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HANDBOOK OF PHYSIOLOGY. 



The air-cells, or air vesicles, may be placed singly, like recesses from 

 the intercellular passage, but more often they are arranged in groups or 

 even in rows, like minute sacculated tubes ; so that a short series of ves- 

 icles, all communicating with one another, open by a common orifice 

 into the tube. The vesicles are of various forms, according to the mu- 

 tual pressure to which they are subject ; their walls are nearly in contact, 

 and they vary from ^ to -fa of an inch in diameter. Their walls are 

 formed of fine membrane, similar to that of the intercellular passages, 

 and continuous with it, which membrane is folded on itself so as to form 

 a sharp-edged border at each circular orifice of communication between 

 contiguous air-vesicles, orbetween the vesicles and the bronchial pas- 



FIG. 152. From a section of the lung of a cat, stained with silver nitrate. A. D. Alveolar duct 

 or intercellular passage. S. Alveolar septa. N. Alveoli or air-cells, lined with large flat, nucleated 

 cells, with some smaller polyhedral nucleated cells. U. Unstriped muscular fibres. Circular mus- 

 cular fibres are seen surrounding the interior of the alveolar duct, and at one part is seen a group 

 of small polyhedral cells continued from the bronchus. (Klein and Noble Smith.) 



sages. Numerous fibres of elastic tissue are spread out between contig- 

 uous air-cells, and many of these are attached to the outer surface of the 

 fine membrane of which each cell is composed, imparting to it addi- 

 tional strength, and the power of recoil after distention. The cells are 

 lined by a layer of epithelium (Fig. 152), not provided with cilia. Out- 

 side the cells, a network of pulmonary capillaries is spread out so dense- 

 ly (Fig. 153), that the interspaces or meshes are even narrower than the 

 vessels, which are, on an average, -^fan of an inch in diameter. Between 

 the atmospheric air in the cells and the blood in these vessels, nothing 

 intervenes but the thin walls of the cells and capillaries ; and the ex- 



