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 cooerdinated 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.

The parasitic theory.  This supposes that a living parasite invades the body, and by its presence excites the cells of certain tissues to grow in tumor form.  It is known that active growth of the cells of the body can be excited in a number of ways, by chemical substances such as certain of the coal tar products, and that it often takes place under the influence of bacteria.  It is further known that parasites can produce tumor-like growths in plants.  The large, rough excrescences on the oaks are produced by a fly which lays its eggs in or beneath the bark, and the larva which develops from the egg secretes a substance which causes the cells about it to multiply, and a huge mass is formed which serves the developing insect for both food and protection.  Large tumor-like masses are formed on the roots and stalk of cabbages as the result of the invasion of the cells by a minute organism:  the tumors of olive trees are due to a bacterium; the peculiar growths on cedar trees, the so-called “witches’ brooms,” are produced by a fungus, and there are many other such examples.  These have many analogies with