The intrapulmonary bronchi divide and subdivide throughout the entire organ, the smallest subdivisions constituting the lobular bronchioles.
The larger divisions consist of:
(1) an outer coat of fibrous tissue in which are found at intervals irregular plates of hyaline cartilage, most developed at the points of division;
(2) internal to the fibrous coat, a layer of circularly disposed smooth muscle fibers, the bronchial muscle;
and (3) most internally, the mucous membrane, lined by columnar ciliated epithelium resting on a basement membrane. The corium of the mucous membrane contains numerous elastic fibers running longitudinally, and a certain amount of lymphoid tissue; it also contains the ducts of mucous glands, the acini of which lie in the fibrous coat. The submucosa lies between the the mucosa and the muscular coat (so only in the medium sized and small bronchi).
The lobular bronchioles differ from the larger tubes in containing no cartilage and in the fact that the ciliated epithelial cells are cubical in shape. The lobular bronchioles are about 0.2 mm. in diameter.
The fibromusuculocartilaginous layer is the outer wall of the intrapulmonary bronchi and is composed by the outer coat (fibrous tissue and hyaline cartilage) and the bronchial muscles.