PULMONARY CAPACITY 147 



"pull" of the elastic lungs upon the chest walls the intrathoracic 

 pressure remains negative even at the end of expiration. But it 

 can be made to become positive under forced action of the 

 expiratory muscles, as in coughing, blowing, etc. The constantly 

 increasing negative condition of intrathoracic pressure is evi- 

 denced by a drawing in of the intercostal tissues during inspira- 

 tion; when the pressure assumes a positive character, as in the 

 expiratory acts of the pulmonary emphysema, these tissues 

 bulge outward. 



Pulmonary Capacity. It is evident that the most forcible 

 expiration cannot completely empty the lungs of air. The air 

 remaining after such an effort is the residual air. It amounts 

 to about 100 cubic inches. But in ordinary respiration at the 

 end .of the expiratory act there is more than 100 cubic inches 

 of air in the lungs, because in such cases all the air possible is 

 not forced out. In fact about 200 cubic inches usually remain; 

 this consists of the residual plus another 100 cubic inches, which 

 is called the reserve or supplemental air. It can be forced out, 

 but is not in tranquil respiration. The amount of air which is 

 taken into the lungs by an ordinary respiratory act amounts to 

 about 20 cubic inches, and is termed tidal air. It is the only 

 volume used in quiet breathing. At the end of the inspiratory 

 act in tranquil respiration it is obvious that the expansion may 

 continue still farther, and a certain amount of air, over and above 

 the tidal air, be taken into the lungs. The maximum amount 

 which can be so inspired (beyond the tidal) is about no cubic 

 inches, and is the complemental air. 



It is seen, then, that the entire lung capacity is equal to about 

 330 cubic inches. But the residual air cannot under any cir- 

 cumstances be called into use, and consequently the vital capac- 

 ity is equal to the total capacity minus the residual air (100 cubic 

 inches), or 230 cubic inches. It is the volume which can be ex- 

 pelled by the most forcible expiration after the most forcible 

 inspiration. 



