the
presence of a dark brown or black pigment within the cells.
Haemorrhages within them are not infrequent, and they may be colored by
the blood or by pigments formed from it. Usually they have a gray
color modified by their varying vascularity, or the cut surface may be
mottled due to areas of cell degeneration. The consistency varies;
some tumors are so soft that they can be pressed through a sieve,
others are of stony hardness. There is no distinct shape, this being
influenced by the nature of the tumor, the manner of growth and
situation. When the tumor grows on or near a surface, it may project
from this and be attached by a narrow band only; in the interior of
the body it may be irregular in outline, round or lobular, the shape
being influenced by many factors. Tumors like the tissues of the
normal body are nourished by the blood and contain blood vessels often
in great numbers.

A tumor arises by the cells of a part of the body beginning to grow
and taking on the characteristics of a tumor. Its growth is
independent, the cells of the adjoining tissue taking no part in it.
The tissue in the vicinity of the tumor is partly pushed aside by the
mass, or the tumor grows into it and the tissue disappears as the
tumor advances. The destruction of the surrounding tissue is brought
about partly by the pressure which the tumor exerts, partly by the
compression of the blood vessels or the blood supply of the organs is
diverted to the tumor.

The characteristics of a tumor are due to the cells which it contains
(Fig 14). These often become separated from the main mass and are
carried by the blood into other parts of the body, where they grow and
form tumors similar in character to the parent tumor. In the
extraordinary capacity for growth possessed by tumor cells, they
resemble vegetable rather than animal cells. There is no limit to the
growth of a tumor save by the death of the individual who bears it,
thus cutting off the supply of nutrition. The cells of tumors peculiar
to man show a narrow range of adaptation. They will grow only in the
body of the individual to whom the tumor belongs, and die when grafted
on another individual. In the case of tumors which arise in animals,
pieces of the tumor when grafted on another animal of the same species
will grow, and in this way the growth capacity of the tumor cells has
been estimated. Thus, by transplanting a small section of a mouse
tumor into other mice, the small trans