154 



THE RESPIRATORY SYSTEM 



fibers arranged lengthwise of the tube and supports the mucosa of 

 ciliated columnar epithelium lining the lumen. 



The lung is composed of many lobules which are somewhat 

 conical or pyramidal in shape and are composed of a number of 

 terminal bronchioles and their branches. Adjacent lobules are 

 separated from each other by interlobular connective tissue. 



Each terminal bronchiole divides into two or more respiratory 

 bronchioles. (Fig. 97.) Near its origin, the mucosa of this subdivision 

 has ciliated columnar epithelium; distally the cilia are lacking and 



Terminal bronchiole 



Mm 



Connectne -— ^ - 



tissue 

 Arteri/- 



-vy 



i) / 



Aluoh 



Alveoli 



Fig. 96 Fig. 97 



Fig. 96. — Diagram of section of a terminal bronchiole, the lumen lined with 

 ciliated columnar epithelium. 



Fig. 97. — Terminal bronchiole of mammal and its branches. 



the cells are cuboidal. Outside the mucosa the wall is very thin, 

 consisting of fibrous tissue containing numerous elastic fibers and 

 smooth muscle cells. It has a few small saccular diverticula, or 

 pulmonary air sacs, whose wall apparently consists of flat, squamous 

 epithelium invested with a rich capillary network where gaseous 

 exchanges can take ])lace. 



At its distal end the respiratory bronchiole di\'i(les into two or 

 more short, thin-walled, irregidar alveolar ducts. The simple 

 squamous epithelium of the thin wall is supported by a very small 

 amount of fibrous tissue with a few scattered elastic fillers and 

 smooth uniscle cells. The wall is broken along its length b\- man^• 



