

STRUCTURE OF THE BRONCHI AND BRONCHIOLES. l6l 



pass into the lung they divide very frequently, and the branches do not anastomose. In the 

 intra-pulmonary bronchi the subdivisions become finer and finer, the finest branches being 

 called terminal bronchi, or bronchioles, which open separately into clusters of air- vesicles.] 



[Eparterial and Hyparterial Bronchi. As the bronchi proceed, one main trunk passes into 

 the lung, running towards its base, and from it are given off branches dorsally and ventrally, 

 and these branches again subdivide. In man one main branch comes off from the right 

 bronchus and proceeds to the upper right lobe, above the place where the pulmonary artery 

 crosses the bronchus. Such branches are called eparterial, and they are more numerous in 

 birds. In man, all the branches, both on the right and left side, come off below the point 

 where the pulmonary artery crosses the bronchus, and are called hyparterial bronchi (C. Aeby).] 



[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. 130,/). 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 longi- 

 tudinally, exist under the basement membrane, d. They are continuous with those of the 

 trachea, and are prolonged onwards into the lung. The mucous membrane of the larger intra- 

 pulmonary bronchi consists of the following layers from within outwards : 



(1) Stratified columnar ciliated epithelium (fig. 130, b). 



(2) Debove's membrane (fig. 130, c). 



(3) Transparent homogeneous basement membrane (fig. 130, d). 



(4) Areolar tissue with longitudinal elastic fibres (fig. 130, e). 



(5) A continuous layer of non-striped muscular fibres disposed circularly {bronchial muscle, 



fig. 130,/). 



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

 (fig. 130, g), sometimes arranged in the form of cords, the lymph-follicular cords. 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. The branches of the pulmonary artery and of the 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 bronchi" (about 

 0*5 to 1 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 " 

 (Kolliker), 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. 133, e) which are continuous on the one hand with the sub-plural connective- 

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



[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 njot continued over and between the 

 air-cells. Elastic fibres, continuous, on the one hand, with those in the smaller bronchi, and 

 on the other with those in the walls of the air-cells, lie outside the muscular fibres in the bron- 

 chioles and infundibula. In the respiratory bronchioles, the ciliated epithelium is reduced 

 to a single layer, and is mixed with the stratified form of epithelium, while, where the alveolar 

 passages open into the air-cells or alveoli, the epithelium is non-ciliated, low, and polyhedral.] 



Alveoli or Air-Ceils. The form of the cells, which are 250 /x (^ inch) in diameter, may be 

 more or less spherical, polygonal, or cup-shaped. They are disposed around and in communi- 

 cation with the alveolar passages. Their form is determined by the existence of a nearly 

 structureless membrane, composed of slightly fibrillated connective-tissue containing a few 



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