THE LUNG 



309 



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

 n-sls upon a thin fibromuscular 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, 



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.; <l, terminal bronchioles in transect ion, they have 

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

 /, a bronchial artery; g, a bronchial vein; h, inter lobular 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 bnmrhiules, pass over 

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



The respiratory bronchioles are short branching tubules leading to 



