Pulmonary tuberculosis, which is the most frequent of all the tuberculoses, is one of the most common and likewise the most fatal of the diseases of the lungs.

For general information on tubercle and tuberculosis we must refer to what has been stated in the first volume; we shall here endeavor to apply those general principles to a special case, and at the same time shall attempt to elucidate certain points, which, from presenting some peculiarity, require a fuller notice.

Tubercle does not primarily occur in the lungs in the numerous forms which have been described, but only in two forms, which are most essentially connected, both in relation to its mode of formation and its seat. To these two forms we apply the respective terms of interstitial tuberculous granulation and infiltrated tubercle or tuberculous infiltration.

A. Interstitial Tuberculous Granulations

Interstitial Tuberculous Granulations occur in the pulmonary texture in the form of roundish, originally gray, semi-transparent bodies, varying from the size of a hemp or millet-seed to that of a barleycorn; these minute bodies either occur singly and in an isolated state, or several are collected into a group, or finally they may coalesce and form a large continuous mass. They are seated, as is shown both by special anatomical investigations and by numerous analogies, in the interstitial tissue between the smallest lobules and the air-cells, and on the walls of the cells themselves; that is to say, they are altogether external to the cell-cavities; but by pressure on the cell-wall they sometimes induce a corresponding internal prominence, or, if they be of larger size, they exert such pressure on the walls, that in every group or confluent mass of tubercles we find a number of cells more or less completely obliterated. It is the result of a chronic or acute tuberculous process, which is accompanied by local congestion or hyperemia. We have now sufficiently indicated this form of tubercle, but we shall hereafter return more fully to it. For the sake of brevity we shall always name it tubercle, or tuberculous granulation, and it must be carefully distinguished from the second form, which we shall invariably term tuberculous infiltration, or infiltrated tubercle.

B. Infiltrated Tubercle

Infiltrated Tubercle, unlike interstitial tubercle, is actually deposited in the cavities of the air-cells. It arises from a more or less extensive croupous pneumonia whose products, under the influence of a tuberculous infiltration, become variously discolored, and converted into yellow tubercle, instead of being absorbed or dissolving into pus. Hence tuberculous infiltration presents the form of hepatization, or more strictly speaking is hepatization, induced by a tuberculous product. The pneumonic product, which was at first red and granular, gradually becomes of a paler and grayish-red color with a tinge of yellow, and is dry and fragile; it finally becomes yellow, moist, of a soft, fatty, cheesy character, and sooner or later becomes disintegrated into tuberculous pus. The granular texture, in the mean time, gradually disappears, whilst the tissue forming the air-cells becomes tuberculous, and the diseased portion of lung appears to be actually changed into a connected fatty-cheesy tuberculous mass, - a condition which Lobstein doubtless observed, and mistook for fatty metamorphosis of the lung-substance.

This form of tuberculosis may attack a whole lobe uniformly, or even a whole lung, according to the extent of the local pneumonic process; it is, however, much more frequently confined to one or several larger or smaller separate portions of lung, and very often occurs as a lobular tuberculous infiltration, and in both these cases it is generally sharply defined; finally, it may occur as vesicular tuberculous infiltration, in which case it is the same thing as Bayle's pulmonary granulations, regarding which there has been much discussion.

It very often attacks the superficial parts of the lungs, as lobar and lobular infiltration, and may then be at once recognized by its external characters, by the pneumonic tendency, and the peculiar color of the diseased portion.

It is always the result of a high degree of tuberculous dyscrasia, and hence it only rarely occurs as primary tuberculosis, but is as a general rule, associated with advanced stages of interstitial tubercle. It gives rise to a form of phthisis which is tumultuous and acute, is accompanied with repeated attacks of pneumonia, and is attended with much pain and distress.

It is especially frequent in young persons and children, and presents an analogy with bronchial tuberculosis, with one of the forms of tuberculosis of the intestinal mucous membrane, with the tuberculous metamorphosis of exudations on serous membranes, etc. It is always combined with a high degree of tuberculosis of the bronchial glands, and very often with tuberculosis of the intestinal mucous membrane.

These are the two principal forms of pulmonary tuberculosis, and all other varieties of tubercle, such, for instance, as depend on physical peculiarities, however important they may individually be, are unimportant in reference to the local process, depending either on different modifications of the general disease or on mere changes in the tuberculous matter.

There are no organs excepting the spleen and serous membranes in which tubercles occur in such great numbers as in the lungs. They appear either as separate granulations or several of them are accumulated into one group. In the first case each granulation is isolated from the others by an extent of lung-tissue proportional to the number of the tubercles. This takes place either in a comparatively uniform or in an irregular manner; the latter occurring when in one part of the lung we find a large number of tubercles with little intervening parenchyma, and in another a few tubercles interspersed among much healthy tissue. When the tubercles are present in large numbers they become pressed upon one another, and finally coalesce in the form of irregular masses, as may be especially observed in the apices of the lungs, where the disease is usually the most developed. In many of the more common cases we find an uniform increase in the number of tubercles, and a corresponding approximation of them to one another as we advance from the lower portions of the lungs towards their apices.

This accumulation of tubercles into irregular masses, such as occur in the apices of the lungs, which are the usual starting points of pulmonary tuberculosis, and occasionally at other spots, must be carefully distinguished from the primary development of tubercle in tolerably regular groups. Under certain local and general conditions which are not yet altogether understood, tubercles are originally deposited at different spots in groups of a roundish form, and of the size of a pea, a bean, or a hazel-nut, or even larger, while around them there are usually other isolated tubercles in greater or less number. In extreme cases of this kind the tubercles are deposited around a central nucleus of pulmonary tissue, from which processes run into the tuberculous groups, dividing them into several compartments.