In this class are included tumours composed of bone, not mere new-formations due to inflammation, nor tumours in which bone exists as a subordinate element with other tissue.

The large majority of bony tumours grow from bone, and are hence called Exostoses. In regard to their structure, some fire like spongy bone with the interstices filled with ordinary bone-marrow, and some are composed of dense bone such as forms the shaft of a long bone, and are called ivory exostoses.

There are also tumours which originate in the teeth. When these are composed of cement, they are properly called dental osteomas, but when formed of dentine, odontomas. The latter term, however, is often applied to either form. The tumour may be little more than a local enlargement of the fang, rendering extraction difficult, or there may be a distinct tumour, growing sometimes to the size of a walnut. In the latter case it is usually a true odontoma, composed of a structure like dentine.

Cartilaginous spongy exostosis of femur.

Fig. 77. - Cartilaginous spongy exostosis of femur.

(VlRCHOW).

Of the proper exostoses several forms are distinguished.

(1) The Spongy Exostosis, Or The Exostosis Cartilaginea (Fig. 77)

These tumours occur mostly at the epiphyses of the long bones, and are derived primarily from the epiphysial cartilage. A rather favourite seat is the dorsal aspect of the last phalanx of the great toe, where they project beneath the nail and produce great pain and discomfort. They grow during childhood, and, just as the cartilage from which they originate ossifies, so do they, and the bony tumour formed is directly continuous with the bone beneath. The tumour begins as a small outgrowth, and so the first bone formed is a narrow piece. The cartilage, as it goes on growing, enlarges in every direction, and so overhangs its base, the tumour thus becoming larger as it grows outwards, and consequently pedunculated. The tumour consists of spongy bone with a thin layer of cartilage on its surface. It is enough to snip through the base in order to remove the tumour.

(2) The Ivory Exostosis

The Ivory Exostosis is mostly met with on the bones of the head, but also on the pelvis, scapula, great toe, etc. The tumours are usually rounded in form, and may be tuberculated on the surface. In their favourite seat on the head they may grow from the external table and project externally, or from the internal table and project internally (as in Fig. 78), in which case they may produce irritation of the brain substance beneath. It sometimes happens that an ivory exostosis grows from corresponding parts of both external and internal tables. These tumours are sometimes multiple (as in Fig. 79).

(3) Hyperostosis And Periostosis

Hyperostosis And Periostosis are names applied to growths of bone which are not properly tumours. They are localized thickenings of bones or portions of bones. This growth of particular parts of the bones of the head may become so independent in its manner that the characters of a tumour are simulated.

Internal ivory exostosis of the frontal bone.

Fig. 78. - Internal ivory exostosis of the frontal bone. The surface is tuberculated, and the tumour has a narrow base. It was situated to the left of the falx (f). Natural size. (Virchow).

Osteomas of other parts than bones are rather rare. It is remarkable, however, that bony masses occur sometimes in the central nervous system. They are met with in the arachnoid, where they used to be regarded as evidences of chronic irritation, and are hardly tumours. Actual tumours occur in the dura mater, and even in the brain substance. They are also met with in the eyeball, in the lungs, and, as little bony granules, in the skin.

Multiple exostoses of the frontal bone.

Fig. 79. - Multiple exostoses of the frontal bone. Natural size. (Virchow).

Literature

Weber, Die Knochengeschwiilste, 1856; Virchow, Geschwiilste, ii., 1 and 53 (Odontoma). Bland Sutton (Odontoma), Tumours Innocent and Malignant, 1893.