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 transplanted fragments will in two weeks grow to the size of filberts, and each of these will furnish material to engraft upon ten mice.  These new tumors are similar in character to the original tumor, and really represent parts of it in the same way that all the Baldwin apples in the world are parts of the original tree which was found in Baldwinville many years ago, and as all the Concord grape vines are really parts of the original vine.  It has been estimated that if all the growth capacity of this mouse tumor were availed of by the successive inoculation of other mice, a mass of tumor several times the diameter of the sun would grow in two years.  The condition of the individual seems to exert no influence upon the growth of the tumor.  Growth may be as rapid when the bearer is in a condition of extreme emaciation as it is when the bearer is well nourished and robust.

[Illustration:  FIG 14.—­PHOTOGRAPH OF A MICROSCOPIC PREPARATION FROM A CANCER OF THE UTERUS.  A large mass of cells is extending into the tissue of the uterus which is shown as the fibrous structure.  Such a cell mass penetrating into the tissue represents the real cancer, the tissue about the cell masses bear the blood vessels which nourish the tumor cells.]

Those tumors which grow rapidly and invade and destroy the surrounding tissue are called malignant tumors or cancers, but in a strict sense no tumor can be regarded as benign, for none can serve a useful purpose.  A tumor after a period of slow growth can begin to grow rapidly.  Tumors may arise in any part of the body, but there are certain places of preference particularly for the more malignant tumors.  These are places where the cells naturally have a marked power of growth, and especially where growth is intermittent as in the uterus and mammary gland.