Armstrong’s discovery gave way to a major milestone in aviation medicine. Working with Heim on more tests, he inserted a viewing tube into the artery of a test animal. The two men took data on what happens to a mammal’s body at forty thousand, fifty thousand, and finally sixty-five thousand feet. They were the first to witness that body fluids boil at sixty-three thousand feet. This point would become known as the Armstrong line. This is the altitude beyond which humans cannot survive without a pressure suit.
