Central or medullary tumours as they increase in size replace the surrounding bone, and simultaneously new bone is formed on the surface; as this is in its turn absorbed, further bone is formed beneath the periosteum, so that in time the bone is increased in girth, and is said to be “expanded” by the growth in its interior.

#Primary Tumours—­Osteoma.#—­When the tumour projects from the surface of a bone it is called an exostosis.  When growing from bones developed in membrane, such as the flat bones of the skull, it is usually dense like ivory, and the term ivory exostosis is employed.  When derived from hyaline cartilage—­for example, at the ends of the long bones—­it is known as a cartilaginous exostosis.  This is invested with a cap of cartilage from which it continues to grow until the skeleton attains maturity.

An exostosis forms a rounded or mushroom-shaped tumour of limited size, which may be either sessile or pedunculated, and its surface is smooth or nodulated (Figs. 138 and 139).  A cartilaginous exostosis in the vicinity of a joint may be invested with a synovial sac or bursa—­the so-called exostosis bursata.  The bursa may be derived from the synovial membrane of the adjacent joint with which its cavity sometimes communicates, or it may be of adventitious origin; when it is the seat of bursitis and becomes distended with fluid, it may mask the underlying exostosis, which then requires a radiogram for its demonstration.

[Illustration:  FIG. 138.—­Radiogram of Right Knee showing Multiple Exostoses.]

Clinically, the osteoma forms a hard, indolent tumour attached to a bone.  The symptoms to which it gives rise depend on its situation.  In the vicinity of a joint, it may interfere with movement; on the medial side of the knee it may incapacitate the patient from riding.  When growing from the dorsum of the terminal phalanx of the great toe—­subungual exostosis—­it displaces the nail, and may project through its matrix at the point of the toe, while the soft parts over it may be ulcerated from pressure (Fig. 107).  It incapacitates the patient from wearing a boot.  When it presses on a nerve-trunk it causes pains and cramps.  In the orbit it displaces the eyeball; in the nasal fossae and in the external auditory meatus it causes obstruction, which may be attended with ulceration and discharge.  In the skull it may project from the outer table, forming a smooth rounded swelling, or it may project from the inner table and press upon the brain.

The diagnosis is to be made by the slow growth of the tumour, its hardness, and by the shadow which it presents with the X-rays (Fig. 138).

An osteoma which does not cause symptoms may be left alone, as it ceases to grow when the skeleton is mature and has no tendency to change its benign character.  If causing symptoms, it is removed by dividing the neck or base of the tumour with a chisel, care being taken to remove the whole of the overlying cartilage.  The dense varieties met with in the bones of the skull present greater difficulties; if it is necessary to remove them, the base or neck of the tumour is perforated in many directions with highly tempered drills rotated by some form of engine, and the division is completed with the chisel.