BRONCHIOLES 239 



than one or two deep. As the bronchi diminish in size by divi- 

 sion, the cartilage plates are no longer of sufficient size to com- 

 pletely encircle the wall but leave broad intervals in which this 

 coat is only represented by fibrous tissue. In tubes of a diameter 

 of 0.85 to 1 mm., bronchioles, the cartilages entirely disappear, 

 and in these or somewhat smaller bronchioles the mucous glands 

 are, likewise, no longer found. 



The outer surface of the cartilages is invested with a clothing of 

 loose fibrous tissue of varying thickness sometimes known as the 

 outer fibrous coat in which the branches of the pulmonary artery 

 and veins and also many nerve trunks and ganglia are found. In 

 the larger bronchi the two vessels, pulmonary artery and pulmo- 

 nary vein, are found on opposite sides of the tube ; in the bron- 

 chioles only one vessel, the artery, is in relation with the tube, 

 the vein pursuing an independent course within the pulmonary 

 tissue. 



^ear the root of the lung many small lymphatic glands are 

 found in the outer fibrous coat. In the smaller bronchi these are 

 represented by single lymphatic 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 lymphatic 

 glands and nodules are deeply pigmented, the vol ume of the pig- 

 ment being dependent upon the age and occupation of the indi- 

 vidual. It is apparently derived by absorption from the surface 

 of the bronchi and is therefore absent in infancy, deficient in 

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

 individuals whose occupations have necessitated the inhalation of 

 a dusty atmosphere. 



The small BRONCHIOLES possess neither cartilage, mucous 

 glands, nor lymphatic nodules. Their epithelium, though still 

 ciliated, is low short columnar, or, in the smaller bronchioles, 

 cuboidal. The tunica propria contains many lymphatic corpuscles 

 and the elastic tissue forms an almost complete layer of longitudi- 

 nal elastic fibres. 



The muscularis mucosae is relatively more highly developed 

 than in the larger bronchi ; it completely encircles the wall and is 

 invested with an adventitious layer of fibrous tissue which contains 

 the small arteries, nerves, lymphatics, a capillary plexus with elon- 

 gated meshes, and occasional venules. 



The fibrous coat of the bronchiole here and there blends with 

 the fibrous bands of interlobular tissue and is in contact with the 



