HUMAN LIMITS IN FLIGHT—MATTHEWS 283 
Bubbles forming in the body fluids have long been a difficulty in deep 
diving where men have been subjected to much increased pressures of 
air. The body fluids then dissolve a large quantity of nitrogen and 
if the diver comes to the surface too rapidly it cannot escape from his 
lungs in time to prevent bubbles forming and he gets decompression 
sickness or “bends” (caisson disease, compressed-air illness), with 
severe pain, cramps, occasionally unconsciousness and even death. A 
diver can get severe bends coming up from a depth where the pressure 
is 4 atmospheres to the surface where it is only 1 atmosphere, but for- 
tunately an airman does not get into such serious difficulties if he 
goes from ground-level to one-quarter ground-level pressure at 33,000 
feet. Bends as they occur in the air are rarely experienced at alti- 
tudes below 25,000 feet. They come on slowly and are rarely of a 
serious nature. Unconsciousness can result if the warning symptoms 
of pain in the joints are neglected. The pains are cured almost 
instantaneously if descent is made to about 25,000 feet where the air 
pressure compresses the nitrogen bubbles sufficiently to drive them back 
into solution in the blood. 
Much research has been carried out on men in decompression cham- 
bers to find ways of alleviating these effects. One method is to breathe 
pure oxygen before ascent, so replacing the nitrogen in the blood with 
oxygen. The oxygen is then used up in the tissues before it can form 
bubbles. This method has long been used to displace nitrogen from 
_ the blood in diving. 
There are other disturbances to man with rapid changes of altitude 
resulting from the change in air pressure. Behind the ear drum is a 
cavity filled with air which communicates through a small canal with 
the throat and it is necessary for air to leave and enter it with ascent 
and descent lest the ear drum be collapsed. The canal to the throat 
will normally open on swallowing and in a dive a pilot clears his ears 
almost unconsciously, but should he fail to do so or have severe catarrh, 
he may damage his ear drums. Enclosed gas elsewhere in the body, as 
in the sinuses surrounding the nose, has to equalize its pressure as the 
altitude changes or severe pain may result. Again on ascent the gas 
normally present in the intestines expands to a larger and larger vol- 
ume as the outside pressure falls when climbing but this is rarely a 
serious problem. 
Thus the human safety limit in height is some 10,000-16,000 feet 
breathing air and 40,000-42,000 feet breathing oxygen; heights much 
in excess of the latter are only achieved by enclosing the pilot in an 
artificial atmosphere. 
But it is clear that starting with fit pilots on the ground much must 
be done to keep them efficient in the air and the efficiency of the man 
may often be of even greater importance than that of the machine. 
