eton. Two varieties are recognised--the spongy or
cancellous, and the ivory or compact. The _spongy_ or _cancellous
osteoma_ is really an ossified chondroma, and is met with at the ends of
the long bones (Fig. 52). From the fact that it projects from the
surface of the bone it is often spoken of as an _exostosis_. It grows
slowly, and rarely causes any discomfort unless it presses upon a
nerve-trunk or upon a bursa which has developed over it. The Rontgen
rays show a dark shadow corresponding to the ossified portion of the
tumour, and continuous with that of the bone from which it is growing
(Fig. 138). Operative interference is only indicated when the tumour is
giving rise to inconvenience. It is then removed, its base or neck being
divided by means of the chisel. The multiple variety of osteoma is
considered with the diseases of bone.

The bony outgrowth from the terminal phalanx of the great toe--known as
the _subungual exostosis_--is described and figured on p. 404. Bony
projections or "spurs" sometimes occur on the under surface of the
calcaneus, and, projecting downwards and forwards from the greater
process, cause pain on putting the heel to the ground.

[Illustration: FIG. 52.--Cancellous Osteoma of lower end of Femur.]

The _ivory_ or _compact osteoma_ is composed of dense bone, and usually
grows from the skull. It is generally sessile and solitary, and may grow
into the interior of the skull, into the frontal sinus, into the cavity
of the orbit or nose, or may fill up the external auditory meatus,
causing most unsightly deformity and interference with sight, breathing,
and hearing.

Bony formations occur in _muscles and tendons_, especially at their
points of attachment to the skeleton, and are known as false exostoses;
they are described with the diseases of muscles.

#Odontoma.#--An odontoma is composed of dental tissues in varying
proportions and different degrees of development, arising from
tooth-germs or from teeth still in process of growth (Bland Sutton).
Odontomas resemble teeth in so far that during their development they
remain hidden below the mucous membrane and give no evidence of their
existence. There then succeeds, usually between the twentieth and
twenty-fifth years, an eruptive stage, which is often attended with
suppuration, and this may be the means of drawing attention to the
tumour. Following Bland Sutton, several varieties of odontoma may be
distinguished according to the part of t