294 HUMAN PHYSIOLOGY 



It has lately been shown that oxygen is distinctly 

 helpful to athletes in certain trials of strength and speed. 

 This is best evidenced by middle-distance runners. 

 They do not greatly improve their time under its in- 

 fluence but they finish with a minimum of distress. How 

 can we find any condition common to the runner in the 

 height of condition and the sick man fighting for his 

 life? This is easier than might be supposed. Both are 

 tending to suffer from a falling of the blood below its 

 normal composition as regards oxygen. In the case of 

 the athlete this is due to the great demands which his 

 muscles are making upon the blood. In the case of the 

 sick man it is not huge consumption of oxygen but 

 restricted supply which is responsible for a somewhat 

 similar chemical disturbance. 



The healthy, resting man, if he breathes oxygen, adds 

 something to the amount circulating in his system. But 

 his tissues were already having offered to them so liberal 

 a supply that they returned much unused in the venous 

 blood. No important results will follow if they are 

 offered more. The tissues will not burn under " forced 

 draft." This is an important difference between the 

 relation of oxygen to living things and to fires. 



Compressed Air. In many engineering operations, 

 especially in constructing tunnels or laying foundations 

 far below water, it is necessary to fill the spaces in 

 which the men are working with highly compressed 

 air. The atmospheric pressure is about 15 pounds to 

 the square inch. At 34 feet below water twice this 

 pressure must be provided to keep the water pressure 

 balanced. At 68 feet the air pressure must be three 

 times the atmospheric, three atmospheres, as we ex- 

 press it. Such works have often been carried on at 

 greater depths than this and pressures of four or more 

 atmospheres have been employed. Divers can descend 

 for short periods to levels at which the air pumped into 

 their helmets has to be compressed to six or seven times 

 the normal density. 



