PERIOSIETIM (Gr. peri, around, and osteon, bone), a tough fibrous membrane which surrounds the various bones. It is highly vascular, and is the means by which the outer layers of the shafts and the greater part of the spongy portions of the bones are supplied with blood. "From the internal surface of the periosteum also is produced a layer of soft blastema (or plastic fluid in which cells are developed), by mean of which additions are made to the exterior of the growing bones. The process of ossification going on in the inner part of this blastema contributes to the thickness of the hone, while a fresh supply is continually being added to the exterior of the blastema, through the medium of the vessels of thp periosteum."—Ilumphry, On the human Skeleton. page 19. In young bones this membrane is thick, and in consequence of the intervening blastema is very easily detached from the bone; but in the bones of the adult it is less thick and vascular, while its connection with the bone becomes closer, in consequence of the blas tema being less; while in aged persons it is very thin, its vessels are scanty, and there is no blastema. Numerous experiments show that the formation of bone is essentially due to the action of this membrane; and that, by transplanting detached portions of perios tenni into muscular or other tissues, bony tissue is generated in those parts. Iu most cases in which this membrane has become detached in consequence of a wound or of disease, the exposed bone (except in the instance of the skull, which derives most of its nutrient matter from the dura mater) perishes; but this is not invariably the case.
Amongst its other offices, it serves, by isolating the bone from the surrounding tissues, to prevent the spread of disease from them to it. The shin-bone, or tibia, is thus indebted to the periostemn for its ordinary immunity, in cases of ulcer in that region. In those parts in which the bone is not so completely isolated from the surrounding tissues, as at the ends of the bones of the fingers and toes, inflammation of the soft parts not unfre quently extends to the bony structure.
PERIOSTr TIS, or INFI„iMMATION OF 'rum PERIOSTEUlf, generally occurs on the surface of thinly-covered bones, such as the tibia, clavicles, and cranial bones. Its chief causes are (1) a syphilitic taint, in which oval swellings, called nodes (q.v.), are produced; (2) rheumatism ; and (3) scrofula. In the two latter cases there' is a periosteal swelling around the whole circumference or surface of the bone. The affection, especially when (Inc. to the first or second of the above causes, is usually accompanied with considerable nocturnal pain. If the disease occurs in an acute form, it must be treated with leeches, fomentations, and the other ordinary antiphlogistic (or lowering) When it becomes chronic, the must be mainly directed to the cause has originated it. In almost all cases, the nocturnal pains are best relieved by somewhat large doses (five to ten grains) of iodide of potassium, taken three times a day on an empty stomach.