STRUCTURE OF THE AIR-CELLS. 



221 



on the other, with those in the walls of the air-cells, lie outside the muscular 

 fibres in the bronchioles and infundibula. In the respiratory bronchioles, the 

 ciliated epithelium is reduced to a single layer, and is mixed with the stratified 

 form of epithelium, while, where the alveolar passages open into the air-cells or 

 alveoli, the epithelium is non-ciliated, low, and polyhedral.] 



Alveoli Or Air-Cells. The form of the air-cells, which are 250/i ( r ^ inch) 

 in diameter, may be more or less spherical, polygonal, or cup-shaped. They are 

 disposed around and in communication with the alveolar passages. Their form 

 is determined by the existence of a nearly structureless membrane, composed of 

 slightly fibrillated connective-tissue containing a few corpuscles. This is sur- 

 rounded by numerous fine elastic fibres which give to the pulmonary parenchyma 

 its well-marked elastic characters (Fig. 99, e, e). These fibres often bifurcate, 

 and are arranged with reference to the alveolar wall. They are very resist- 

 ant, and in some cases of lung- disease may be recognised in the sputum. 

 A few non-striped mus- 

 cular fibres exist in the 

 delicate connective - tissue 

 between adjoining air- ves- 

 icles (Moleschott). These 

 muscular fibres sometimes 

 become greatly developed in 

 certain diseases (W. Stir- 

 ling). The air-cells are lined 

 by two kinds of cells (1) 

 large, transparent, clear poly - 

 gonal (nucleated?) squames 

 or placoids (22-45/x) lying 

 over and between the capil- 

 laries in the alveolar wall 

 (Fig. 98, a); (2) small irre- 

 gular "granular" nucleated 

 cells (7-15/u) arranged singly 

 or in groups (two or three) 

 in the interstices between 

 the capillaries. They are 

 well seen in a cat's lung 

 (Fig. 98, d). [When acted 

 on with nitrate of silver the 

 cement-substance bounding 

 the clear cells is stained, 

 but the small cells become of 

 a uniform brown granular 

 appearance, so that they are 

 readily recognised. Small 

 holes or "pseudo-stomata" 

 seem to exist in the cement-substance, and are most obvious in distended alveoli 

 (Klein). They open into the lymph-canalicular system of the alveolar wall (Klein), 

 and through them the lymph-corpuscles, which are always to be found on the 

 surface of the air- vesicles, migrate, and carry with them into the lymphatics par- 

 ticles of carbon derived from the air. ] In the alveolar walls is a very dense plexus of 

 fine capillaries (Fig. 99, c), which lie more towards the cavity of the air-vesicle 

 (Rainey), being covered only by the epithelial lining of the air-cells. Between two 

 adjacent alveoli there is only a single layer of capillaries (man), and on the 

 boundary line between two air-cells the course of the capillaries is twisted, thus 

 projecting sometimes into the one alveolus, sometimes into the other. 



Fig. 98. 



Air- vesicles from a kitten whose lungs were injected 

 with silver nitrate ( x 450) a, outlines of fully- 

 developed squamous epithelium; &, alveolar wall; 

 e, young epithelial cell losing its granular appear- 

 ance; d, aggregation of young epithelial cells 

 germinating. 



