In the _central_ form, it is usually situated at or about the articular ends, but always affects the whole of the bone by infiltration. In the _peripheral_, the more common form of cancer of the bone, the osseous tissue is not so completely invaded; for although the disease may be located upon, or in intimate contact with the outer layers of the bone, which are incorporated in it, it does not extend into the cancellous tissue or the medullary canal. In this form, the muscles attached to the affected portion of bone will often be found extensively infiltrated with cancer cells. Encephaloid of bone is harder and more fibrous looking than the same affection elsewhere. The cancer cell also is not so well marked, and may indeed be absent altogether. Occasionally some colloid, and more rarely melanotic matter, is intermixed, but scirrhus is never found in bone. Of the Medulla. “It is yet to be determined in how far the medulla is liable to be primarily affected. It varies in consistency according to the vigor of the individual; while in dropsical and phthisical cases we find it thin and serous, or yellow in icterus, or very scanty in ivory condensation of a bone. It exhibits greater firmness, and a richer pink hue, in habits tending to an inflammatory character. The real seat of inflammation in bone is the membrane that lines its cavities. It is, therefore, fair to infer that, in all diseases dependent upon the state of the vascular system, whether of an ordinary or of a malignant character, the medulla is affected coincidently with, if not previously to, the bony tissue itself.” _Morbid growths_, of various kinds, may be found within the medulla. The Cysticercus cellulosæ, and Echinococcus, are said to have been detected within the medulla and periosteum. CHAPTER II. DISEASES OF THE JOINTS. =Malformations.= Cases of congenital anchylosis, the joints being absent, have been met with; also imperfectly developed joints, with a partial or total absence of the ligaments. Supernumerary joints also occur, either with the normal, or with an excessive number of bones. =Morbid Conditions of the Synovial Membrane.= _Inflammation_ may be acute, sub-acute or chronic. It usually results from exposure to cold, especially in rheumatic or syphilitic constitutions.