Some of the causes are external, while others are internal or constitutional. Sometimes the life of the bone is instantaneously destroyed by them; but in other instances, the bone is first stimulated, and its death is preceded by true inflammation. The external causes which injure the perios teum and medullary structure, and thus produce necrosis, are wounds, contusions, pressure, fractures, comminutions, acrid substances, caustics, extreme degrees of heat or cold.

When the outer covering of bone, in consequence of an external cause, inflames and sloughs, or is at once deprived of its vitality, as may be by the action of caustic, fire, or intense frost, the vessels which conveyed nourishment to the bone are destroyed, and as a result the death and exfoliation of the denuded portions of the bone are inevitable. But if the detachment of the periosteum (that is, the covering) is of little extent, the patient young and healthy, and the treatment calculated to prevent inflammation, and preserve uninjured the vessels distributed to the bone, hopes may be entertained that no part of the bone will die, but that granulations will very soon arise from its surface, being adherent to it as the periosteum was, and that they will grow too, and cicatrize with the surrounding parts.

On the other hand, when the detached piece of the periosteum is extensive, when the bone itself is contused, or when it has been long exposed to the air, when the inflammation is violent and extensive, when the patient is old and worthless, when the constitution is bad, and more especially when improper applications are used, necrosis cannot be avoided.

Now the inflammation arising from the causes which excite necrosis may be acute or chronic. It is chronic when it begins and passes through its different stages slowly, and when the mildness of the symptoms may lead us to mistake the nature of the case. This sort of inflammation chiefly happens in debilitated constitutions, in which the necrosis only affects the external part of a bone, and originates from some chronic cause, such as scrofula, etc. But when necrosis attacks the interior, and the disease occurs in a strong, irritable, plethoric animal, inflammation is immediately kindled, attended with the most acute symptoms, severe pain, fever, restlessness, etc. Again, chronic inflammation is more supportable, but its duration is longer; acute inflammation is more afflicting, but sooner comes to a crisis.

The part in which a necrosis is situated is affected with swelling. What has been observed respecting the inflammation is also applicable to this tumour, which most frequently forms gradually, but sometimes with great rapidity. In the first case the accompanying pain is dull and inconsiderable, in the second it is violent. The swelling has not, like that of abscesses, an elevated apex. On the contrary, it is so widely diffused, that the limits which circumscribe it can hardly be distinguished. This diffusion of the swelling is the greater in proportion as the diseased bone is more deeply buried in soft parts; it may extend over the whole morbid bone, or even over the whole limb. Again, the swelling comes on at the very beginning of the disorder, and continues to increase until the matter which it contains finds its way out, when the evacuation is followed by a partial subsidence of the tumour.

When the inflammation is acute, purulent matter of good quality soon collects in the vicinity of the necrosis. In the contrary case, the pus forms slowly, and is thinner and less healthy.

The abscess which accompanies a necrosis naturally soon bursts when it arises from intense inflammation, and is situated near the skin, which is itself inflamed. But when the bone is surrounded by a great thickness of soft parts, and the inflammation is chronic, the quantity of matter daily increases, the cavity which it occupies becomes larger and larger, while considerable pressure is made by the abscess on every side. The bones and tendons resist for a long time the progress of the matter, but the cellular tissue yields, and different sinuses form, which sometimes run to a vast distance from the main collection of matter, especially when the abscess lies under a fascia.