136 RESPIRATION. 



less than three hundred cubic feet, an amount nearly equal ' 

 to the contents of sixty barrels. 



26. Of this large volume of air five per cent, is absorbed 

 in its transit through the lungs. The loss thus sustained is 

 almost wholly of oxygen, and amounts to fifteen cubic feet 

 daily. The quantity of carbonic acid exhaled by the lungs 

 during the day is somewhat less, being twelve cubic feet. 

 Under the influence of excitement or exertion, the breath- 

 ing becomes more frequent and more profound ; and then 

 the internal respiratory work increases proportionately, 

 and may even be double that of the above estimate. It 

 has been estimated that in drawing a full breath, a man 

 exerts a muscular force equal to raising two hundred 

 pounds placed upon the chest. 



27. Impurities of the Air. The oxygen in the at- 

 mosphere is of such prime importance, and its proportion 

 is so nicely adjusted to the wants of man, that any gas or 

 volatile substance which supplants it must be regarded as 

 a hurtful impurity. All gases, however, are not alike in- 

 jurious. Some, if inhaled, are necessarily fatal ; arsen- 

 uretted hydrogen being one of these, a single bubble of 

 which destroyed the life of its discoverer, Gehlen. Others 

 are not directly dangerous, but by taking the place of 

 oxygen, and excluding it from the lungs, they become so. 

 Into this latter class we place carbonic acid. 



28. Most of the actively poisonous gases have a pungent 

 or offensive odor; and, as may be inferred, most repugnant 

 odors indicate the presence of substances unfit for respira- 

 tion. Accordingly, as we cannot see or taste these impu- 

 rities, the sense of smell is our principal safeguard against 

 them ; and we recognize the design which has planted this 

 sense, like a sentinel at the proper entrance of the air- 



26. Air absorbed in its transit through the lungs ? The loss ? Carbonic acid 

 exhaled ? Effect of excitement or exertion ? What estimate ? 



27. Importance of the oxygen in the atmosphere? Injurious character of 



28. Pungency of gases ? The inference ? Our safeguard ? 



