EESPIKATION. 585 



will be greater than the volume of air displaced in gentle respiration ; 

 and, of course, the deeper the inspiration or the more forced the expira- 

 tion the more air will be displaced. There is a limit present in both 

 cases beyond which the quantity of inspired and expired air cannot be 

 increased. In the case of the domestic animals it has not as yet been 

 established what is the average quantity of air displaced in respiration, 

 but that it depends upon the size of the animal and the size and mode 

 of structure of the thorax is self-evident. From the few experiments 

 which have been made on this subject it may be assumed that in a quiet 

 respiration about one-sixth of the total quantity of air capable of being 

 contained within the lungs is displaced. It would follow from this that 

 every six or seven respiratory movements would serve to completely 

 renew the air within the lungs. In man, the volume of air changed in 

 different respiratory movements has been subjected to close. study. It 

 was found that in an ordinary respiratory effort about thirty cubic inches 

 of air enter the lungs. With this accepted as a fact, and placing the 

 average number of respiratory movements at twenty in a minute, the 

 whole amount of air passing through the lungs in a minute will amount 

 to six hundred cubic inches ; in an hour, to thirty-six thousand cubic 

 inches; and in a day, to eight hundred and sixty-four thousand cubic 

 inches, or five hundred cubic feet. Extending these periods, one 

 hundred and eighty-two thousand five hundred cubic feet of air may be 

 estimated to pass through the lungs of an adult man in a year, and to 

 produce this displacement nine million respiratory movements are 

 required. This quantit}' of air is employed in the aeration of about 

 three thousand five hundred tons of blood sent out by the heart to the 

 lungs in the same period. With these facts before us, we may obtain 

 valuable suggestions upon the subject of ventilation. Though about 

 five hundred cubic feet of air passes through the lungs in twenty-four 

 hours, yet this amount of atmosphere is insufficient to sustain life for 

 that period, for after the introduction of carbon dioxide into the air 

 the liberation of gases in the lungs is to a certain degree rendered more 

 difficult. There must be at least five hundred cubic feet of pure air sup- 

 plied, and it has been found that to attain this at least eight hundred 

 cubic feet of air should be provided for each individual. 



The amount of air taken into the lungs at each gentle inspiration 

 and displaced by the reverse movement constitutes the so-called pressure 

 or tidal volume, and has been placed at about thirty cubic inches in man. 

 At the end of every gentle inspiratory effort the lungs may be capable of 

 still further inflation, and the additional quantity of air so inspired 

 amounts to about one hundred and ten cubic inches (complemental 

 volume). At the end of every gentle expiratory effort the lungs may be 

 still further compressed ; the amount of air so displaced by this 



