578 



THE TISSUES. 

 Fig. 404. 



Fig. 405. 



Thin slice from the pleural surface of a cat's lung, considerably magnified. At the thin edge (b, c, 

 d), cells (alveoli) are seen. In the centre (at a), where the slice is thicker, cells are seen on the walls 

 of infundibula, and opening into the latter. (From Rossignol.) 



to the infundibula, in sections of the latter. (Fig. 404.) There are 

 about 18,000 air-cells in communication with each terminal air- 

 tube, and the number of cells in both lungs is estimated at 600 

 millions. (Rochoux.) The size of the cells varies considerably; 

 they being after death, and when not distended with air, from ^ig 



to ^3 of an inch in diameter. 

 They may, however, be dis- 

 tended to two or three times 

 this diameter without rup- 

 turing ; and are, probably, at 

 least one-third larger during 

 life than after death. In a 

 collapsed lung, they are usu- 

 ally of a rounded oval form; 

 when inflated, they are round- 

 ed angular ; and those on the 

 surface of the lung are inva- 

 riably polygonal, and their 

 external sides almost always 

 plane. 



In structure, they present 

 merely a wall (fibrous mem- 

 brane) and an epithelium. 



Walls of the air-cell., a. Epithelium, fc Elastic The foi>mer haS beGn S &td - 

 trabeculae. c. Membranous walls with fine elastic fibres. ed as the attenuated HIUCOUS 



