They may originate on the surface of the bone, or within the cancellous tissue. In the former case, they exhibit a lobulated arrangement, and are surrounded by a fibrous sheath; in the latter, the bone gradually expands with the development of the tumor. The rapidity and extent of their growth vary. In their microscopic characters, the enchondroma resembles normal cartilage. The central variety presents a semi-elastic feel, and, on section, the knife passes through a thin, crackling shell of bone, and then exhibits a white, cartilaginous mass, which is occasionally found to contain some small cells; while in some tumors there is an interlacement of fibrous tissue, in which cartilage is imbedded. The superficial variety is microscopically and chemically identical with the central form, but has no osseous shell. It is met with chiefly in the pelvis, on the cranium, and on the ribs. There may be a partial ossification. The disease is chiefly met with in early life. =Osseous Growths=, consisting of true bone, are divided into _exostoses_ and _osteophytes_. The surface of the former is smooth; their outline generally a segment of a circle or of an ellipse; their cause: an idiosyncracy of the individual, not referable to any definite constitutional taint. Of the latter, the surface is rough; they do not form any well defined _local_, circumscribed tumor; are referable to rheumatic or gouty inflammation, to syphilis or other causes. Exostoses are of two kinds: the one, hard and compact; the other, softer and more spongy. The _hard_ or _ivory exostosis_ is extremely dense, and whiter than the bone from which it springs, but possesses a true bony structure. It generally grows from flat bones, and is of small size. It has been known to necrose and to slough away from the parts on which it has been situated. _Spongy exostoses_ often attain a considerable size, and are very commonly multiple. They differ from the compact variety in being composed of cancelli, containing medullary matter, and surrounded by a shell of bone. They spring from the cancellous, or compact tissue of the bone, and their surface is continuous with that of the latter. In some cases the cavity of the exostosis communicates directly, or is continuous, with the medullary cavity of the bone. Their most common seats are the tibia, fibula and humerus. The _osteophyte_ chiefly affects the more vascular portions of bones, as their articular ends, their rough lines, or, in the skull, the sutural cartilages; being generally the product of an inflammatory process in the superficial part of the bone, and in the periosteum. =Fibrous Growths= always develop in the cancellous structure. All the long bones and many of the flat bones are liable to this disease. They present more or less elasticity, are of a gray and opaque appearance, and yield gelatin on boiling. They may attain an enormous size. =Cystic Tumors= are of rare occurrence, and are generally met with in adults.