PULMONARY CAPACITY 147 



tion and positive during expiration. Now, owing to condi- 

 tions already referred to, as the chest and lungs expand dur- 

 ing inspiration, the pressure between the adjacent walls of 

 the two (intrathoracic) becomes less and less and reaches 

 a minimum at the end of that act. Furthermore, owing to 

 the continuous "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 cough- 

 ing, blowing, etc. The constantly increasing negative con- 

 dition of intrathoracic pressure is evidenced by a drawing 

 in of the intercostal tissues during inspiration; when the 

 pressure assumes a positive character, as in the expiratory 

 acts of the pulmonary emphysema, these tissues bulge out- 

 ward. 



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 respira- 

 tion 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 ordi- 

 nary respiratory act amounts to about 20 cubic inches, and is 

 termed tidal air. It is the only volume used in quiet breath- 

 ing. At the end of the inspiratory act in tranquil respira- 

 tion it is obvious that the expansion may continue still far- 

 ther, 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 



