560 STRUCTURE OF BRONCHIA. [BOOK n. 



320. In passing from the trachea to the bronchi and larger 

 bronchia, the chief changes to be observed are that the cartilages 

 are no longer in the form of regular hoops, but are plates placed 

 irregularly, becoming smaller and more irregular in disposition the 

 smaller the tube, and that the transverse muscular fibres become 

 more and more prominent, forming a distinct circular coat of some 

 thickness. The cartilages, supported by a fibrous coat of con- 

 nective tissue, lie entirely outside the muscular coat, and the small 

 glands have their ducts lengthened so that the bodies of the glands 

 instead of lying in the submucous tissue, lie outside the muscular 

 layer which is pierced by their ducts. The tube becomes now 

 distinctly a muscular tube, though the patency of its bore and a 

 certain amount of rigidity combined with flexibility is still secured 

 by the scattered plates and flakes of cartilage. After death, owing 

 to the contraction of the circular muscular fibres, the mucous 

 membrane, like the internal coat of an artery in the same circum- 

 stances, is thrown into longitudinal folds. 



In the smaller bronchia the cartilages disappear altogether, and 

 the tube then consists of an outer coat of connective tissue with 

 abundant elastic fibres and a considerable number of circularly 

 disposed muscular fibres, and an inner coat of mucous membrane 

 with its own elastic layer; the supply of small glands still con- 

 tinues. 



As one of these bronchia plunging into a lobule divides into 

 bronchioles, the columnar cells of the mucous membrane lose their 

 cilia, become shorter so as to be cubical, and are disposed in 

 a single layer or at most in two layers only. At the same time 

 the muscular fibres become more scanty, and are disposed not as a 

 continuous coat but in scattered rings, the connective tissue coat 

 becomes thinner, and the glands disappear. 



In the bronchioles themselves as they prepare to open into 

 infundibula, the epithelium cells become flat though still retaining 

 granular cell-bodies. Among these however may now be seen 

 patches in which the cells are flat transparent plates, many of 

 which do not possess a nucleus; and towards the infundibulum 

 these patches increase in number until the epithelium assumes the 

 character which we previously described as characteristic of the 

 alveoli. The muscular fibres disappear or spread out longitudi- 

 nally, and the previously compact layer of elastic fibres now becomes 

 scattered and spread out over the alveoli of the infundibulum and 

 bronchiole. In this way the structure of the bronchiole gradually 

 merges into that of an alveolus. 



321. In an infundibulum and in each of its constituent 

 alveoli what we may consider as the original wall of a pulmonary 

 passage, namely, a mucous membrane separated by submucous 

 connective tissue from a muscular coat, is reduced to a thin sheet 

 of connective tissue in which bundles of fibrillse are scanty or even 

 absent, and which is rather to be considered as a membrane 



