A casual inspection of a bone shows it to consist of several structures. Outwardly will be noticed a thin fibrous membrane (periosteum). This not only covers the exterior of the bone, but serves as a bed in which blood-vessels break up into small branches before entering it through the minute openings provided on the surface. With these small vessels fine fibres from the periosteum itself also pass into the tissue of the bone, and become connected with another membrane lining it within termed the endosteum. The several structures-may separately suffer from inflammation, but the intimate connection existing between them renders it impossible for one to suffer without the others being soon involved in the disease. Inflammation occurring in the periosteum is known as periostitis, in the bone as ostitis, and in the lining membrane of the bone as endostitis.