The chondromas that are met with at the ends of the long bones in children and young adults form a group by themselves. They are usually related to the epiphysial cartilage, and it was suggested by Virchow that they take origin from islands of cartilage which have not been used up in the process of ossification. They are believed to occur more frequently in those who have suffered from rickets. They have no malignant tendencies and tend to undergo ossification concurrently with the epiphysial cartilage from which they take origin, and constitute what are known as _cartilaginous exostoses_. These are sometimes met with in a multiple form, and may occur in several generations of the same family. They are considered in greater detail in the chapter dealing with tumours of bone. Minute nodules of cartilage sometimes form in the synovial membrane of joints and lining of tendon sheaths and bursæ: they tend to become detached from the membrane and constitute loose bodies; they also undergo a variable amount of calcification and ossification, so as to be visible in skiagrams. They are further considered with loose bodies in joints. 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 Röntgen 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.