Mat 9, 1919] 



SCIENCE 



433 



duced by rarefied air — so-called mountaiu 

 sickness — from the conditions resulting 

 from work in compressed air — so-called 

 caisson disease. It is clear that it is from 

 the fonner, and not at all from the latter, 

 that aviators suffer; but, as the two dis- 

 orders are sometimes confused, a few 

 words regarding the latter are in place 

 here. 



Caisson disease — known also as the 

 "bends," "diver's palsy," and by other 

 names — depends upon the fact that, under 

 the high pressure necessary for diving, 

 tunneling, and other work below water, tlie 

 the nitrogen of the air dissolves in the 

 blood and in the other fluids and tissues of 

 the body in amounts proportional to the 

 pressure. This in itself does no harm, and 

 has in fact no effect upon the body, until 

 the subject comes out of the pressure lock 

 or caisson, or rises from the depth of the 

 sea where he has been working. Then the 

 nitrogen which has been dissolved begins 

 to diffuse out of the body. This also does 

 no harm and has no effect unless the pres- 

 sure under which the man has been work- 

 ing is so high, and the lowering of the ex- 

 ternal pressure is so rapid, that the dis- 

 solved nitrogen separates in the form of 

 bubbles. Such bubbles may form in the 

 blood, in the synovial fluid of the joints, 

 and even in the brain. They induce in- 

 tense pain, and even paralysis and death. 

 In order that bubbles may be formed it is 

 essential, however, that the pressure with 

 which the tissues are in equilibrium should 

 have been lowered considerably more than 

 half its absolute amount in a few seconds. 



In the present state of the art of flying 

 it is scarcely possible for an aviator to rise 

 to a height of more than 20,000 feet, where 

 the barometer would be less than half of 

 that at sea level, in a period sufficiently 

 short to allow bubbles of nitrogen to form 

 in this wav. The disorders from which 



aviators suffer are, therefore, of a dift'erent 

 class from those to which workers in com- 

 pressed air are exposed. 



When the study of the effects of lowered 

 barometric pressure was begun, it was sup- 

 posed tliat the circulation might be pri- 

 marily disturbed. The blood in the arteries 

 of a healthy man is under such a pressure 

 that, if a glass tube were inserted ver- 

 tically into one of the arteries of his neck, 

 and the blood were allowed to flow up the 

 tube, the column of blood would oome to 

 rest at a height of 4 or 5 feet above his 

 heart, corresponding to pressures of 120 

 to 150 mm. mercury. Knowing that the 

 air pressure is reduced at great altitudes, 

 some of the earlier writers made the mis- 

 take of supposing that such a column of 

 blood would rise higher, and the blood 

 vessels would be under a greater strain, 

 and more likely to burst therefore, at a 

 great altitude that at sea level. That 

 which they looked for they found. One 

 writer has left a lurid description of how, 

 while crossing a pass in the Andes, he got 

 off his mule and walked for a time to rest 

 the animal. On the least exertion his 

 breathing became oppressed, "his eyes 

 bulged and his lips burst. ' ' The odd part 

 of this is that in reality the blood vessels 

 are mider no greater strain at a high alti- 

 tude that at sea level. When the air 

 pressure upon the exterior of the body and 

 in the lungs is reduced, a part of the gas 

 — at least the nitrogen dissolved in the 

 ])lood — rapidly diffuses out through the 

 lungs, so that the gas pressure within and 

 without the blood vessels are again equal 

 just as at sea level. The idea is stiU pre- 

 valent that hemorrhages occur under low 

 barometric pressures. However, among 

 thousands of people whom I had an oppor- 

 tunity to observe on Pike's Peak during a 

 five weeks stay at the summit, I saw not a 

 single nose bleed, except one which was 



