It is well known from experiments on animals that various living, normal tissues when transplanted to remote parts of the same individual or to other individuals may continue to grow. Cohnheim claims, as has been previously stated, that a distinction is to be drawn in this respect between the tissues of the adult and the foetus, where the genesis of tumors is concerned. This observer, in connection with Maas,80 has found that the transplanted material (periosteum), although growing for a while, disappears at the end of five weeks, and it is asserted that fragments of tumors, when transferred, suffer a similar fate. Wile,81 on the contrary, who has experimented with reference to the fate of transplanted tissues and portions of tumors, reports that one hundred days after the transfer of periosteum the lung was found to contain several centres of ossification. He regards the latter as proceeding from the fragments of periosteum introduced into the jugular vein, and his results thus widely differ from those of Cohnheim.
80 Virchow's Archiv, 1877, lxx. 161.
81 The Pathogenesis of Secondary Tumors, reprint from Philadelphia Med. Times, July, Aug., and Sept., 1882.
Notwithstanding the numerous experiments which have been made in various parts of the world to excite the growth of transplanted bits from tumors, most of them have terminated unsuccessfully. Although a temporary growth of fragments of tumors has taken place after transplantation, their eventual disappearance has usually occurred. Cohnheim lays stress upon this fact in connection with his theory of the origin of tumors. He considers that the fragments of tissue and tumors disappear in consequence of the inability of the foreign particles to withstand the metamorphosis of physiological tissues. If this opposition is neutralized, the existing germs of tumors become capable of development. Wile, however, found that eight weeks after the introduction of a bit of cancer into the lung of an animal the fragment had increased nearly twice in size. He also refers to the positive experiments of Newinsky,82 who transplanted a bit of cancer from a dog to the subcutaneous tissue of another, young dog, and found, after five months, not only an ulcerating cutaneous cancer at the place of inoculation, but also a metastatic nodule of the size of a hazel-nut in an axillary lymphatic gland.
82 Allgem. medicinische Central-Zeitung, 1876, lxxi. 875.
For the present consideration it may be borne in mind that fragments of normal (foetal) tissues, as shown by the experiments of Zahn and Leopold, when introduced into the organs of animals, may become enlarged. It is also certain that bits of tumors, after their introduction into the tissues and organs of animals, have become increased in size. What their eventual fate might have been does not appear; and herein lies the weak point of the experiments with reference to the production of secondary tumors. For such experiments to be regarded as crucial it is necessary that a large number of previously healthy animals, after inoculation with fragments of morbid growths, should present in various parts of the body well characterized tumors whose structure should be like that of the particles introduced.
The experiments above referred to are of value in confirming the views concerning the generalization of tumors which have been generally admitted since Virchow's discoveries with regard to the phenomena of embolism.
Tumors are said to become generalized when they appear not only in various systems of the body, but in various organs and tissues. They are found usually in considerable numbers, and with such differences in size, shape, and appearance as to indicate different ages. Such tumors are regarded as arising directly or indirectly from a common source. This source is called the primitive or primary tumor, and its derivatives the secondary tumors. The latter are usually considered as the direct descendants of the former, although their relation may be that of several successive generations.
The primitive tumor in its growth may extend into lymphatics and blood-vessels, as has already been suggested. Such an extension may be so little obvious when the tumor is removed by the surgeon that all diseased tissues are apparently separated from the body. A recurrence of the tumor is said to take place when the growth returns in the cicatrix, frequently in a multiple form. The explanation of such a recurrence is based upon the probable presence, at the time of the operation, of fragments of the tumor within the tissues forming the base and edges of the wound. During and after the healing of the wound their growth is supposed to continue till they become apparent as small tumors. The progress of these recurrent tumors is at times extremely rapid, and they may attain a considerable size in the course of a few weeks. Such nodules are secondary in point of time, although they were actually a part of the primary growth.
Secondary nodules in descent as well as time are those which appear at distant parts, often after the discovery of the primary tumor. Such nodules are regarded as resulting from the transfer of particles of various size from the primitive growth, either through the lymph-vessels or blood-vessels. If the invasion of the body takes place through the former, the fragments may be floated along to the nearest lymphatic gland, where it remains when too large to pass through. If it retains the capacity of growth or of stimulating a like growth, there results a more or less complete transformation of the gland into a morbid tissue like that from which the fragments came. Adjoining lymph-glands may become infected from the first, until eventually an entire series becomes more or less completely transformed into morbid growths. A like invasion of the lymphatic glands may take place through a continuous extension along the lymph-vessels; and it is not rare to find the sub-pleural or sub-peritoneal lymphatics as an elevated meshwork in consequence of the neoplastic growth within them. Such a method of extension may take place when a cancer of the stomach or liver is associated with a cancer of the pleura, the intervening lymphatics of the diaphragm offering a direct and continuous communication.