Let us next endeavour to trace the signs by which we may not only ascertain the presence of the disease, but its modifications.

In the first place, we should make ourselves acquainted with everything which may have predisposed to the disorder; as, for instance, what accidental circumstances have occurred, and what symptoms followed them. We should also inquire into any previous treatment which may have been adopted, because injudicious remedies have caused many a necrosis that would not have occurred at all, if the case had been properly treated, or even if Nature had been allowed to take its own course.

The kind of inflammation with which the disease commences may afford grounds for suspecting that necrosis will happen. It is generally slow and deeply seated, passing through its stages tardily, and the attendant symptoms are severe. The matter does not reach the skin till a considerable time has elapsed, and when the abscess bursts, the inflammatory symptoms are still slow in subsiding. When the inflammation is acute, the patient suffers intolerable pain a long time.

There are also other symptoms of necrosis, namely, the swelling which accompanies when the inflammation is situated upon a bone, or rather the bone is included in the tumour. The swelling at the same time is very diffused, and the suppuration lies deeply, and can only be felt in an obscure way. Now none of the preceding symptoms convey such information as leaves no doubt of the positive existence of necrosis. The touch is the only reliable thing in the absence of seeing which can give us this knowledge when the bone is not too deeply situated, and the sinuses not tortuous, nor obstructed with fungous growths.

When the openings of the ulcers are considerable, the finger may be introduced. If in this way the bone can be felt to be extensively uncovered by the periosteum, you have just reason for concluding that all such portions of the bone have perished. And you will still be more certain of the fact when you find the edges of the bone unequal and rough. The examinations made with the finger give the most correct and exact information of the state of the bone; but the orifice of the sores are sometimes so small, that the finger cannot be introduced without causing great pain. A probe must then be used for the purpose of ascertaining the extent of the denudation of the bone, whether its edges are rough, whether the dead portion is loose, and likely to separate soon. It is also necessary to distinguish with the greatest attention the different stages of the disease.

The first stage may be considered as existing when the attack is yet recent, and the inflammation and its concomitant symptoms, the pain, swelling, and symptomatic fever, prevail in a high degree, and no suppuration has taken place, or at least no discharge of matter.

The second stage, in which the dead bone is undergoing the process of separation, is indicated by a diminution of the inflammation, a partial subsidence of the swelling, and the discharge of purulent matter. When a probe is passed into the ulcers, the bone is felt bare and dry, and towards the limits of the swelling it is rough, where, as will be afterwards noticed, an excavation is formed. Every part of the bone, however, which is to be detached, still continues adherent to the rest of the living bone. At length, we know that the disease has reached the last stage, or that in which the dead portion of bone is entirely separated, when sufficient time for the completion of this separation has transpired, and when the dead bone can be distinguished with the finger, probe, or even the eye, to be loose and free from all connections.

When a portion of bone dies, Nature uses all her efforts to bring about the separation from the part of the bone that still remains alive. This process has been denominated exfoliation, which resembles the separation of the soft parts affected with sphacelus from the living. The exfoliation of bone, however, happens much more slowly than the separation of a slough. Again, exfoliations are not completed at any regular and fixed periods, for they proceed most quickly when the animal is young, when the system is usually more full of energy, the bones are more vascular, and less replete with solid, inorganic, earthy matter. On the other hand, the process is slower in old subjects, whose vitality is less active. A thin small scale of bone separates sooner than a large, thick portion, and the most tedious exfoliation is that of a thick bone from which a portion, including its entire diameter, is coming away. The separation of a necrosis takes place more expeditiously in bones of a light texture than in those of a solid structure, and sooner in the less compact parts of bones, such as the spongy substance, than in those of greater density. The separation occurs precisely at the different points where the living and dead parts of the bone come into contact, and it is obvious that the particles of the dead bone which are at a distance from the part that retains its vitality cannot be acted upon by it.

A variety of opinions have been entertained concerning the means employed by Nature in effecting this separation. Hippocrates believed that the dead part was pushed away by a fleshy substance which grew underneath it. And many others adopted the same idea. Another theory was, that the dead part was forced away by the incessant beating of the arteries. Others supposed that the exfoliating piece of bone became loosened partly by the suppuration, and partly by the rising of new granulations.

Now when a piece of bone perishes, it is then with regard to the rest of the body completely an extraneous substance, and, as such, proves a source of irritation to the surface of the living hone, which becomes inflamed, and acquires increased vascularity.

The next stage of the process is the formation of a groove between the dead and living bone, the earthy matter of which is first taken away and then the animal substance. This has been often noticed in cases where the exfoliation of the end of a fracture was taking place. When it had just begun, the living bone immediately adjacent to the dead portion was found softened by absorption of its earthy particles, as if it had been immersed in diluted acid. A channel was soon formed in it, and as this became gradually deeper, the dead was separated from the living hone. As this groove grows deeper it is filled by granulations arising from the living hone, and hence, on separating a piece of necrosed bone, there is seen next to it, not the surface of the living hone, hut the layer of very vascular granulations, by which it is completely covered, and whose soft velvety-like appearance one never forgets. And in correspondence with the granulations that have sprung up from the living bone, we have the well-known rough surface of the dead, with its multitude of prominences and excavations fitted to the granulations. In every one of its processes, therefore, the exfoliation of dead bone is but the repetition of that of the separation of a slough from soft parts. In both may be recognised-

Firstly, increased vascularity in the contiguous parts.

Secondly, the groove between the dead and the living parts.

Thirdly, the granulations from the surface exposed by the removal of the dead bone.