CHANGES ON THE AIR IN THE LUNGS. 99 



experimental results, as the lungs are not completely emptied 

 at each expiration, and the cells always retain a quantity of 

 air, and this quantity is greater in proportion as the respiration 

 is calm and shallow. 



Changes on the air in the lungs. It is clear from the fore- 

 going, that the air which is expired has neither the same 

 volume nor the same proportion of constituent elements as 

 the air which is inspired. In fact, an adult man absorbs by 

 respiration from 450 to 550 grains of oxygen in an hour. 

 He exhales in the same time 632 grains of carbonic acid; a 

 less quantity of nitrogen, amounting to about a hundredth of 

 the oxygen absorbed; and lastly, about 9720 grains of water 

 in the form of vapour. This exhalation of water by the lungs 

 constitutes the pulmonary perspiration, a function analogous 

 to the perspiration of the skin. The expired air, as already 

 stated, is deprived of a portion of its oxygen, and is charged 

 with carbonic acid gas. The proportion of this gas is about 

 4 parts in 100. From 350 to 400 cubic feet of air are taken 

 into the lungs in 24 hours, and rapidly changed, and the 

 gravest consequences result from placing a man under condi- 

 tions in which the air cannot be renewed. During the Eng- 

 lish war in India in the last century, one hundred and forty- 

 six prisoners were shut up in a room scarcely large enough to 

 hold them, into which the air could only enter by two narrow 

 windows; and at the end of eight hours only twenty-three 

 remained alive, and these were in a most deplorable condi- 

 tion. Percy relates that after the battle of Austerlitz, three 

 hundred Russian prisoners were confined in a cavern, and 

 two hundred and sixty of these unfortunates perished in a 

 few hours from asphyxia. 



Influence of the pressure of the atmosphere on respiration. 

 Mountain-sickness. It is well known that the density of the 

 air diminishes with the atmospheric pressure, that is, in the 

 lower regions of the air, on the sea-coast for example, the 

 air is denser than in elevated regions. Thus in order to 

 absorb the quantity of oxygen necessary for sanguification, it 

 is necessary to respire oftener upon high mountains than 

 when on plains, but this acceleration of respiration is 

 perceptible only when the height is considerable and the 



