MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 235 



quantity the name of supplemental or reserve air has been given. 

 After the deepest expiration there always remains 1,000 to 1,200 c.c. 

 of air in the lungs (Durig), and this is called the residual air. After 

 a normal expiration following a normal inspiration the lungs still 

 contain stationary air to the amount of about 2,500 c.c. 



The term vital or respiratory capacity is applied to the quantity 

 of air which can be expelled by the deepest expiration following the* 

 deepest inspiration, and amounts in an adult of average height to 

 3,500 or 4,000 c.c. The maximum quantity of air which the lungs 

 can contain is evidently equal to vital capacity plus residual air. 

 At one time the vital capacity was thought to be capable of affording 

 valuable information in the diagnosis of chest diseases; but little 

 stress is now laid upon it, as it varies from so many causes. For 

 instance, it can be increased by practice with the spirometer. It is 

 greater in mountaineers than in +he inhabitants of lowland plains. 



It is clear from the figures we have given that in ordinary breath- 

 ing only a small proportion of the air in the lungs comes in direct at 

 each inspiration from the atmosphere, and only a small proportion 

 escapes into the atmosphere at each expiration. The greater part 

 of the air in the lungs is simply moved a little farther from the upper 

 respiratory passages, or a little nearer them; and fresh oxygen 

 reaches the alveoli, as carbon dioxide leaves them, mainly by diffu- 

 sion, aided by convection currents due to inequalities of temperature, 

 and to the churning which the alternate expansion and shrinking 

 of the lungs, and the pulsations of their arteries, must produce. 

 But that some of the tidal air strikes right down to the alveoli is 

 evident enough. For the respiratory ' dead space ' that is, the 

 capacity of the upper air-passages and the bronchial tree down to 

 the infundibula is only 140 c.c., or one-third of the amount of the 

 tidal air (Zuntz, Loewy). There is no direct way of determining 

 whether any respiratory exchange goes on through the walls of the 

 upper air-passages. But by indirect methods it has been estimated 

 that about 30 per cent, of the volume of the tidal air is pure air 

 (Haldane and Priestley). This, of course, corresponds to the ' effec- 

 tive ' dead space. Taking the average tidal air at 460 c.c. (p. 234), 

 it is clear that the effective corresponds very closely with the ana- 

 tomical dead space that is to say, the respiratory function of the 

 air-passages above the point where the infundibula are given off is 

 negligible. Although such calculations can only be approximately 

 correct, the agreement is of interest. The immense extent of the 

 pulmonary surface, and the extreme thinness of the layer of blood 

 in the capillaries of the lungs and of the alveolar walls, facilitate 

 the interchange between the gases of the blood and the gases of the 

 alveoli. 



The Amount and Variations of the Intrathoracic Pressure. In the 

 deepest expiration the lungs are never completely collapsed; their 



