The frequency of tuberculosis in the bony system is unquestioned. The tubercle either assumes the granular form, or, as very frequently occurs, it is a product of inflammation of the bone, and presents the characters of softening tubercle. Tuberculosis affects chiefly cancellous bones, and portions of bone; the bodies of the vertebrae; the spongy articular extremities of the long bones, - especially the lower end of the femur and upper end of the tibia, and the ends of the bones which compose the elbow-joint; the carpal and tarsal, the metacarpal and metatarsal bones, and the phalanges; and the sternum: more rarely the ribs are attacked, and the cranium: while the parts least frequently affected are the shafts of the long bones.

Its seat is sometimes the outer layer of the bone and the periosteum, and sometimes the deeper bony tissue.

Moreover it very commonly occupies several adjoining bones at once, as for instance, the ends of the bones which form a joint, the whole apparatus of the carpal and tarsal joints, the vertebrae, etc. Young persons are especially subject to it in the years of childhood, and at puberty; but it is also frequent in later, and even in advanced life.

a. Tubercle in the state of gray crude separate granulations can be detected only by close examination of the spongy tissue of a bone in the vicinity of a tubercular abscess. It usually occupies the membrane which lines the Haversian canals and the cells. As the granulations accumulate, they form larger masses of tubercle, and partly compress the bony structure, and partly include necrosed fragments of it amongst them. The aggregate morbid growth is sometimes found as a yellow, lardaceous and cheesy mass; much more frequently it is softened, and consists of a cream-like grayish-yellow pulp, or a thinner, flocculent, tubercular pus. It is contained within a more or less complete lardaceo-callous cyst, which is, in fact, the tissue surrounding the softening tubercle, infiltrated with lardaceo-gelatinous material. If, as is frequently the case, the tuberculous disease should occupy the outer part of a bone and the periosteum, the latter, with the cellular and ligamentous tissues upon it, partakes in the formation of this cyst; and if the tuberculosis advance deeply into the bone, it is mostly the only rudiment of the cyst that can be clearly proved to exist. The best opportunity to observe it, is frequently afforded by the vertebrae.

The degree of congestion which gives rise to primary tubercle, may vary in bone as well as in other tissues; for the development of the disease is, in many cases, unnoticed, while in others it is ushered in with very marked symptoms.

The usual metamorphosis of tubercle in bone is softening; but it sometimes also becomes cretaceous.

(1.) When it softens, a tubercular ulcer is formed in the bone, which corresponds in extent with the quantity of substance the bone may have lost. The loss of substance arises from necrosis of the portion included in the tubercular mass; and it may die either when first involved in the mass of tubercle, or at a later period: but in either case, it is in consequence of its vessels becoming obstructed, or destroyed in the suppuration.

Softening of tubercle at the surface of a bone, produces a superficial breach of its substance, which has the appearance of being unevenly corroded; when a larger and more deeply situated mass softens, the bone is excavated, and a cavern - a tubercular cavern in bone - results. The greater the number of caverns which are found in a macerated bone, or set of bones, the more safely may it be concluded that they originated in the softening of tubercle.

The cavern contains a fluid, which presents the character of tubercular matter, and is mixed up with numerous particles of bone. Sometimes the particles are small, and resemble crumbled mortar; when larger, they are seen distinctly to be necrosed bone; they are usually of a dirty white color, soaked through with tubercular matter, and not so brittle as the sequestrum produced by other processes in a spongy bone.

The various processes which are usually found in the neighborhood of a softening tubercle, occur in bone also.

First, there is a secondary deposition of tubercles, which, as they soften, increase the size of the cavern. The congestion to which the secondary deposition of tubercle is owing, usually advances to the degree of inflammation (reaction), and leads to the formation of a gelatinous granulating product which lines the wall of the cavern, and as the tubercle softens, always breaks down too. Should the tuberculosis have attacked the peripheral layer of a bone, the congestion, vascularity, and product just spoken of, are seen with remarkable distinctness in the periosteum which immediately covers the diseased spot, and may be found also in the adjoining cellular and fibrous tissues. The periosteum is covered and infiltrated with this product, which, gelatinous at first, gradually assumes a lardaceous appearance. The tubercular matter collected under the membrane swells it out like a saccular appendage.

