R E SPIRA TION 257 



conditions are very different. For the blood-pressure, the 

 intra-thoracic pressure, and the intra-alveolar pressure, all 

 fall together when the pressure of the atmosphere is 

 diminished, and all rise together when it is increased. It is 

 possible not only to live, but to do hard manual labour, at 

 very different atmospheric pressures. Loewy found that 

 the quantity of oxygen absorbed by a man breathing air in 

 the pneumatic cabinet remained constant at all pressures 

 between about two atmospheres and half an atmosphere. 

 At 440 mm. of mercury dyspnoea became evident ; but if 

 the person was now made to work, the dyspnoea passed 

 away, and did not again manifest itself till the pressure was 

 reduced to 410 mm. There are towns on the high table- 

 lands of the Andes, and in the Himalayas, where the 

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

 the inhabitants feel no ill effects. And in the caissons of 

 the Forth Bridge the workmen were engaged in severe teal 

 under a maximum pressure of over three atmospheres, while 

 in the caissons of the St. Louis Bridge in America a maximum 

 pressure of about four atmospheres 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 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 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. 



But that the action of oxygen under a high pressure is not merely 

 mechanical seems to follow from the experiments of Bert. He dis- 

 covered the singular fact that in pure oxygen at a pressure of 3 to 5 

 atmospheres, which corresponds to air at 15 to 25 atmospheres, 

 animals die in a short time in convulsions like those produced by 

 strychnine. Even seeds and vegetable organisms in general are 

 killed in a short time ; and an atmosphere of pure oxygen, equal to 

 five atmospheres of air, hinders the development of eggs. Lorrain 

 Smith has recently shown that in small birds and mice exposure for 



