The effect of a tumor upon its bearer depends upon its character and
situation. Pain is very commonly present, and is due to the pressure
which the growing tumor exerts upon the sensory nerves. Pain may,
however, not be present or appear only at the last. A condition of
malnutrition and emaciation often results due to the passage into the
blood of injurious substances formed in the tumor, or to the
destruction of important organs by the growing tumor. The growth of a
tumor in the intestine may obstruct or close the canal and thus
interfere with nutrition.

The cause or causes of tumors are unknown. We know that the tumor
represents essentially an abnormal growth, and that this growth is due
to new formation of cells. In certain cases the tumor repeats the
structure of the organ or tissue in which it originates, in others it
departs widely from this; always, however, its structure resembles
structures found in the body at some period of life. The tumor cells,
like all other cells of the body, grow by means of the nutriment which
the body supplies; they have no intrinsic sources of energy. The great
problem is what starts the cells to grow and why the growth differs
from that of normal tissue, why it is not regulated and coördinated as
are other forms of growth. When a small piece of the skin, for
instance, is cut out growth as rapid as that in tumors takes place in
the adjoining cells, _but it ceases when the loss is restored_.
The same is true when a piece of the liver is removed.

Various hypotheses have been formed to explain the tumor, all of them
of interest, and they have had great importance in that the attempt to
prove or disprove the hypothesis by continued observation and
experiment along definite lines has produced new knowledge. The
various theories as to cause may be divided into three heads.