Oftener still, the inflammatory process becomes more intense, and under the influence of a highly advanced state of the general disease, pervades the osseous tissue throughout with a yellow cheesy product, which breaks down at once. Further remarks on this subject will be given below. It occasions a rapid enlargement of the ulcer in the bone, and extensive destruction, not only of the bone in which it originates, but of other tissues into which it may advance.

When the circumstances of the case are more favorable, and the formation of tubercle has ceased, the inflammatory product at the wall of the cavern becomes organized into a fibroid, lardaceo-callous tissue, and that in the bone itself, into bone; and the cavern changes into a thick firm capsule, which becomes surrounded by an indurated (sclerosed) bony tissue. Its contents are then partly reabsorbed, and partly, as the capsule shrivels and diminishes in size, they become inspissated, and form a greasy calcareous pulp, of a grayish-yellow color, or a mortar which incrusts the walls of the capsule, or a chalky concretion: and thus the tubercular disease is cured.

(2.) Under favorable circumstances, tubercle in bone becomes converted into chalk. A chalky concretion is found in the interior of the bone, enclosed within endurated osseous tissue: if the tubercle have been situated on the surface of the bone, the concretion is covered on the outside by thickened periosteum.

B. A form of tuberculosis, common in young persons, is comprised, in great part, of what are known by the general term of scrofulous inflammations.

The inflammation furnishes a tuberculous product by which the actually inflamed spot becomes infiltrated. It may be a primary affection of the bone, or may come on around an abscess already formed from the tubercle above described.

Spongy bones affected with this disease are found at first partly of a dark-red color, injected, and extruding a fatty or gelatinous matter from their cells; and partly pale, and having their cells filled with softening tubercular exudation: both bone and periosteum are frequently swollen, and the former is elastic and soft, and yields easily to pressure or the knife. Ulceration presently begins to destroy it, and the fluid discharged is either thin, grayish, or yellowish in color, and mixed with cheesy flakes, and with particles of necrosed bone; or it is colored of a dirty brown by hemorrhagic exudation, or else is highly discolored, and blackish-green, extremely offensive to the smell, and mixed with black fragments of bone, and with particles of the soft parts destroyed by sloughing.

When this process affects a compact bone, a bone of the skull for instance, its cavities appear filled with tubercular exudation; it becomes of a dirty yellowish-white color, and is, in fact, necrosed - a tuberculous sequestrum. Similar tubercular product is exuded between the surface of the bone and the periosteum. If the process involve only a superficial layer, the bony tissue is partly lost amid the softening of the tuberculous product, and is partly thrown off in particles which are sufficiently large to be palpable; and thus an uneven rugged surface of bone is exposed, from which the process extends more deeply; the osseous tissue previously becoming indurated, and the bone increasing in volume and thickness.

The abscesses which are produced by the process just described, advance in various directions from the bone into the soft tissues, which are infiltrated with the gelatino-lardaceous matter; after having given rise to other secondary (congestive) abscesses in those tissues, they open externally at a part which is often very far removed from their original seat. This is noticed particularly in abscesses in and about the vertebrae.

After the contents of the abscess have been evacuated, or perhaps have partly cretified, the tuberculous caries heals, leaving an indurated cicatrix in the bone, which deforms it in proportion to the amount of substance it has lost: the cicatrix has a rugged and nodular, streaked, and radiated or knitted appearance, and adheres to the thickened and callous periosteum.

The inflammation of the bone which is attended with the production of tubercle, and the caries which thence ensues in the spongy articular portions of the long bones, in the carpal and tarsal bones, and in the phalanges, passed among old writers by the name of Paedarthrocacia, while the same affection of the vertebrae is known by the name of "Pott's disease".