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Rh own accord without any treatment, and a natural cure may be said to have occurred. But these form such an infinitesimal proportion Of the whole that they do not affect the general truth of the statement that .the universal tendency of a malignant tumour is to cause death.

Although the separation of the new growths into two groups is supported by certain fairly definite characteristics, both clinical and histological, yet it seems likely that the difference between them is one of degree rather than of kind. There is every reason to believe that the same perverted impulse may give rise either to an innocent or a cancerous growth, the issue depending in part on the intensity of the impulse, and in part on the resisting powers of the tissues in which the incipient tumour cells lie. Such a hypothesis is supported by the analogy of the microbial infections, where the final outcome of life or death depends no less on the defensive mechanism of the individual than on the virulence of the infecting organism. Again, it is beyond doubt that occasionally a tumour, which for years has been void of the least taint of malignancy, may become converted into an active cancer. Moreover, certain tumours seem to lie on the border line, for example, rodent ulcers and cancers of the parotid gland. These are malignant in that they are undoubtedly infiltrating tumours, they are innocent in that they never form metastatic deposits. Therefore it seems that malignancy or the reverse is not to be regarded as an absolute and constant attribute of any particular tumour or class of tumours, but rather as an expression of the balance struck in the conflict between the opposing forces of the tumour and its host. .

Histology ol/ Tumours.-On examining a microscopical preparation of an epithe ial tumour it is found to be built up of two distinct elements. There are the epithelial cells, which form the essential part of the tumour; there is a network of fibrous connective-tissue cells, which acts as a supporting framework to the epithelial elements, and is known as the stroma of the tumour. This twofold structure is seen in all the epithelial tumours, both non-malignant and malignant, and in the case of the latter it is a general rule that the greater the proportion of epithelial to connective-tissue elements the faster will the tumour grow. On the other hand in the connective tissue tumours (with the exception of the sarcoma ta) this compound structure is absent and there is only one type of cell present; thus a fatty tumour consists merely of fat cells; a bony tumour of bone cells, and so on.

To understand clearly the differences and likenesses that obtain between the malignant and the non-malignant new growths it is necessary to compare the histology of the two groups. Figs. Ia, Ib represent an innocent tumour (adenoma) of the breast. Figs. 211, 2b a cancer (spheroidal-celled carcinoma) of the breast. Fig. 3 an innocent tumour (papilloma) of the skin. Fig. 4 a cancer of the skin.

Fic. la.-Diagram to show the relations of an innocent tumour (adenoma) of the breast.

a, Tumour; b, normal breast tissue; c, underlying muscular tissue.

FIG. Ib.-Microscopical appearances of an adenoma of the breast. (Drawn from an actual specimen.) a, Tumour cells; b, fibrous connective tissue. In the adenoma the individual cells bear the closest resemblance to the glandular cells of the normal breast from which they are derived. In addition they tend to follow the normal very closely in their arrangement, so that at times it is difficult or impossible to decide which is tumour and which is healthy breast substance. Finally the growth is surrounded by a well defined capsule of fibrous tissue.

FIG. za.-Diagram to show the relations of a malignant tumour (spheroidal cell carcinoma) of the breast. Note the in drawing of the nipple by the growth and the infiltration of the underlying muscle.

a, Tumour; b, normal breast tissue; c, muscle.

Fig. 2b.-Microscopical appearances of a carcinoma of the breast. (Drawn from an actual specimen.) a, Tumcur cells; b, stroma.

In the carcinoma, the individual resemblance is present, though less conspicuous, as many of the cells are irregular in size and shape. But the similarity of the arrangement is very hard to make out or even absent. The cells are arranged in disorderly masses; they'are not enclosed by any semblance of a capsule, but tend to transgress their proper boundaries and invade the underlying muscles. Figs. 3 and 4 show analogous changes in an innocent and in a malignant tumour of the skin.

FIG. 3:*'NOU-malignant tumour (papilloma) of the skin. The tumour is formed by an outward proliferation of the cells of the epidermis, but these cells show no tendency to invade the underlying connective tissue or muscle. (Semidiagrammatic) a, Normal skin. d, Muscular tissue. b, Epithelium or epidermis. e, Papilloma. c, Connective tissue.

Speaking generally it may be said that the cells of an adenoma are fully differentiated and typical of the normal, whereas the cells of a carcinoma show less perfect differentiation, are in some degree and resemble rather the actively growing cells found at an early § tage of embryonic life. . But it is in the cells of a sarcoma that the widest departure from type is seen. A sarcoma is a malignant growth arising from connective tissue, but the resemblance to adult