34 2 RESPIRATION. [CH. xxiv. 



only by minute and scattered cartilaginous flakes. When the 

 bronchial tubes, by successive branchings, are reduced to about 

 T V of an inch (-6 mm.) in diameter they lose their cartilaginous 

 element altogether, and their walls are formed only of a 

 fibrous elastic membrane with circular muscular fibres; they are 

 still lined, however, by a thin mucous membrane with ciliated 

 epithelium, the length of the cells bearing the cilia having become 

 so far diminished that the cells are now cubical. In the smaller 

 bronchial tubes the muscular fibres are relatively more abundant 

 than in the larger ones, and form a distinct circular coat. 



The Lungs and Pleurce. The lungs occupy the greater por- 

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

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



Fig. 312. Transverse section of the chest. 



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) floats in water ; no 

 other tissue 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. 312. 

 The appearance of a space, however, between the pleura which 

 covers the lung (visceral layer) 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 



