the transformation of a benign into a
malignant tumour must be admitted. Such a transformation implies a
change in the structure of the growth, and has been observed especially
in fibrous and cartilaginous tumours, in tumours of the thyreoid gland,
and in uterine fibroids. The alteration in character may take place
under the influence of injury, prolonged or repeated irritation,
incomplete removal of the benign tumour by operation, or the altered
physiological conditions of the tissues which attend upon advancing
years.

After a tumour has been removed by operation it should as a routine
measure be subjected to microscopical examination; the results are often
instructive and sometimes other than what was expected.

#Varieties of Tumours.#--In the following description, tumours are
classified on an anatomical basis, taking in order first the
connective-tissue group and subsequently those that originate in
epithelium.

INNOCENT CONNECTIVE-TISSUE TUMOURS

#Lipoma.#--A lipoma is composed of fat resembling that normally present
in the body. The commonest variety is the _subcutaneous lipoma_, which
grows from the subcutaneous fat, and forms a soft, irregularly lobulated
tumour (Fig. 45). The fat is arranged in lobules separated by
connective-tissue septa, which are continuous with the capsule
surrounding the tumour and with the overlying skin, which becomes
dimpled or puckered when an attempt is made to pinch it up. As the fat
is almost fluid at the body temperature, fluctuation can usually be
detected. These tumours vary greatly in size, occur at all ages, grow
slowly, and, while generally solitary, are sometimes multiple. They are
most commonly met with on the shoulder, buttock, or back. In certain
situations, such as the thigh and perineum, they tend to become
pedunculated (Fig. 46).

A fatty tumour is to be diagnosed from a cold abscess and from a cyst.
The distinguishing features of the lipoma are the tacking down and
dimpling of the overlying skin, the lobulation of the tumour, which is
recognised when it is pressed upon with the flat of the hand, and, more
reliable than either of these, the mobility, the tumour slipping away
when pressed upon at its margin.

[Illustration: FIG. 45.--Subcutaneous Lipoma showing lobulation.]

The prognosis is more favourable than in any other tumour as it never
changes its characters; the only reasons for its removal by operation
are its unsightliness and its probable increas