RESPIRATION 209 



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 the inhabitants of 

 lowland plains. 



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

 breathing 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 diffusion, 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). 

 The immense extent of the pulmonary surface, and the 

 extreme thinness of the layer of blood in the capillaries of 

 the lungs, facilitate the interchange between the gases of 

 the blood and the gases of the alveoli. 



The Amount and Variations of the Intra-thoracic Pressure. In 

 the deepest expiration the lungs are never completely 

 collapsed ; their elastic fibres are still stretched ; and the 

 tension of these acts in the opposite direction to the external 

 atmospheric pressure, and diminishes by its amount the 

 pressure inside the thoracic cavity. In the dead body 

 Bonders measured the value of this tension, and therefore 

 of the negative pressure of the thorax, by tying a mano- 

 meter into the trachea, and then causing the lungs to 

 collapse by opening the chest. It varied from 7-5 mm. of 



14 -I 



