o parts of the organism together; bony tumors add
nothing to the supporting framework of the body; the tissue of fatty tumors
never serves as a storehouse of feed and energy; the cells of an adenoma,
or gland tumor, furnish no secretion; a tumor composed of muscle tissue
produces no increase to the strength of the individual--its muscle cells
are not contractile.

Tumors arise from cells of preexistent tissue. Tumor tissue is not a new
variety. Whatever the structure of a tumor, its counterpart is found among
the tissues of the body, the lawlessness of the tumor, however, showing
itself in more or less departure from the normal type. This departure is
usually a reversion to a more elementary or embryonic stage, so that the
tumor tissues may be said to be structurally immature.

Tumors arise without obvious cause. Concerning the ultimate cause of tumor
formation we are absolutely ignorant. Various theories have been advanced
from time to time, but none of them have been applicable to more than a
limited number of cases. The most important theories may be briefly
mentioned.

(1) _The theory of tumor diathesis._--Bilroth taught that tumors are caused
by a peculiar predisposition consisting of a diseased state of the fluids
of the body. This constitutional taint might be acquired, but, having been
acquired, is also hereditary. This theory is known also as the heredity
hypothesis, but, while it is true that heredity appears to play some role
in the causation of certain neoplasms, its application is too limited to
make it of value.

(2) _The mechanical or irritant theory._--Virchow assumed that tumors arise
as the result of previous irritation of the part. This has been noticed
particularly in the case of certain cancers. They frequently develop on the
edges of old ulcers, thus being dependent apparently on chronic irritation.
Cancer of the lip in pipe smokers is a case in point. Cancerous tumors of
the skin often develop on the arms of workers in paraffin, tar, or soot,
the chemical irritation of these substances being the cause. On the
contrary, the proportion of those thus affected among the exposed is very
small and forces the conclusion that if the real cause were in the
irritation vastly more cases would occur.

(3) _The theory of nervous influence._--That is based upon (_a_) the
observed fact that tumors occur more frequently in man and the higher
animals than in those lower in the scale, among which the nervous sy