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Elementary Biology. 



buccal cavity, and immediately beneath the opening ol the 

 oesophagus, may be seen a slit-like aperture, already men- 

 tioned, known as the glottis, the walls of which are strength- 

 ened by cartilage. The glottis leads into a cylindrical 

 chamber from which the two lung-sacs open. Each lung 

 is a hollow tapering bag, externally covered by a layer of 



FIG. 155. SECTION OF INJECTED LUNG. (Quain.) 



a, a, free edges of the pulmonary sacs (alveoli); c, c, partitions between 

 neighbouring alveoli in section ; b, small artery giving off numerous 

 ca i laries which form network over the alveolar wall. 



the same membrane which has already been described as 

 covering the viscera generally, here, however, known under 

 the special name of pleuron. The inner surface of the 

 lung is thrown into folds and ridges, giving the wall the 

 appearance of being honeycombed. Manifestly the super- 

 ficies of the lung-wall is thus greatly increased. The pul- 



