THE LUNG 



309 



plates. It has a diameter of 0.5 millimeter or less. The epithelium 

 rests upon a thin nbroinuscular coat, the continuation of the mucous 

 membrane of the bronchioles. The muscle still forms an almost com- 

 plete though very thin investment of circular fibers; the muscle fibers, 



a 





FIG. 292. FROM A SECTION OF A CHILD'S LUNG. 



a, a small tertiary bronchus; b, bronchioles; c, bronchioles ending in terminal 

 bronchioles, alveolar ducts, etc.; d, terminal bronchioles in transection, they have 

 a more regular contour and thicker wall than the alveoli; e, pulmonary arteries; 

 /, a bronchial artery; g, a bronchial vein; h, interlobular fibrous septum. Hematein 

 and eosin. Photo. X 62. 



however, are not continued into the wall of the pulmonary air sacs. The 

 elastic fibers, derived from the elastic layer of the bronchioles, pass over 

 to the alveolar walls in which they form a delicate network. 



The respiratory bronchioles are short branching tubules leading to 



