Caseation. A striking characteristic of tubercle is the occurrence of coagulation necrosis, beginning in the centre of the specific nodule as a whitish or pale softening and degeneration of the cell elements and gradually extending toward the circumference. The cells, and even the proximate tissue elements, die and disintegrate passing into a structureless, granular or hyaline debris, which has been named from its supposed resemblance to old, ripe, soft cheese. Baumgarten has observed that the small lymphoid cells are the first to degenerate, followed by the epithelioid and giant ones.
While the formation of tubercle is at first a productive inflammation of which the cell clusters and nodules are the result, yet the tendency to necrosis, and caseation is so great that it must be looked upon as one of its most prominent features, and is rarely absent, whether the lesion exists in the connective tissue of the lungs, liver or other organ, in the lymph glands, in the respiratory or intestinal mucosa or submucosa, in the osseous cancelli, brain or skin. It is, therefore, largely pathognomonic, yet it is not peculiar to tubercle, being common in glanders, and other infective inflammations. Its presence should always lead to a search for the primary tubercle nodule, with its nonvascular cluster of lymphoid and giant cells and above all for the specific bacillus resistant to staining.
The tendency to extensive degeneration and caseation is especially marked in swine, in which the resulting debris is often so liquid that the tubercles bear a strong resemblance to abscesses.
Calcification. The deposition of lime salts (mainly phosphates) in the tubercle is a common feature of advanced cases in man and pig, but especially in cattle. It is unknown in rabbits and Guinea pigs. The tubercle assumes a hard cretaceous aspect and feeling, grates under the knife and crepitates when pressed. This is always an evidence of chronicity, but it has been observed in swine in 3½ months.
Fibroid Degeneration. Fibrosis. In this case a productive fibroid inflammation takes place in the tissue of the tubercle, and it is resolved more or less extensively into a hard, white, resistant body. This is the pearly mass which has given rise to the name of the pearl disease (perl-zucht) in Germany. It may be fibrous throughout, but usually a number of the nodules show a caseated centre while the external zone, that nearest to healthy tissue, has alone taken on the conservative fibroid development. Like cretifaction, fibrosis is an indication of chronicity in the lesion. It is often seen in man, but still more so in cattle, in which it affects particularly the abdominal cavity, but also to a lesser extent the chest and other parts. It is remarkable for the paucity of bacilli to be found in its substance, often requiring many sections and infinite patience to reveal the presence of the microbe. This comparative lack of actively multiplying bacilli is doubtless one factor, operating in the direction of conservative processes, chronicity and even partial recovery. In experimental tuberculosis the extent and rapidity of development of tubercle, as also of the degeneration are found to be closely allied to the number of bacilli introduced.
Cattle: Pulmonary Tuberculosis. One of the most frequent seats of tuberculosis, the lung, may in acute cases show a diffuse miliary tuberculosis, a considerable part of a lobe or of several lobes being congested, infiltrated, and filled with small individual tubercles, grayish or transparent in the midst of the general redness. A second form, often of old standing, is in the shape of hard masses, often circumscribed or isolated, and easily felt when the soft lung tissue is manipulated. They are respectively formed by the local aggregation, and confluence of the small miliary nodules and may vary in size from a pea to a mass of ten or twelve inches in diameter. On section the miliary tubercle may not yet show central caseation, but the larger ones do so as a rule. The caseated nucleus may be soft, cheesy and somewhat homogeneous; it may be dry, granular and yellow, resembling damp farina of maize; it may be calcified in its outer portion and invested by a firm fibrous envelope. The tubercle may be the seat of general fibroid degeneration, constituting the pearl nodules (perl-knoten) dense as cartilage and either with or without a soft caseated centre; it may show an agglomeration of nodules in all stages from the early congested nodule to advanced caseation or cretefaction, the whole embedded in a solid congested and infiltrated tissue, largely fibroid. There may be extensive abscesses, the seats of complex infections, having thick uneven walls, often showing grape like tuberculous outgrowths, and containing thick, caseopurulent, viscous, granular, yellowish or greenish and comparatively odorless contents; in other cases the abscesses have burst into the bronchia, leaving vomicæ and with the new resultant infections the contents have acquired an offensive putrid odor; finally, there may be extensive inflammatory infiltrations, affecting lobules or lobes, and interspersed with tubercles in the form of the early grayish or transparent nodule, or even more advanced caseous or cretaceous centres. These latter serve to distinguish the condition from broncho-pneumonia, the grayish centres of which might suggest miliary tubercle. The broncho-pneumonia, however, has the whitish centres confined to the bronchioles and their terminal air sacs, and represent their thickened walls and muco-fibrinous contents; they do not show the same tendency to caseation, nor necrosis of the tissue, nor to caseation of the dependent lymph glands, and, above all, they do not contain the bacillus tuberculosis.
A noticeable feature in the tuberculous lung is the great frequency of tubercles of all different ages from the initial transparent nodule to the caseated or calcified mass side by side. A tuberculous bronchitis is a familiar accompaniment, with lines of miliary tubercles, ulcers and a flocculent (sometimes gritty) muco-purulent discharge, containing elements of the necrotic tissue and bacilli. Emphysema and interlobular œdema are also met with.
The Pleuræ often suffer by continuity of tissue from the diseased lung, but they may be affected primarily through infection of the circulating blood. The earliest pleural lesions may be congestion, exudation and the formation of false membranes in fringes, but soon these become the seats of minute nodules or forming tubercles which steadily encrease to form pale red cauliflower-like growths—which have been spoken of as grapes, from their supposed resemblance to bunches of that fruit. Extensive areas of the mediastinal, costal or pulmonic pleuræ are often completely covered by these productions. Dense, and thick adhesions sometimes form, holding in their substance characteristic tubercles.
The bronchial and mediastinal lymph glands receiving as they do the afferent trunks from the lungs and the great lymph sac of the pleura, offer in their sluggish currents the most favorable culture vessels, and almost always become affected in consequence. Not unfrequently they are found to be tuberculous when the tissues which they drain appear to be sound, and we must therefore, conclude that the primary slight lesions in the latter have recovered, or that the bacilli have passed through the tissues and lymph channels without establishing any centre of disease. The glandular lesions are primarily congestion and redness, with more or less infiltration and swelling, followed by a nodular induration, with enlargement or exceptionally shrinking. When bisected they may show all stages of the tubercle from the miliary granule to the caseous, dry, yellow, granular or cretaceous necrosis. By aggregation these often grow to a large size, a long diameter of 6 to 12 inches being not uncommon.
Pericardial and Cardiac Lesions. The pericardium may be implicated from the pleuræ or independently, and though tuberculosis of the heart is rare, it may be the seat of primary tuberculosis or of extension from the pericardium or endocardium. In the N. Y. State Veterinary College Museum is a cow’s heart, greatly enlarged, and completely invested and invaded by tubercle several inches thick.