196 APPLIED PHYSIOLOGY 



it. In normal circumstances this vacuum is filled partly 

 by blood which enters the large venous trunks and right 

 heart, but mainly by air which rushes into and inflates 

 the lungs. Should there be any obstacle to the entrance 

 of air, the vacuum in the alveoli tends to be filled by the 

 transudation of fluid. It is in this way that oedema of 

 the lungs is produced in laryngeal or tracheal obstruc- 

 tion. If the chest wall is very soft, it may be unable to 



FIG. 11. CROSS SECTION OF THE EIGHT LUNG, SHOWING 

 DIRECTIONS OF EXPANSION. (KEITH.) 



withstand the atmospheric pressure on its outer surface 

 when the internal pressure is lessened by inspiration, 

 and it will then sink in at its softest parts. It is in this 

 manner that the chest deformity of rickets is produced. 

 As the greatest increase in the capacity of the thorax 

 during inspiration occurs in its lower part, it will readily 

 be understood that the expansion of the lungs is freest 

 in their lower and lateral portions ; indeed, they cannot 



