This is a disease worthy of serious investigation - one, in fact, that we cannot lightly slip, because it is of great importance. It is a disease of the bones, supposed to be very analogous to ulceration of the soft parts. This comparison is one of great antiquity.

Now it must be borne in mind that the bones, like other parts of the body, are composed of vessels that are endued with vitality; they are nourished, they grow, they waste, and are repaired; they undergo various changes, and are subject to diseases analogous to those of the soft parts. To the phosphate of lime, which is more or less abundantly distributed in their texture, they owe all their solidity, and perhaps it is to the same inorganic substance that the difference in their vital properties and in their diseases from those of the rest of the body is to be referred. In fact, this particular organisation and inferior vitality of the bones are generally supposed to account for the small number, peculiar character, and usually slow progress of their diseases.

Bones of a spongy texture are more frequently attacked by caries than such as are compact. Hence the vertebrae and other bones, the knee, the pelvis, and the heads of the long bones are often affected, and the bones of young animals are unquestionably more frequently the seat of caries than those of old subjects. But although the soft and spongy bones are most subject to caries, they sometimes suffer a degree of injury sufficient to produce the death of a portion of their texture.

Now in necrosis the bone is entirely deprived of life. In caries the vital principle exists, but a morbid action is going on whereby the texture of the bone is altered, and rendered softer and lighter than natural. But although these disorders are essentially different from each other they often occur together in the same part.

In the most common species of caries a loose fungous flesh grows out of the interstices formed on the surface of the diseased bone, and bleeds from the slightest causes; while in the soft parts a sinus generally leads down to the caries, and emits a fetid, dark-coloured fluid. These symptoms, however, as well as the tendency in the accompanying ulcer or sinus to produce large fungous granulations, are more constant in cases of necrosis than in those of caries, some of which may remain a considerable time unattended with any outward sore, abscess, or sinus, as is illustrated in caries produced by various diseases of the joints, and indeed particular forms of caries (if they deserve that name) are rarely accompanied with suppuration.

"The absorption of bone, like that of soft parts," says Dr. Thomson, "may be distinguished into interstitial, progressive, and ulcerative." We have ample proofs of the interstitial absorption, or that which is daily and hourly taking place from every part of the substance of bone, in the deposition and removal of phosphate of lime that has been tinged with madder. If too much earth be removed, the quantity of animal matter will be relatively increased, and a disposition given to softness of the bones, a state which exists in the disease called rickets, which we have already noticed.

Again, hydatids in the brains of sheep, etc., are often the cause of the whole substance of the bone being removed layer after layer by progressive absorption, without the formation of a single particle of pus. This state of the bone has often been confounded, but improperly, with that state of the bone which arises from ulcerative absorption, the state which is properly denominated caries, and in which one or more solutions of continuity may be produced upon the surface or in the substance of the bones.

In caries absorption is preceded by a change in the bone, which (with very few and doubtful exceptions) has a well-marked inflammatory character. The same condition exists during the progress of the absorption. There is further present an imperfect restorative action, which is shown in the more or less partial growth of unwholesome granulations from the ulcerated surface. Of these changes the inflamed condition of the bone is the primary and most important. The absorption is secondary and accidental.

Absorption may be prevented by subduing the inflammation, or may, after having begun, be arrested, and the crop of unwholesome granulations converted into a healthy restorative growth if the case is of such a nature as to allow of the suppression of the inflammatory or specific action.

Caries has been divided into several kinds, according to the nature of its causes.

Firstly, caries from external causes. Secondly, caries from constitutional disease. Besides local remedies, it is necessary to employ such medicines as are calculated to obviate the particular affection of the system whence the diseased state of the bone has originated.

If the situation of the bones, the nature of their organisation, and the slowness of their diseases, would let an attentive observer trace the formation, development, and progress of caries, no doubt there would be noticed a diversity in its symptoms corresponding to its different species, and probably it would be found that a scrofulous caries would vary in its origin and progress as much from a caries arising from a purely local cause as a scrofulous ulcer differs from the kind of ulceration that follows a common abscess.

The worm-eaten caries, as it has been termed, which penetrates the whole substance of a bone, and gives it the appearance as if it had been bored in hundreds of places, is a very different affection from some other forms of the disease, whether superficial or extending to the deeper texture of the bone.

Around the carious part there is always a deposit of new osseous matter, in the form of tubercles, extending to a considerable distance, and greatly increasing the thickness of the bone. The new bone, on superficial inspection, appears rough and porous, the pores being for the transmission of bloodvessels. A carious bone, after maceration, looks as if it had been burned, being harder, whiter, and more brittle than usual, and always attended with more or less excavation, so as to expose the cellular structure. It resembles a piece of loaf sugar that has been partially dissolved by momentary immersion in hot water.

"Abscesses situated in the vicinity of bones are frequently thought to be the cause of caries and necrosis." This was the ancient doctrine, and it has found various advocates in modern times; hence the rule to open such abscesses at an early period in order to prevent the bone from being affected. "When, therefore, caries is fairly established, and the integuments have given way, the indications point to the immediate removal of the bone, or the employment of means calculated to make it be thrown off by the constitution. The first indication is to be accomplished by the proper use of such instruments as trephines, perforators, saws, forceps, etc, for dividing or extracting, the second "by cauteries; but in general a combination of both is required.