350 RESPIRATION [CH. XXVI. 



the thorax. They are of a spongy elastic texture, and are composed 

 of numerous minute air-sacs, and on section every here and there the 

 air-tubes may be seen cut across. Any fragment of lung (unless 

 from a child that has never breathed, or in cases of disease in which 

 the lung is consolidated by inflammation or pneumonia) floats in 

 water ; no other tissue (except fat) does this. 



Each lung is enveloped by a serous membrane the pleura, one 

 layer of which adheres closely to its surface, and provides it with its 

 smooth and slippery covering, while the other adheres to the inner 

 surface of the chest-wall. The continuity of the two layers, which 

 form a closed sac, as in the case of other serous membranes, will be 

 best understood by reference to fig. 286. The appearance of a space, 

 however, between the pleura which covers the lung (visceral layer) 



FIG. 286. Transverse section of the chest. 



and that which lines the inner surface of the chest (parietal layer) 

 is inserted in the drawing only for the sake of distinctness. It does 

 not really exist. The layers are, in health, everywhere in contact 

 one with the other; and between them is only just so much fluid as 

 will ensure the lungs gliding easily, in their expansion and contrac- 

 tion, on the inner surface of the parietal layer, which lines the chest- 

 wall. 



If, however, an opening is made so as to permit air or fluid to 

 enter the pleural sac, the lung, in virtue of its elasticity, recoils, and 

 a considerable space is left between it and the chest-wall. In other 

 words, the natural elasticity of the lungs would cause them at all 

 times to contract away from the ribs were it not that the contraction 

 is resisted by atmospheric pressure which bears only on the inner 

 surface of the air-tubes and air-sacs. On the admission of air into 

 the pleural sac, atmospheric pressure bears alike on the inner and 



