THE PHYSIOLOGY OF THE AVIATOR 99 



pressure was begun, it was supposed that the circulation 

 might be primarily disturbed. The blood in the arteries 

 of a healthy man is under such a pressure that, if a glass 

 tube were inserted vertically into one of the arteries of 

 his neck, and the blood were allowed to flow up the tube, 

 the column of blood would come 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 mistake 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 than at sea level. That which they 

 looked for they found. One writer has left a lurid de- 

 scription 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 under no 

 greater strain at a high altitude than 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 blood 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 still prevalent that hemorrhages occur under low- 

 barometric pressures. However, among thousands of 

 people whom I had an opportunity 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 caused by the 

 forcible application of a hard object to the organ in 

 question. 



The only direct effects of changes of pressure are those 

 which are felt in the ears, and occasionally in the sinuses 

 connected with the nose. The ear drums are connected 

 with the throat and contain air at the prevailing pres- 

 sure. If the pressure is lowered this air expands, and 



