338 RESPIRATION 



outside. The passage is specially liable to be blocked if any 

 catarrh of the air passages is present ; and if the warning pain is 

 disregarded the membrane may burst, though this is not a very 

 serious accident. In men accustomed to compressed air the Eus- 

 tachian tubes open easily, so that no inconvenience is felt, and a 

 diver goes quite easily within two minutes to a pressure of seven 

 atmospheres or more, while one who is not accustomed to com- 

 pressed air may have a long struggle with his Eustachian tubes 

 before he can reach an extra pressure of half an atmosphere. It 

 also happens occasionally that there is trouble with the frontal 

 sinuses. The same difficulties with the middle ear may, of course, 

 be met with by airmen during rapid descents, or even, to a minor 

 extent, in descending a deep mine shaft. 



A man who has reached a pressure of six or seven atmospheres, 

 and is breathing pure air, is perfectly comfortable if he has es- 

 caped ear trouble. His voice is, however, altered by the com- 

 pressed air, and this is so marked that it is often difficult to make 

 out through the telephone what he is saying. At first sight it 

 might seem that an increased mechanical pressure of several 

 atmospheres would in itself be expected to have an appreciable 

 effect on a man or animal. It was commonly supposed, for ex- 

 ample, that the increased pressure on the skin must at first tend to 

 drive blood into the internal organs, producing congestion of the 

 brain, etc., with a converse effect on diminishing the atmospheric 

 pressure. The pressure is, however, transmitted instantly through- 

 out all the liquid and solid tissues of the body, so that this idea 

 was totally fallacious, and indeed ridiculous. As will be seen 

 below, many divers have lost their lives owing to well-meant in- 

 junctions to descend and ascend slowly. As regards other possible 

 effects of a few atmospheres of mechanical pressure, it should be 

 remembered that the intrinsic pressure of water is calculated to 

 be over 10,000 atmospheres. As the tissues are largely composed 

 of water, the addition to this of a few atmospheres of mechanical 

 pressure in the liquid or semi-liquid parts of the body cannot be 

 of much account. 



As Paul Bert showed experimentally, the serious inconveni- 

 ences and dangers to which workers in compressed air are ex- 

 posed are due (apart from easily avoidable effects on the ears) 

 not to the mechanical pressure, but to the increased partial pres- 

 sures of the gases in the air breathed. If the air breathed is pure, 

 the only gases which come into consideration in this connection 

 are nitrogen and oxygen; but if the air is rendered impure by 



