BRONCHI AND LUNGS. 



209 



vestibula, which are 0.2 mm. in diameter. Three to six of 

 these arise from the end of each terminal bronchus. Smooth 

 muscle fibres do not extend beyond the vestibule, but are found 

 surrounding it like a sphincter. Several atria communicate with 

 each vestibule. These are thin- walled chambers resembling 

 the air sacs in their possession of a network of blood capillaries. 

 Opening from each atrium are two or more air-sac passages, 

 which average 0.143 mm. in diameter. The atria and air-sac 

 passages contain no muscle cells. The air sacs are irregular in 

 shape, with an average size of 0.511 mm. by 0.313 mm. The 



FIG. 1G1. 





Terminal bronchus of a mammalian lung. (Miller.) S, air sac; A, atrium; B, terminal 

 bronchus; V, vestibule; P, air-sac passage. The artery is shaded and the vein is in 

 outline. 



walls are thin, and are made up of capillaries and a little con- 

 nective tissue covered by flat epithelium. Irregular, thin- 

 walled diverticula from the air sacs are the air cells. These 

 are lined with cells of two kinds : delicate irregular cells lying 

 over the blood-vessels, and small flat polygonal cells over the 

 meshes of the capillary network. Similar air cells may arise 

 from the bronchus and from the atrium. Those on the bronchi 

 have an average diameter of 0.047 mm., and those on the 

 atrium and air sac 0.113 mrn. 



The respiratory bronchiole of some authors leads into two 



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