Tubercle affects chiefly the larger divisions of the serous system; the peritoneum, pleura, and pericardium. It is ordinarily the product of a general constitutional disease, which has been already localized in some parenchymatous organ, and in this sense the tuberculosis of serous membranes usually has some definite starting-point, or prior cause (Ausgangsherd). It sometimes, however, occurs independently of any such previous and causal deposition, and is the primary and only local affection in which the general tubercular diathesis expresses itself. It is, with very few exceptions, the result of a high degree of the general disease, and hence is associated with tuberculosis occurring simultaneously with, or soon after, it in organs which stand in immediate connection with the membrane. The starting-point for tuberculosis of serous membranes is, in general, a previous affection of the absorbent glands, or of the lungs; that for peritoneal tubercle is tuberculosis in the abdominal lymphatic system, in the internal sexual organs of the female, or in the intestines; the cause of tubercle in the pleura and pericardium is found in the bronchial glands and lungs; tuberculosis of the tunica vaginalis testis has the starting-point in the lymphatics of the genital organs, and in the testicle itself; and so on. Peritoneal tubercle is, almost as a rule, associated with the same disease in the spleen, or liver; that of the pleura with recent deposition of tubercle in the lung; and further, tuberculosis not unfrequently appears in nearly all the serous membranes at once, or almost at once, and either in one and the same form, or in the various forms to be described presently.

In some few cases the tubercle may occupy the tissue of the membrane itself, and the subserous cellular structure. Generally, however, its site is manifestly the free, smooth surface of the membrane, or it is seated quite within the surface in a false membrane of cellular or cellulo-serous structure that lines the serous membrane. In the former case it may be stripped or broken off from the serous membrane, and leaves behind it a spot of corresponding size, dull, lustreless, often distinctly opaque, and deprived of its epithelium. If it have been of large size, its pressure may have formed a pit, and then it appears as if it had been seated in the tissue of the serous membrane itself.

Tubercle presents itself upon serous membranes in the following forms: a. One form is that of the gray, semi-transparent, crude, granular tubercle, the size of which is about that of coarse sand, or millet-seed. When chronic, this form of tuberculosis may originate at several parts of the membrane. Commencing at one or more of these starting-points at the same time, it gradually extends over large portions, or even over the whole of the surface: its advance, however, is not uniform, and hence the original depositions may still be recognized by the close grouping, and by the appearance of the granulations. In the acute form, the tubercles are usually abundant, and are sown evenly and close together over the whole expanse of the membrane, or at least over a very considerable part of it. They consist of granular tubercle of the size of millet-seed, or, as is often the case, of transparent, crystalline granulations, resembling vesicles, and so fine as to be perceptible only when the light falls favorably. Acute tuberculosis ordinarily arises out of a more or less lingering (chronic) tuberculosis of the membrane: and in that case, as well as in the rarer instances in which it commences on perfectly healthy membranes, it is usually but a partial manifestation of a general tubercular diathesis, which is exhibiting itself in several structures, either together or consecutively. This fact is one of great importance, from the absolutely unfavorable prognosis which it establishes. Dropsy of the serous cavity co-exists with the tubercle, and is directly proportioned in amount to the extent of the deposit over the membrane: general cachexia and dropsy of other cavities and organs follow in the same proportion. The oedema of the serous and adjoining cellular tissues, the infiltration of the parenchyma with serum of the blood, and the loss of its color, as well as the thin fluidity and defibrination of the blood generally, are all proportioned to the acuteness of the disease.

This kind of tubercle undergoes scarcely any metamorphosis, for the local disease which gives rise to it, and still more the general, and already far-advanced, constitutional affection, prove too speedily fatal: sometimes, however, when the course of the disease is chronic, the tubercle is found here and there obliterated (obsolete).

