid 

 decompression. 



When the air-pressure is diminished below a certain limit, death 

 takes place from asphyxia, more or less gradual according to the 

 rate at which the pressure is reduced. The haemoglobin cannot get 

 or retain enough oxygen to enable it to perform its respiratory func- 

 tion; its dissociation tension is no longer balanced by an equal or 



