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undifferentiated tissue elements or "rests," and so determine the growth
of a tumour, or that it may alter the characters of a tumour which
already exists, causing it to grow more rapidly.

The popular belief that there is some constitutional peculiarity
concerned in the causation of tumours is largely based on the fact that
certain forms of new growth--for example, cancer--are known to occur
with undue frequency in certain families. The same influence is more
striking in the case of certain innocent tumours--particularly multiple
osteomas and lipomas--which are hereditary in the same sense as
supernumerary or webbed fingers, and appear in members of the same
family through several generations.

INNOCENT AND MALIGNANT TUMOURS

For clinical purposes, tumours are arbitrarily divided into two
classes--the innocent and the malignant. The outstanding difference
between them is, that while the evil effects of innocent tumours are
entirely local and depend for their severity on the environment of the
growth, malignant tumours wherever situated, in addition to producing
similar local effects, injure the general health and ultimately cause
death.

_Innocent_, benign, or simple tumours present a close structural
resemblance to the normal tissues of the body. They grow slowly, and are
usually definitely circumscribed by a fibrous capsule, from which they
are easily enucleated, and they do not tend to recur after removal. In
their growth they merely push aside and compress adjacent parts, and
they present no tendency to ulcerate and bleed unless the overlying skin
or mucous membrane is injured. Although usually solitary, some are
multiple from the outset--for example, fatty, fibrous, and bony tumours,
warts, and fibroid tumours of the uterus. They produce no constitutional
disturbance. They only threaten life when growing in the vicinity of
vital organs, and then only in virtue of their situation--for example,
death may result from an innocent tumour in the air-passage causing
suffocation, in the intestine causing obstruction of the bowels, or in
the vertebral canal causing pressure on the spinal medulla.

_Malignant tumours_ usually show a marked departure from the structure
and arrangement of the normal tissues of the body. Although the cells of
which they are composed are derived from normal tissue cells, they tend
to take on a lower, more vegetative form; they may be regarded as
parasites living at the expense of t