Introduction

In order to appreciate the changes which occur in affections of the bronchi, it is necessary to refer to some points in the structure of the tubes. The mucous membrane is covered with epithelium whose superficial layer is cylindrical and ciliated. Beneath the epithelium there is a basement membrane consisting of a translucent homogeneous membrane which has few nuclei and is believed by some to be traversed by infrequent channels or canaliculi. Beneath the basement membrane there is the mucosa, consisting of a highly vascular connective tissue, with bands of elastic fibres. In the superficial parts of the mucosa are also found little masses of lymphoid tissue not having the denned form of the closed follicles of the intestine, but similar in structure though much smaller. In the deeper parts of the mucosa there is a layer of smooth muscle, the muscularis mucosae, consisting chiefly of circular fibres, but with some longitudinal ones. Outside this we have the sub-mucosa or adventitia, containing the mucous glands whose ducts penetrate the mucosa and open on the surface by trumpet-shaped orifices. The sub-mucosa is continuous with, and really forms a part of, the general interstitial connective tissue of the lung. In this loose adventitia the cartilages are embedded, and there are abundant serous and lymphatic spaces which are in communication with those throughout the lung, the perivascular, and others.

The diseases of the bronchi stand in close relation on the one hand to those of the larynx and trachea, and on the other to those of the lungs, and it is impossible to draw an absolute line of distinction on either side. The affections of the larger and middle bronchi are essentially like those of the larynx and trachea, and this is consistent with the fact that in structure these parts are similar. As we pass down the bronchial tree, however, the structure changes considerably. The cartilaginous plates become irregular and smaller, and finally disappear; the elastic tissue becomes more completely incorporated with the mucous membrane, so as to form a single layer which becomes thinner as the tube diminishes in calibre; the bronchial glands altogether disappear in the finer tubes. The bronchial tube thus hecomes more simple in structure, and approximates to that of the lung alveoli So it happens that in their diseases the finer bronchi are more allied to the lung parenchyma, and are often involved with it. We shall afterwards see that in acute catarrhal inflammation of the lungs the disease often begins in a capillary bronchitis, and is sometimes described under that name.