THE LUNGS 169 



laginous rings and acquire additional muscle fibres. 

 The lining mucous membrane also loses its cilia and 

 consists of thin broad flat cells. The bronchial tubes 



PIG. 99. Two alveoli of lung, considerably magnified, b, b, air sacs ; c, c, ter- 

 minal branches of bronchial tube. 



themselves are very thin-walled and each leads to sev- 

 eral expanded funnel-shaped soft-walled passages, which 

 serve as entrances to numerous rounded cavities, the 

 air sacs. 



Air sacs. The thousands of air sacs with their con- 

 necting passages make up the main bulk of the expanded 

 lungs. In an adult, they contain when normally ex- 

 panded about three quarts of air, but are so elastic 

 that they can be stretched to hold one or two quarts 

 more. They have a lining membrane made up of very 

 thin broad cells, which is an extension of that in the 

 bronchial tubes. Under this lining membrane, there is 

 a close meshwork of capillaries, which is so arranged as 

 to expose the largest possible surface 1 of blood to the air 

 contained within the air sacs. 



1 About 800 sq. ft. in extent. 



