THE RESPIRATORY ORGANS. 



253 



FIG. 289. 



alveoli are placed closely side by side, and by mutual pressure be- 

 come polyhedral. Around the opening or base of the alveolus, 

 where it communicates with the air-sac wall, the elastic tissue of 

 the latter is arranged as a ring, from which elastic fibres pass in all 

 directions over the air-sac to form its framework. 



The wall of the alveoli comprises the epithelium, the con- 

 nective-tissue framework, and the capillary net-work. 



The epithelial lining is represented chiefly by a single layer 

 of large plates, closely resembling endothelium in silvered prepara- 

 tions, among which 

 small polyhedral 

 cells lie scattered as 

 isolated elements or 

 in groups of two or 

 three. Originally, 

 in the embryonal 

 condition of the tis- 

 sue, only the smaller 

 polyhedral cells are 

 present in the air- 

 sacs and the closed 

 alveoli, the large 

 plate-like elements 

 first appearing after 

 the tissue has been 

 expanded following 

 inflation of the 

 organ. The small 



cells, therefore, are to be regarded as genetically identical with 

 the larger, the only difference being that the smaller have never 

 undergone the expansion to which their neighbors have been sub- 

 jected ; during forced expiration the larger cells become diminished 

 in size. Between the cells, frequently at the juncture of the angles 

 of several, minute pseudo-stomata may exist ; they usually con- 

 nect with microscopic passages leading into the lymphatic channels. 

 By means of these channels particles of inhaled foreign matters, 

 often deeply pigmented, are carried from the air-sacs into the lym- 

 phatics, and become lodged within the interlobular connective tis- 

 sue. Additional particles are carried into the tissues by means of 

 the wandering lymphoid cells which occur within the epithelium 

 of the air-sacs and air-passages. 



The framework of the alveoli is composed almost entirely of the 

 elastic fibres springing from the annular bundle surrounding the 

 mouth of the sac. These fibres unite to form a net-work which 



Section of silvered lung of kitten, including portions of two alveoli 

 and atrium : a, small polyhedral epithelial cells covering distal 

 part of atrium ; b, nbro-elastic framework ; c, large flattened epi- 

 thelial plates lining alveoli, among which lie small groups of the 

 small cells rf. 



