Part 61

Tumors arise without obvious cause. Concerning the ultimate cause of tumor formation we are absolutely ignorant. Various theories have been advanced from time to time, but none of them have been applicable to more than a limited number of cases. The most important theories may be briefly mentioned.

(1) The theory of tumor diathesis.--Bilroth taught that tumors are caused by a peculiar predisposition consisting of a diseased state of the fluids of the body. This constitutional taint might be acquired, but, having been acquired, is also hereditary. This theory is known also as the heredity hypothesis, but, while it is true that heredity appears to play some role in the causation of certain neoplasms, its application is too limited to make it of value.

(2) The mechanical or irritant theory.--Virchow assumed that tumors arise as the result of previous irritation of the part. This has been noticed particularly in the case of certain cancers. They frequently develop on the edges of old ulcers, thus being dependent apparently on chronic irritation. Cancer of the lip in pipe smokers is a case in point. Cancerous tumors of the skin often develop on the arms of workers in paraffin, tar, or soot, the chemical irritation of these substances being the cause. On the contrary, the proportion of those thus affected among the exposed is very small and forces the conclusion that if the real cause were in the irritation vastly more cases would occur.

(3) The theory of nervous influence.--That is based upon (a) the observed fact that tumors occur more frequently in man and the higher animals than in those lower in the scale, among which the nervous system is less highly developed; (b) that certain formations seem to be directly connected with nerve distribution, while others have been associated with alternations in neighboring nerve trunks.

(4) The embryonal theory.--This is known also as Cohnheim's hypothesis. In early fetal life there occurs a production of cells in excess of those required for the construction of the various parts of the body, so that a certain number of them are left over in the fully developed tissue or become misplaced during the sorting of cells for future development of tissues and organs. These cells lie dormant until favorable conditions arise or until some sufficient stimulus is applied, when, released from their inactivity, they begin to reproduce and grow. Not being normally related to their site, they lack the controlling and limiting influences of the part, and, their embryonic character enduing them with a most potent proliferating power, they develop in a lawless and unrestrained manner. There are tumors whose existence can be explained only on these grounds. Still, this theory falls far short of answering the question as to the origin of tumors.

(5) The parasitic theory.--This is not only one of the latest, but, merely as a hypothesis, it is the most attractive and plausible of all. The serious objections to it, however, are the almost uniform failure that has met the attempts to transplant these tumors from one animal to another and the absence of any constant variety of organism in them. Several forms of parasites have been found in certain tumors, but nothing definite has been shown with reference to the relation they bear to the causation of the neoplasm.

CLASSIFICATION OF TUMORS.

In Senn's work on tumors occurs the following: "A uniform system of classification of tumors is one of the great wants of modern pathology, and all attempts in this direction have proved failures." It would be folly, therefore, to burden the pages of a work of this kind with one or several of the proposed systems which have, admittedly, at some important point, failed of their purpose. Since the value of this chapter depends chiefly upon its practical character, which in turn is measured by its aid in diagnosis, prognosis, and treatment, the old but important clinical division is here adopted.

Tumors are either malignant or benign. The essential difference between the two classes is that while benign tumors depend for their ill effects entirely upon their situation, malignant neoplasms wherever located inevitably destroy life. The clinical features of each group are in many cases sufficiently marked to distinguish them.

MALIGNANT TUMORS.

(1) These are invariably pernicious, and from the beginning tend to destroy life.

(2) The cellular element predominates; therefore they grow rapidly.

(3) Possessing no capsule, they infiltrate surrounding tissues.

(4) They infect adjacent lymph glands.

(5) They recur even after complete removal.

(6) They give metastasis; that is, they become disseminated in different organs.

(7) Their presence develops a progressive emaciation.

BENIGN TUMORS.

(1) These in and of themselves do not tend to produce death.

(2) As the cellular element is not liable to predominate, they grow slowly.

(3) They are encapsulated, and when diffuse do not infiltrate surrounding tissues.

(4) They do not infect adjacent lymph glands.

(5) They do not recur after complete removal.

(6) They do not manifest metastasis.

Benign tumors, though harmless, may, by the accident of their location, indirectly produce death. Mere pressure on the brain substance of an otherwise innocent tumor, compression of the blood supply for vital organs, growth in such manner as to cause obstruction in the alimentary tract or pressure upon nerves, may cause death, or, prior to death, so combine the effects of anemia (deficiency of blood), starvation, and pain, with its consequent restlessness, as to produce a veritable cachexia (condition of general ill health).

On the other hand, a malignant tumor in its primary growth may so implicate a vital organ as to destroy life before metastasis can occur or even before cachexia can develop. Thus, to the untrained observer, environment may so operate as to cause these two classes of new growths to simulate each other. The boundary lines may seem to overlap. It is here that the microscope, as the court of last appeal, adjudicates positively in the diagnosis between these two clearly marked divisions.

It may almost be asserted that a true classification of tumors can not be made until we know more about the cause of them. The arrangement here presented is offered to meet the practical needs of the veterinarian, student, and farmer rather than of the pathologist.

We may roughly divide the tissues of the body into structural and lining tissues. The structural tissues are composed of the tissues of special function and simple connective tissues. The lining or covering tissues, both internal and external, are known as epithelium.