VENTILATION OF AIR-PASSAGES. 337 



THE VOLUME OF AIR. 



During ordinary respiration the volume of the inspiratory and 

 expiratory stream of air is surprisingly small when compared with 

 the volume of air sojourning in the lungs. 



After an ordinary expiratory act we can force out a great 

 quantity of air by a voluntary effort; but even after this is got 

 rid of the lungs are still well filled. Some of this residual air, 

 which never leaves the chest during the life of the animal, is 

 pressed out by the elasticity of the lungs when the pleura is 

 opened. But a certain amount of air cannot be removed in any 

 way from the alveoli. Even when the lung is cut out of the chest 

 and divided into pieces, enough air is retained in the air cells to 

 render it buoyant. This fact is relied on by medical jurists as an 

 evidence that an infant has breathed after birth and distended 

 the lungs with air, for, except breathing has been well estab- 

 lished, the tolerably fresh lung of an infant will sink in water. 



In order to have a clear idea of the volumes of air at rest and 

 in motion during pulmonary ventilation, it is convenient to follow 

 the classification from which the nomenclature in common use 

 has been borrowed. 



Tidal air is the current of air which passes into and out of the 

 chest in quiet natural breathing. It amounts to about 500 cc. 

 (30 cubic inches). 



Reserve air is that volume which can be voluntarily emitted 

 after the end of a normal tidal expiration, and which, therefore, 

 during ordinary respiration remains in the lungs; it is estimated 

 at about 1500 cc. (or about 100 cubic inches). 



Complemental air is that which can be voluntarily taken in 

 after an ordinary inspiration by a forced inspiration ; it also 

 amounts to about 1500 cc., but is not used during ordinary 

 breathing. 



Residual air is the air volume which remains in the lungs after 

 a forced expiration, that is to say, which no voluntary effort can 

 remove from the lungs ; it includes the air which leaves the lungs 

 when the pleura is opened after death and the air which persist- 



