A tumor is an abnormal mass of cells or tissues resembling those normally present, but arranged atypically. It grows without any definite limit at the expense of the organism, without serving any useful function.

The cause of such growths is as yet unknown. They are made up of tissues that have their counterpart either in the embryonal or adult development. They differ in having a more or less atypical arrangement, in occurring in tissues in which they are heterologous, and in not having any mechanism to control their growth and function. Inflammation is unessential to their occurrence, and their structure is dissimilar to that of inflammatory lesions. There is no hyperemia, no exudation, no leukocytic invasion, no granulation tissue, no cicatrization. Tumors tend to increase and persist, while most inflammations tend to recover and disappear. Inflammatory growths always consist of connective tissue, regardless of the tissue or organ in which they occur.

Theories Of Origin

They are numerous, but as yet no one answers in every case.

1. Spermatic Influence

It was thought that the normal tissue where the growth occurred had become directly transformed into the tissue of the tumor, but this is not in any way supported.

2. Mechanical Irritation Theory Of Virchow

By this it is claimed that new growths arise in tissues that have been the seat of injury or chronic irritation. Such cases as the development of epitheliomata on the lower lips of pipe-smokers, carcinoma of the gall-bladder associated with gall-stones, scrotal cancer in chimney-sweeps, x-ray cancer, cancer of mouth in Ceylon, Kangri cancer in natives of Kashmir, etc., would seem to uphold this theory. It is probable, however, that the injuries and irritation are not the causative, but are predisposing, factors.

3. Theory Of Embryonic Remnants (Cohnheim)

'The author of this theory believed that "in an early stage of embryonic development more cells were produced than were required for the formation of the tissue involved, so that there remained unused a number of cells, possibly very few, which, on account of their embryonic character, were endowed with the power of marked proliferation." These remnants are frequently spoken of as "rests." Cohnheim thought that they could lie latent for many years and develop in after-life if conditions should become favorable. No explanation is given, however, as to what is meant by "favorable conditions".

Such groups of cells have been observed not infrequently in various tissues and organs of the body. Adrenal rests are not uncommon. In certain forms of tumors this theory seems to hold good: in enchondromata of the testis and parotid glands and of other organs, and particularly in the case of the dermoid cysts.

4. Parasitic Or Infective Theory

It has been claimed by many investigators, especially concerning the carcinomata and sarcomata, that tumors are caused by the presence of living micro-organisms. Bacteria were first supposed to be the cause, but protozoa and blastomycetes have also been suspected. The general opinion, however, is that these cell inclusions are portions of broken-down nuclei or else secretions of the cells. Attempts to grow these bodies have, as a rule, resulted in failures, or, if grown, have not reproduced the disease in other animals. Up to the present no specific micro-organism has been demonstrated in cancer or in any other spontaneous new growth. Attempts to transplant human carcinoma from one person to another have not as yet been successful, although transplants of tumors have been made in many generations of mice and rats. Success occurs only when the growth has been implanted in other animals of similar kind. Such experiments, nevertheless, do not show the necessity of any low form of organism. It is well known that portions of skin can be transplanted from one person to another. More recently attempts have been successful in causing sarcoma-like tumors to grow in hens that have been inoculated with a filtered extract of the growth.

5. Theory Of Decreased Tissue Resistance

Ribbert's theory-is that the connective tissue loses its normal resisting power or "tissue tension," and by doing so allows the epithelial cells to undergo abnormal proliferation. The essential feature is that the cells must become separated from their normal relation to the surrounding tissue and then take on an active growth.

This theory is not satisfactory, as in the healing of wounds scattered groups of epithelial cells are found constantly which have actively pushed over and into the underlying granulation tissue, yet tumor formation in such cases is exceptional.

6. Nervous Theory

This was to the effect that through disturbances of the trophic nerves the tissues were able to undergo an overgrowth. Certain investigators have shown what seems to be a definite correspondence between the occurrence of skin cancer and the distribution of certain cutaneous nerves.

Of other theories, that of Adami is interesting. According to him the cell, instead of adhering to the habit of function, has reverted to an earlier stage, one in which the habit of growth predominates. The energy, therefore, that primarily was devoted to the performance of function is now directed to growth, and there is then formed a mass of uncontrolled cells. This view gains support in that many of the malignant tumors appear at a time when the function of the tissue is at a decline.

A general survey of the field would indicate that true tumors are not parasitic in nature. That the condition is one in which the potential activity of the cell is sufficient to give rise to unlimited growth if the restricting barrier, whatever that may be, be removed. To that extent the cancer cell itself may be considered in the light of a parasite. It has been shown that in the rat certain tissues ordinarily resistant to the implantation of bits of rat tumor may become susceptible as a result of some pre-existing irritation. To this condition has been applied the term precancerous stage. In the human being there are certain lesions of the mammary gland that although in themselves non-cancerous may later become so.