(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.