In studying the changes in the lung it will be necessary to give descriptions of the various processes separately, and to a certain extent these processes are separable, but at the same time it is to be understood that many of them go on together and by their simultaneous occurrence frequently mask each other. It may be said in general that the disease, beginning in the finer bronchi, tends to spread in one of two directions, or in both of them at once, namely, along the bronchi to the lung alveoli, or else from the bronchi to the surrounding connective tissue and on into the general connective tissue of the lungs. In both cases we find tubercles developed in all stages of the process, and in both there is inflammation, but of different kinds according to the structures involved. In the one case there is chiefly catarrh of the lung alveoli, although the alveolar wall and surrounding connective tissue also show new-formation. In the other case the inflammation produces new-formation of connective tissue and consequent induration. It is 'important to observe that the tubercles which occur in both cases partake, to a considerable extent, in the peculiarities of the inflammations. In the case of extension to the connective tissue the tubercles tend to undergo fibrous transformation. In the other case they are liable rather to caseous necrosis.

(A) The Caseous Form

When the lungs are examined in an ordinary case of this kind after death they are found adherent to the chest wall, and they ere usually seen to be the seats of several cavities, chiefly in the upper lobes. The cavities are of various sizes and very irregular in outline (see Fig 363). There are very often one or more large ones divided by partial septa, evidently formed by the coalescence of several smaller ones. If the disease be in active progress, as is mostly the case at the time of death, the internal surface of the cavities is rough and irregular as if from ulceration. The cavities may contain a curdy pus, or they may be comparatively empty, but their internal surface is usually coated more or less with a yellow curdy matter. If the cavities are at all smooth on the surface, or free from the appearance of ulceration, these are signs that the disease is to some extent in a condition of retrogression. The tissue immediately around is usually condensed and pigmented.

Fig. 303. - Upper part of lung in an advanced case of caseous phthisis. There is a large cavity with many irregular septa. The surface of the cavity is rough and irregular. Compare this condition, which is that of the active disease, with Fig. 374, in which the disease has retrograded.

It is necessary to look away from the cavities in order to observe indications of the Initial lesion. One can nearly always distinguish in the midst of the crepitant and comparatively normal tissue isolated areas of condensation, such as those illustrated in Fig. 364. They can be felt as solid masses in the soft tissue, and on section they stand out above the general level of the cut surface. They are usually more or less rounded in area and present in section indications of a lobular arrangement, there being a central stem and bodies grouped round it like grapes on a bunch, or, more correctly, like the carpels of a berry. The central parts are generally whitish or yellow and opaque, and this appearance may involve the whole area, but the peripheral parts have usually a grey translucent character. The coalescence of areas having these characters gives rise to considerable condensations, in which no such lobular arrangement may be visible, although at the margins there are generally indications of it.

On microscopic examination of the more recent of the initial lesions, appearances will be found which may be illustrated by Fig. 365. The lesion begins and centres in a small bronchus. The bronchus is plugged with what is at first merely an inflammatory exudation (b) consisting of desquamated epithelium and round cells. The outline of the tube is preserved (a) but its wall is considerably infiltrated with round cells (c). The plugged bronchi form the central stems and branching twigs of the areas under consideration, but there is an extension to the lung alveoli, and here also we have the effects of inflammation. The inflammation is generally of the parenchymatous' or catarrhal character, and the alveoli are occupied by large catarrhal cells which are the derivatives of the alveolar epithelium. Sometimes the epithelium itself is seen enlarged and it may be desquamating. Blood is very often present along with the catarrhal cells, sometimes in such abundance as to fill the alveoli. In the specimen of which Fig. 365 is a drawing, for example, the alveoli contained much blood (see further on under Haemorrhage).

Fig. 364. - Lobular condensations in the caseous form of phthisis. Rounded areas are seen which, being solid, stand out from the general level. From piece of lung in the fresh state.

Fig. 365. - Caseous phthisis; recent centre, a, wall of bronchus with pigment externally; b, plug in bronchus; c, round cells infiltrating wall of bronchus; d, alveoli filled with blood and catarrhal cells, x 60.

The exudation in the alveoli is sometimes more like that of an acute inflammation, consisting to some extent of round cells, and there are eases in which even fibrine is present.

Besides these inflammatory conditions we have Tubercles present in the affected parts. The bronchial wall, as we have seen, is infiltrated with leucocytes which, as they accumulate, obscure the structure of the wall. They also extend to the connective tissue outside the bronchial wall, and to the alveolar wall. In the midst of these evidences of inflammation definite tubercles are visible, sometimes typical in every respect (see Fig. 366). But they are apt to be somewhat indefinite from the existence of the inflammatory infiltration, so that only the giant-cells may be definitely distinguilhable. The giant-cells not infrequently take into their substance the black pigment of the lung tissue, and so may be somewhat strikingly manifest.