274 A MANUAL OF PHYSIOLOGY 



of the Andes, and in the Himalayas, where the barometric pres- 

 sure is not more than 1 6 to 20 inches, yet the inhabitants feel 

 no ill effects. And in the caissons of the Forth Bridge the work- 

 men 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 the caissons the men sometimes suffer from pain and noise 

 in the ears, due to excessive pressure on the external surface of the 

 tympanic membrane. If the pressure in the tympanum is raised by 

 a swallowing movement, which opens the Eustachian tube and per- 

 mits air to enter it, the symptoms generally disappear. The sudden- 

 ness 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 atmo- 

 sphere 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 

 mechanical 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 verte- 

 brate 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 five atmospheres of air, hinders the develop- 

 ment of eggs. Lorrain 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 respira- 



