170 NORMAL HISTOLOGY. 



The smaller bronchi differ in structure from the trachea in 

 possessing a muscularis mucosse, with its fibres disposed in a 

 circular direction, and having irregular cartilaginous plates in their 

 walls*, instead of C-shaped, imperfect rings. The four coats may 

 be enumerated as follows : 



a. Mucous membrane, covered with ciliated columnar epithelium 

 resting upon a basement-membrane, beneath which is a fibrous 

 tissue containing numerous elastic fibres lying parallel to the axis 

 of the bronchus. Under this are the circular fibres of the mus- 

 cularis mucosae. 



b. Submucous coat, similar to that of the trachea and larger 

 bronchi. 



c. Cartilaginous coat, containing the plates of cartilage that sup- 

 port the walls. 



d. Fibrous coat of areolar tissue, containing a little adipose tissue 

 and passing into the areolar tissue of neighboring structures. 



As the bronchi subdivide and become smaller the coats get 

 thinner, and first the cartilaginous and then the muscular coat dis- 

 appears. Those air-passages which are without cartilage, but have 



FIG. 146. 



Portion of a cross-section of a bronchiole from the lung of a pig. (Schultze.) a, areolar 

 external coat ; b, muscularis mucosse ; c, subepithelial areolar tissue, containing numerous 

 longitudinal elastic fibres, represented here in cross-section ; d, ciliated epithelium, form- 

 ing the most superficial layer of the mucous membrane ; /, walls of the neighboring pul- 

 monary alveoli. In these walls branching and anastomosing elastic fibres are shown ; 

 the capillary plexus has been omitted. 



a muscularis mucosse, are called "bronchioles" (Fig. 146). The 

 still smaller branches, which have lost their muscular tissue, are 

 known as the "alveolar passages." In the latter the columnar 



