rv THE THORAX AXD LUNGS 137 



lung is very readily distended to its full size ; but, on 

 being left to itself, it collapses, the air being driven out 

 again with some force. The abundant elastic tissues of 

 the walls of the air-cells are, in fact, so disposed as to be 

 greatly stretched when the lungs are full ; and when the 

 cause of the distension is removed, this elasticity comes 

 into play and drives the greater part of the air out again. 



Fig. 42. — Trans\'erse Section of the Chest, wtth the Heart and 

 Lungs in pl.\ce. (A little diagrammatic.) 



D.V. dorsal vertebra, or joint of the backbone ; Ao. Ao'. aorta, the top 

 of its arch being cut away in this section ; S.C. superior vena cava ; P. A. 

 pulmonary artery, divided into a branch for each lung ; L.P. R.P. left 

 and right pulmonary veins ; Br. bronchi ; R.L. L.L. right and left lungs ; 

 CE. the gullet or cesophagus ; p. outer bag of pericardium ; pi. the two 

 layers of pleura ; v. azygos vein. 



The lungs are kept distended in the dead subject, so 

 long as the walls of the chest are entire, b}- the pressure 

 of the atmosphere acting down the trachea, bronchi and 

 bronchioles upon the inner surfaces of the walls of the 

 alveoli. For though the elastic tissue is all the while 

 pulling, as it were, at the layer of pleura which covers the 

 lung, and attempting to separate it from that which Lines 



