COMPOSITION OF THE ATMOSPHERE 263 



functions, the presence of muscular fiber in the walls of a hollow viscus, such 

 as a lung, is only what might be expected from analogy with other organs. 

 Subject as the lungs are to such great variation in size, it might be antici- 

 pated that the elastic tissue, which enters so largely into their composition, 

 would be supplemented by the presence of much muscular fiber. 



RESPIRATORY CHANGES IN THE AIR BREATHED. 



Composition of the Atmosphere. The atmosphere we breathe has, 

 in every situation in which it has been examined in its natural state, a 

 nearly uniform composition. It is a mixture of oxygen, nitrogen, carbon 

 dioxide, and watery vapor, with, commonly, traces of other gases, as argon, 

 ammonia, sulphureted hydrogen, etc. Of every 100 volumes of pure at- 

 mospheric air, 79 volumes, on an average, consist of nitrogen and argon, 

 the remaining 21 of oxygen. The proportion of carbon dioxide is extremely 

 small; 10,000 volumes of atmospheric air contain only about 3 of that gas. 



The quantity of watery vapor varies greatly according to the tem- 

 perature and other circumstances, but the atmosphere is never without 

 some. In this country the average quantity of watery vapor in the atmos- 

 phere varies greatly according to the region. In some of our Western arid 

 plains in the dry season the air is almost free of moisture. 



Composition of Air which Has Been Breathed. The changes 

 effected by respiration in the atmospheric air are: i, an increase of tem- 

 perature; 2, an increase in the quantity of carbon dioxide; 3, a diminution 

 in the quantity of oxygen; 4, a diminution of volume; 5, an increase in the 

 amount of watery vapor; 6, the addition of a minute amount of organic 

 matter and of free ammonia. 



Temperature of the Expired Air. Expired air, after its contact with the 

 interior of the lungs, is hotter (at least in most climates) than the inspired air. 

 Its temperature varies between 36 and 37.5 C. (97 and 99.5 F.), the lower 

 temperature being observed when the air has remained but a short time in 

 the lungs. Whatever may be the temperature of the air when inhaled, it 

 acquires nearly that of the blood before it is expelled from the chest. 



The Carbon Dioxide of Expired Air. The percentage of carbon dioxide 

 is increased, but the quantity exhaled in a given time is subject to change 

 from various circumstances. From every volume of air inspired 4 to 5 per 

 cent of oxygen is abstracted; while a rather smaller quantity, 4.38 per cent, 

 of carbon dioxide is added in its place; the expired air will contain, there- 

 fore, 438 volumes of carbon dioxide in 10,000. The total quantity of carbon 

 dioxide exhaled into the air breathed by a healthy adult, calculating that 

 15.4 c.c.of the 35oc.c.of the average air breathed out at each expiration con- 

 sists of carbon dioxide, and that the rate of respiration is on an average 16, 

 would be about 400 liters in the twenty-four hours. From actual experiment this 



