STRUCTURE OF THE BRONCHI. 



219 



(1.) Stratified columnar ciliated epithelium. 



(2.) A layer of flattened cells (D4bove's membrane). 



(3.) A clear homogeneous basement membrane. 



(4.) A basis of areolar tissue, with adenoid tissue and blood-vessels, and out- 



side this a layer of longitudinal elastic fibres. 



Outside this, again, is the sub-mucous coat, consisting of loose areolar tissue, 

 with the larger vessels, lymphatics, nerves, and mucous glands.] 



[The Bronchi. In structure^ the extra-pulmonary bronchi resemble the 

 trachea. As they pass into the lung they divide dichotomously very frequently, and 

 the branches do not anastomose. The subdivisions become finer and finer, the finest 



Fig. 97. 



Transverse section of part of a normal human bronchus ( x 450) a, precipitated 

 mucus on the surface of the ciliated epithelium, 6; b, ciliated columnar 

 epithelium; c, deep germinal layer of cells (De*bove's membrane); d, elastic 

 basement membrane; e, elastic fibres divided transversely (inner fibrous 

 layer);/, bronchial muscle (non-striped); g, outer fibrous layer with leuco- 

 cytes and pigment granules (black) deposited in it. The lower part of the 

 figure shows a mass of a denoid tissue. 



branches being called terminal bronchi, or bronchioles, which open separately into 

 clusters of air-vesicles.] 



