64 HUMAN PHYSIOLOGY. 



During inspiration two sounds are produced; the one, heard in the 

 thorax, in the trachea and larger bronchial tubes, is tubular in character; 

 the other, heard in the substance of the lungs, is vesiciilar in character. 



AMOUNT OF AIR EXCHANGED IN RESPIRATION, AND CAPACITY 

 OF LUNGS. 



The Tidal or breathing volume of air, that which passes in and out of the 

 lungs at each inspiration and expiration, is estimated at from 20 to 30 cubic 

 inches. 



The Complemental air is that amount which can be taken into the lungs 

 by a forced inspiration, in addition to the ordinary tidal volume, and 

 amounts to about no cubic inches. 



The Reserve air is that which usually remains in the chest after the ordi- 

 nary efforts of expiration, but which can be expelled by forcible expiration. 

 The volume of reserve air is about 100 cubic inches. 



The Residual air is that portion which remains in the chest and cannot 

 be expelled after the most forcible expiratory efforts, and which amounts, 

 according to Dr. Hutchinson, to about 100 cubic inches. 



The Vital Capacity of the chest indicates the amount of air that can 

 be forcibly expelled from the lungs after the deepest possible inspiration, 

 and is an index of an individual's power of breathing in disease and pro- 

 longed severe exercise. The combined amounts of the tidal, the comple- 

 mental and reserve air, 230 cubic inches, represents the vital capacity of an 

 individual 5 feet 7 inches in height. The vital capacity varies chiefly with 

 stature. It is increased 8 cubic inches for every inch in height above this 

 standard, and diminishes 8 cubic inches for each inch below it. 



The Tidal Volume of air is carried only into the trachea and larger 

 bronchial tubes by the. inspiratory movements. It reaches the deeper 

 portions of the lungs in obedience to the law of diffusion of gases, which is 

 inversely proportionate to the square root of their densities. 



The ciliary action of the columnar cells lining the bronchial tubes also 

 assists in the interchange of air and carbonic acid. 



The entire volume of air passing in and out of the thorax in 24 hours is. 

 subject to great variation, but can be readily estimated from the tidal 

 volume and the number of respirations per minute. Assuming that an 

 individual takes into the chest 20 cubic inches at each inspiration, and 

 breathes 18 times per minute, in 24 hours there would pass in and out of 

 the lungs 518,400 cubic inches, or 300 cubic feet. 



Chemistry of Respiration. As the inspired air undergoes a change 



