+Acute Periostitis.+ The older text books always laid great stress upon the occurrence of an acute infectious inflammation of the periosteum. Acute suppurating periostitis alone does not occur, and most of the cases which have been described as such are really mild cases of superficial osteomyelitis, with abscess formation beneath the periosteum, and possibly slight inflammation of the periosteum itself. These cases ordinarily lead to only a slight destruction of the outer layer of the cortical bone. +Symptoms.+ These are the same as in acute osteomyelitis, except in a very much milder form. There is usually a rise of temperature, oftentimes with a chill, with circumscribed tenderness over some portion of the shaft of one of the long bones. +Treatment.+ Incision over such an area shows an elevated periosteum, with a small, localized abscess beneath it, with bare, white, somewhat vascular bone cortex. Incision alone in most cases suffices to cure the disease, although if the process has extended sufficiently deep to cause a superficial necrosis of the outer layer of the cortex, removal of a small sliver of necrotic bone may be necessary. +Chronic Periostitis.+ A long-continued and chronic irritation of the periosteum, sufficient to cause a proliferation of the osteogenetic cells of the periosteum, is common in a great many diseases. A chronic thickening of the periosteum with a new formation of bone, is seen frequently after traumatism, blows or contusions; sometimes after the occurrence of superficial abscess of the soft tissues in the immediate vicinity of the shaft of the long bone, described as chronic ulcer of the surface of the tibia; or after certain infectious diseases, notably syphilis. It also may occur after various other local infections. In such cases the thickening of the periosteum ordinarily is pretty sharply localized. A general thickening over the periosteum, and over several or many of the bones of the body, also occurs in the disease known as _toxic osteoperiostitis ossificans_, seen in diseases with long continued suppuration. It also is common after syphilitic disease, either congenital or acquired. +Symptoms.+ The symptoms of chronic periostitis with new formation of bone are invariable. In a certain number of cases there is a constant, heavy, dull pain, at the point of thickening, with at times more or less acute exacerbation; at other times the lesion is associated with no pain whatever, and the patient’s attention is first called to the disease by the presence of the enlargement of bone. Recognition of the condition may depend upon X-ray examinations for indefinite pains in or over the bone. Chronic periostitis is not really a disease itself, but a manifestation of the reaction of the periosteum to some irritant.