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.  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.