al tumours_ are at first situated on one side of the bone, but
as they grow they tend to surround it completely. Innocent periosteal
tumours retain the outer fibrous layer as a capsule. Malignant tumours
tend to perforate the periosteal capsule and invade the soft parts.

_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