B. An inflammatory product, deposited upon a serous membrane under the influence of a constitutional affection, - which affection is usually already localized, and very often is even manifested in established phthisis, - may undergo the metamorphosis into tubercle. The change is induced by some inherent anomaly in the quality of the product, and is effected in various ways. Sometimes the exudation in its whole thickness degenerates into a uniform cheesy, or caseo-purulent, fissured layer, which agglutinates and connects the organs contained in the serous sac to one another, and to the parietal layer of the membrane: sometimes it is partially organized and gradually converted into a cellular or cellulo-serous tissue, while a more or less considerable portion of it becomes tubercle. The layer of exudation is then found in different stages of organization, and interwoven with isolated or confluent, grayish, fawn-colored, or dirty yellow tubercles, of the size of sand, millet-seed, or hemp-seed, and often with still larger shapeless masses of tubercle. Two species of this form of tuberculosis are in several respects remarkable:

(1.) An exudation in the form of a rugged layer, for the most part of considerable thickness, and of fibro-cartilaginous firmness, which consists of a quantity of confluent granular tubercles, and of a grayish-red, moderately vascular, lardaceo-gelatinous, or grayish, pale, slightly vascular, and lardaceo-callous, substance, in which those rugged masses of tubercle are imbedded. A comparative analysis shows that the status of this tubercular layer, as a vascular structure, is secondary, and that it corresponds to the lardaceous infiltration and callous condensation of the tissue of mucous membranes and parenchymatous organs around tubercle, and tubercular ulcers.

(2.) In cellular and cellulo-serous tissue recently formed on serous membranes, especially on the peritoneum, there occur yellow, cheesy or fatty, brittle masses, of round or subovate form, and of the size of peas or beans: sometimes they are shapeless, and are as large as doves' or hens' eggs.

This form of tubercle also rarely undergoes any metamorphosis; as, indeed, might be expected from the high degree which the constitutional disease (the dyscrasia), the preponderating, internal cause of the exudative process, attains; but sometimes the species just noticed - (2) - is seen, on the one hand, softening and leading to suppuration (tubercular phthisis) of the serous membrane, or on the other hand, becoming chalky.

y. Lastly, an exudation upon serous membranes, originally free from tubercle, may, at any stage of its organization, become the nidus of that growth, - a form which, when it is possible, is to be distinguished from that developed in the way described in section B. That such a form exists is probable from two observations, and is not opposed by any positive facts.

(1.) In chronic inflammations of serous membrane, which recur in the exudations, one of the secondary inflammations sometimes furnishes a product upon the free surface of the older exudation; and that product becomes tuberculous in the manner described in section B; that is to say, a serous membrane is sometimes found lined with an exudation, the outer and older layer of which is free from tubercle, whilst the inner - the product of a secondary inflammation of the older layer - is tuberculous.

(2.) In the cellular false membrane lining a serous cavity, especially the peritoneum, we sometimes see tubercle, usually of considerable size as large as hemp-seed or peas - from the highest and central point of which loose-walled bloodvessels project, and passing to the outskirts of the tubercle, sink deeply, and so are lost, or else are seen to anastomose with other vessels of the false membrane. Indeed, in a few apt cases, the tubercle is found upon close examination to be excavated by a canal or cavity, which forms the centre of this small vascular apparatus. But, in most instances, the canal is already obliterated, the circulation is obstructed, and the vascular apparatus is beginning to waste. When the atrophy is accomplished, the tubercle is found imbedded in cellular tissue, which is streaked with blackish-blue lines. Such an appearance may give rise to the assumption that tubercle is supplied with bloodvessels, especially as I have recommended serous membranes as the structure best adapted for the study of tubercle, because in that system it may best be followed in all directions. The appearance, however, may be safely explained in the following manner: The tubercle is thrown out under the influence of a tubercular diathesis by the vascular centres which are forming in the false membrane, and arranges itself around them: the more abundant - the larger - it is, so much the more prominent does it render the vascular apparatus that radiates from its centre.

The tubercle formed upon serous membranes is frequently a hemorrhagic product; especially when it is a result of the processes just described under sections (1) and (2); indeed this is sometimes the case when it is thrown out by a primary exudative process.

The congestion that attends its production not unfrequently degenerates into inflammation, and that, for the most part, furnishes a hemorrhagic exudation, in the same manner, but not to the same extent, as the inflammation of a false membrane in which tubercle is forming. As has before been explained, the hemorrhagic nature of the exudation is owing to the fact of the blood being impoverished in fibrin by the exudation of tubercle, and also in the second case to a local circumstance, viz. the imperfect formation of the vessels in the false membrane.