200 BIOLOGICAL PROBLEMS 



of diminished pressure on the human body. It was 

 shown by Paul Bert, and has been subsequently con- 

 firmed by other observers, that diminution of atmos- 

 pheric pressure has in itself practically no effect upon 

 the human body which is composed of approximately 

 70 per cent fluid any alteration in external pressure 

 being transmitted equally to all parts of the body, so 

 that no effects due to pressure arise within it. 



The diminution in pressure experienced by aviators 

 is quite insufficient to produce gaseous emboli within 

 the circulatory system, and therefore there is no 

 question of any "air disease" akin to divers' palsy. 



The only pressure effects which take place are those 

 due to alteration in the volume of gas occluded in 

 spaces which, although apparently within the body, 

 are in reality situated outside it. For example, there 

 will be a certain effect upon the gases within the ali- 

 mentary tract, but insomuch as the amount of this 

 gas is not normally large, and its expansion produces 

 increased intestinal movement, such gas is soon 

 voided from the body. 



More important, however, from the point of view 

 of the aviator, is the air normally occluded within the 

 passages leading from the back of the pharynx to the 

 tympanic membrane of the middle ear, namely, within 

 the Eustachian tubes. Since the passage leading to 

 this membrane by the external ear is wide and the 

 passage of the Eustachian tube is very narrow, and 

 normally more or less closed, and since also it is im- 

 portant that the pressure on both sides of the tympanic 

 membrane shall always be the same, it is very neces- 

 sary that the equalizing of pressure by way of the 

 Eustachian tubes shall be easily accomplished, other- 

 wise discomfort, pain, and giddiness may arise. 

 Especially is this the case when the pressure is in- 

 creased through the external ear and not equalized 



