The bronchial mucous membrane exposed to the inspired air presents varying degrees of sensitiveness to variations in temperature and otherwise. Much that has been said under nasal catarrh is again applicable here. It is to be added, however, that in many persons there is a special proclivity to recurrent attacks of bronchitis. This may be due to an inherited weakness, but is perhaps more frequently occasioned by an acute bronchitis which has left the bronchi considerably altered in structure, has rendered them, in fact, the least resistant part of the body. In such persons bronchitis may be set up by cold, by disorder of the stomach, or by some other trivial cause. Valvular disease of the heart frequently predisposes to bronchitis by causing a permanent passive hyperemia of the mucous membrane.

Bronchitis mostly occurs as recurrent attacks of acute inflammation, whilst between the attacks the inflammation has a more chronic character. Hence in many instances the lesions of acute and chronic inflammation are more or less conjoined.

(A) Acute Bronchitis

The first phenomenon here is active congestion of the highly vascular mucosa with swelling both from dilatation of the vessels and from exudation. The epithelium undergoes desquamation, but there is at the same time proliferation in the deeper layers. There is, in addition, exudation of leucocytes from the vessels of the mucosa, and the latter is infiltrated with round cells. The mucous glands are much affected. Their interstitial tissue is infiltrated with round cells, so that the gland tissue is somewhat obscured in sections, and at the same time the glandular epithelium shows proliferation and mucus is abundantly secreted. It will be observed that the exudation, so far as the mucosa is concerned, must occur beneath the basement membrane, and no doubt this structure somewhat interferes with the passage of fluid and leucocytes to the surface. But Hamilton is probably in error in regarding it as forming an almost complete barrier. There are channels, as noted above, through the basement membrane. By these and by the mucous glands the exudation reaches the surface of the tube.

The Exudation in bronchitis is important as it forms the Sputum, and gives its characters to the latter. In the normal condition the mucous glands secrete enough mucus to keep the membrane moist, the secretion consisting of a glairy fluid containing a few leucocytes which are here called mucous corpuscles. The fluid owes its glairy or sticky character to the fact that it contains mucin, a substance allied to albumen and secreted by the mucous glands. This normal secretion catches and holds the minute particles forming the dust which we inhale with the air, and the vibratile cilia of the epithelium, acting towards the outlets, carry mucus and dust outwards, to be swallowed or expectorated. In bronchitis the secretion, variously altered, is mixed with the exudation from the vessels. At first there is slight increase of the secretion, and it is chiefly concentrated and tough. As the disease progresses it becomes more abundant but less tough and less transparent, and this is brought about by the increase of the inflammatory exudation consisting of serous fluid and cells. The degree of toughness depends on the proportion of mucin, and the degree of opacity on the quantity of cells.

The Sputum coctum, or ripe sputum, met with at the acme of the disease, is yellowish white or greenish, and opaque. The sputum, as seen in a vessel appears at first sight like pus, but it is much more tenacious. Under the microscope it also resembles pus, the field being crowded with multitudes of leucocytes. But the tenacity of the fluid is often shown by the manner in which the plastic leucocytes are altered in shape, being drawn out into oval or more elongated forms according as the tough mucus is drawn out. On adding acetic acid the usual development of nuclei occurs in the cells, and the intermediate fluid becomes markedly opaque from the precipitation of the mucin, which can be seen now in fine granules. With acetic acid the sputum assumes to the naked eye a whitish, opaque, and almost membranous character.

(B) Chronic Bronchitis

We have here, as in other chronic inflammations, chiefly a new-formation of connective tissue often accompanied by atrophy of the more active structures. In many cases the mucosa assumes more or less permanently a highly cellular character, the inflammation not subsiding sufficiently to allow of much fibrous development. The tissue is like a highly vascular granulation tissue and, like it, exudes abundant pus. In other cases an increase of fibrous tissue occurs, so that under the basement membrane, instead of a succulent vascular mucosa we have strands of fibrous tissue containing elastic fibres. This gives a greatly altered appearance to the tube as seen in microscopic sections. The mucous membrane is greatly reduced in thickness, and the surface shows great irregularity, due to the varying degrees of shrinking. There are even villus-looking projections visible in many cases. The fibrous tissue is largely in longitudinal bands, and these, being hypertrophied, are often vieible to the naked eye as elongated ridges. The mucous membrane so altered is atrophied and indurated, and the atrophy sometimes extends to other structures. The muscularis mucosae is somewhat persistent and may even undergo hypertrophy. The mucous glands and even the cartilages undergo atrophy.

In chronic bronchitis the sputum is often very abundant, and may be sero-mucous with comparatively few leucocytes, but is liable to be intermittently semi-purulent. The sputum is not so tough as in more ' acute cases, and is often very frothy. In some very chronic cases there is an abundant cellular exudation - the sputum is almost like pus and has not the toughness of the sputum coctum. Large quantities of pus may thus be expectorated. In these cases it may be supposed that leucocytes are present in the walls and outside the bronchi as well as in the expectoration, and it is in such cases that we may look especially for dilatation of the tubes.