Cartilaginous tumours in the parotid, submaxillary gland, and testicle belong to a class of “mixed tumours” that will be referred to later.

#Osteoma.#—­The true osteoma is composed of bony tissue, and originates from the skeleton.  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 the tooth-germ concerned in their formation.

The epithelial odontoma is derived from persistent portions of the epithelium of the enamel organ, and constitutes a multilocular cystic tumour which is chiefly met with in the mandible.  The cystic spaces of the tumour contain a brownish glairy fluid.  These tumours have been described by Eve under the name of multilocular cystic epithelial tumours of the jaw.