THE LUNG 



307 



solitary nodules which, it is important to note, are always found in 

 the fibrous connective tissue which forms the outer portion of the 

 bronchial wall. The bronchial lymph nodes and nodules are deeply 

 pigmented, the volume of the pigment being dependent upon the age 

 and occupation of the individual. It is apparently derived by absorption 

 from the surface of the bronchi and is therefore absent in infancy, defi- 

 cient in youth, abundant in adult life, and especially abundant in those 

 individuals whose occupations have necessitated the inhalation of a dusty 

 atmosphere. 



THE BRONCHIOLES 



^x 



The bronchioles possess neither cartilage, mucous glands, nor 

 lymph nodules. Their epithelium, though still ciliated, is low short 

 columnar, or, in the smaller bronchioles, cuboidal. The tunica propria 

 contains many lym- 

 phocytes and the 

 elastic tissue forms 

 an almost complete 

 layer of longitudi- 

 nal elastic fibers. 



The muscularis 

 mucosae is relatively 

 more highly devel- 

 oped than in the 

 larger bronchi ; it 

 completely encircles 

 the wall and is in- 

 vested with an ad- 

 ventitious layer of 

 fibrous tissue which 

 contains the small 

 arteries, nerves, 

 lymphatics a capil- FIG. 290. DIAGRAM OF PRIMARY LOBULE OF LUNG 



lary plexus with (LUNG UNIT). 



elongated meshes ^.r., respiratory (terminal) bronchiole; d. al., alveolar 



, duct; a, atria; s. al., alveolar or air sac; a.p., pulmonary 



alveoli; art., pulmonary artery; v., pulmonary vein. (After 

 ules. Miller.) 



The fibrous coat 



of the bronchiole here and there blends with the fibrous bands of inter- 

 lobular tissue and is in contact with the adjacent pulmonary alveoli. 



