THE RESPIRATORY SYSTEM. 369 
that serves to maintain the surrounding parts in a moist con- 
dition, thus lessening friction. The importance of this ar- 
Fic. 299.—Section of the parenchyma of the human lung, injected through the pulmonary 
artery (Schulze). a, a,c, c, walls of the air-cells ; b, small arterial branch. 
rangement is well seen when, in consequence of inflammation 
of this pleura, it becomes dry, giving rise during each respira- 
tory movement to a friction-sound and a painful sensation. 
It will not be forgotten that this membrane extends over the 
diaphragm, and that, in consequence of the lungs completely 
filling all the space (not occupied by other organs) during every 
position of the chest-walls, the costal and-pulmonary pleural 
surfaces are in constant contact. By far the greater part of 
the lung-substance consists of elastic tissue, thus adapting the 
principal respiratory organs to that amount of distention and 
recoil to which they are ceaselessly subjected during the en- 
tire lifetime of the animal. 
THE ENTRANCE AND EXIT oF AIR. 
Since the lungs fill up so completely the thoracic cavity, 
manifestly any change in the size of the latter must lead to 
an increase or diminution in the quantity of air they contain. 
Since the air within the respiratory organs is being constantly 
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