Chap.xxvur.] ORGANS OF RESPIRATION. 219 



(c) A inucosa, with the terminal networks of 

 capillary blood-vessels, and infiltrated with adenoid 

 tissue. 



(d) A layer of longitudinal elastic fibres. 



(e) A loosely-textured submucous tissue, contain- 

 ing the large vessels and nerves and small mucous 

 glands. Occasionally the gland or its duct is embedded 

 in a lymph follicle. 



287. III. The bronchi and the lungr. 

 The bronchi branch within the lung dendritically 

 into finer and finer tubes. The finest branches 

 are the terminal bronchi. In the bronchi we find, 

 instead of rings of hyaline cartilage, as in the trachea, 

 larger and smaller oblong or irregularly-shaped plates 

 of hyaline cartilage distributed more or less uniformly 

 in the circumference of the wall. Towards the small 

 microscopic bronchi, these cartilage plates gradually 

 diminish in size and number. The epithelium, the 

 basement membrane, the sub-epithelial mucosa, and 

 the layer of longitudinal elastic fibres, remain the same 

 as in the trachea. The submucous tissue contains 

 small mucous glands. 



288. Between the sub-epithelial mucosa and sub- 

 mucosa is a continuous layer of circular non-striped 

 muscular tissue. In the smaller microscopic bronchi 

 this layer is one of the most conspicuous. By the 

 contraction of the circular muscular coat the mucosa 

 is placed in longitudinal folds. 



The state of contraction and distension of the 

 small bronchi bears an important relation to the aspect 

 of the epithelium, which appears as a single layer of 

 columnar cells in the distended bronchiole, and strati- 

 fied when the bronchiole is contracted. 



The distribution of the blood-vessels is the same 

 as in the trachea. Lymph follicles are met with in 

 the submucous tissue of the bronchial wall in animals 

 and man. 



