The difficulties which arise in an endeavor to comprehend normal
growth are greater when the growth of tumors is considered. A tumor is
a mass of newly formed tissue which in structure, in growth, and the
relations which it forms with adjoining tissues departs to a greater
or less degree from the type of the tissue to which it is related in
structure or from which it originates. It is an independent structure
which, like a parasite, grows at the expense of the body, contributing
nothing to it, and its capacity for growth is unlimited. A tumor
cannot be considered as an organ, its activities not being coordinated
with those of the body. A part of the body it certainly is, but in the
household economy it is to be considered as a wild and lawless guest,
not influenced by or conforming with the regulations of the household.
The rapidity of growth varies; certain tumors for years increase but
little in size, while others may be seen to increase from day to day.
The growth is often intermittent, periods of great activity of growth
alternating with periods of quiescence. The nutrition and growth of a
tumor is only slightly influenced by the condition of nutrition of the
bearer. Its cells have a greater avidity for food than have those of
the body, and, like the growing bones of an insufficiently fed animal,
growth in some cases seems to take place at the expense of the body,
the normal cells not obtaining sufficient nutriment to repair their
waste.

A tumor may be of any size: so small as to be invisible to the naked
eye, or its weight may exceed that of the individual who bears it. The
limitations to its growth are extrinsic and not intrinsic. There is no
distinct color. Certain tumors have color which depends upon the
presence of a dark brown or black pigment within the cells.
Hæmorrhages within them are not infrequent, and they may be colored by