220 STRUCTURE OF THE BRONCHI AND BRONCHIOLES. 



[In the middle-sized intra-pulmonary bronchi, the usual characters of the 

 mucous membrane are retained, only it is thinner; the cartilages assume the form 

 of irregular plates" situated in the outer wall of the bronchus ; while the muscular 

 fibres are disposed in a complete circle constituting the bronchial muscle (Fig. 

 97, f). When this muscle is contracted, 'or when the bronchus as a whole is 

 contracted, the mucous membrane is thrown into longitudinal folds, and opposite 

 these folds the elastic fibres form large elevations. This muscle is particularly 

 well-developed in the smaller microscopic bronchi. Numerous elastic fibres, e, 

 disposed longitudinally, exist under the basement membrane, d. They are con- 

 tinuous with those of the trachea, and are continued onwards into the lung. 

 The mucous membrane of the larger intra-pulmonanj bronchi consists of the 

 following layers from within outwards : 



(1.) Stratified columnar ciliated epithelium (Fig. 97, b). 

 (2.) De"bove's membrane (Fig. 97, c). 



(3.) Transparent homogeneous basement membrane (Fig. 97, d). 

 (4.) Areolar tissue with longitudinal elastic fibres (Fig. 97, e). 

 (5.) A continuous layer of non-striped muscular fibres disposed circularly 

 (bronchial muscle Fig. 97, /). 



Outside this is the sub-mucous coat, consisting of areolar tissue mixed with much 

 adenoid tissue (Fig. 97, g), sometimes arranged in the form of cords, the lymph- 

 follicular cords of Klein. It also contains the acini of the numerous mucous glands, 

 blood-vessels, and lymphatics. The ducts of the glands perforate the muscular 

 layer, and open on the free surface of the mucous membrane. The sub-mucous 

 coat is connected by areolar tissue with the perichondrium of the cartilages. 

 Outside the cartilages are the nerves and nerve ganglia accompanying the bronchial 

 vessels. A branch of the pulmonary artery and pulmonary vein usually lie on 

 opposite sides of the bronchus, while there are several branches of the bronchial 

 arteries and veins. Fat cells also occur in the peri-bronchial tissue.] 



In the small bronchi the cartilages and glands disappear, but the circular 

 muscular fibres are well developed. They are lined by lower columnar ciliated 

 epithelium, containing goblet cells. 



Bronchioles. After repeated subdivision, the bronchi form the "smallest 

 bronc/ii" (about O'5-l mm.) or lobular bronchial tubes. Each tube is lined by a 

 layer of ciliated epithelium, but the glands and cartilages have disappeared. 

 These tubes have a few lateral alveoli or air-cells communicating with them. Each 

 smallest bronchus ends in a " respiratory bronchiole" (Kblliker), which gradually 

 becomes beset with more air-cells, and in which squamous epithelium begins 

 to appear between the ciliated epithelial cells. [Each bronchiole opens into several 

 wider alveolar or lobular passages. Each passage is completely surrounded with 

 air-cells, and from it are given off several similar but wider blind branches, the 

 infundibula, which, in their turn, are beset on all sides with alveoli or air-cells. 

 Several infundibula are connected with each bronchiole, and the former are wider 

 than the latter. Each bronchiole, with its alveolar passages, infundibula, and air- 

 vesicles, is termed a lobule, whose base is directed outwards, and whose apex may 

 be regarded as a terminal bronchus. The lung is made up of an immense number 

 of these lobules, separated from each other by septa of connective-tissue, the inter- 

 lobular septa (Fig. 100, e) which are continuous on the one hand with the sub-pleural 

 connective-tissue, and on the other with the peri-bronchial connective-tissue.] 



[It is evident that there is an alteration in the structure of the bronchi, as we 

 proceed from the larger to the smaller tubes. The cartilages and glands are the 

 first structures to disappear. The circular bronchial muscle is well developed in 

 the smaller bronchi, and bronchioles, and exists as a continuous thin layer over 

 the alveolar passages, but it is not continued over and between the air-cells. 

 Elastic fibres, continuous, on the one hand, with those iu the smaller bronchi, and 



