—Under the head of nomenclature I have already endeavored to distinguish as between exostosis, or irregular bone outgrowth, and osteoma, as a distinct tumor which is composed of bone tissue, with the subvariety odontoma, or tumors of dental origin and structure. Osteoma is regarded by some as ossifying chondroma, for it is nearly always found near epiphyseal lines, and is always covered by hyaline cartilage when thus found. Nevertheless it is not invariably such. We speak of compact or ivory osteoma and of a cancellous form. The former is identical with the compact tissue of the shafts of long bones, and may occur anywhere, but is most common about the cranium, at the frontal sinus, the external meatus, and the mastoid process. Osteomas growing into the frontal sinus of oxen form large, lobulated, bony masses, sometimes weighing several pounds, and as dense as ivory. Some of these tumors growing into the cranial cavity have been regarded as ossified brains. Osteomas in connection with the external auditory meatus may partially obscure this channel and cause deafness. They constitute ivory-like growths, which sometimes defy the finest steel instruments with which the surgeon can supply himself.
Fig. 76
Double osteoma of the skull. (Musée Dupuytren.)
Cancellous osteomas grow in the cranium as well as in the long bones, and, like the compact forms, only occasion pain by pressure upon nerve trunks.
Exostoses.
—Exostoses are classed by Sutton as—
1. Those formed by ossification of tendons and their attachments. There should be excluded from this group such natural or evolutionary processes as the supracondyloid process, the third trochanter of the femur, etc. Over or around such exostoses bursæ will form to mitigate as much as possible the effect of friction. Such an outgrowth is known as an exostosis bursa; it is most frequently seen on the inside of the femur immediately above the knee.
2. Subungual exostoses, occurring usually beneath the nail of the big toe.
3. Exostoses due to calcification of inflammatory exudations, including the rare condition known as myositis ossificans.