3. Nature of Termination.—In one or two well authenticated cases a malignant tumour has disappeared of its own accord with out 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 and natural tendency of a malignant tumour is to cause death.
Although the separation of new growths into the malignant and non-malignant groups is support ed by their respective clinical and histological characters, the differ ence between them is probably one of degree rather than of kind. It is beyond doubt that occa sionally a tumour, which for years has been devoid of malig nant characters may suddenly be come cancerous. Moreover, cer tain 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 infil trating tumours, they are inno cent in that they never form metastatic deposits. Therefore it seems that malignancy or the re verse 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 of Tumours.—In tumours it is generally true that the essential cells behave approximately in the same fashion as in the corresponding normal tissue. In a duct or tubular gland the lining columnar cells are arranged in a single layer side by side to leave a lumen, and in tumours formed from duct-like struc tures the same tubular arrangement is recognizable. Similarly, just as in a secreting gland the epithelium multiplies and breaks down into the secretion so in tumours formed from secreting glands the cells multiply and fill the acini (spheroidal cell carci noma) or in part break down into a representative of the normal secretion (adenoma). So also the characteristic of a connective tissue cell that it forms some substance around it, e.g., fibrous tissue, cartilage or bone, is reproduced more or less faithfully in the connective tissue tumours. 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. These are
shown in Figs. 'a, i 2a, 2b, 3 and 4.
In the adenoma the individual gland cells and their mutual re lations resemble those of the normal breast from which the adenoma is derived and follow the normal arrangement very closely though the collections of cells are irregularly disposed throughout the stroma. Finally the growth is surrounded by a well defined capsule of fibrous tissue.
In the carcinoma, the re semblance of cancerous tonormal breast cells is slight ; their mutual arrangement is disturbed the collections of cells are arranged in disorderly masses not enclosed by any semblance of a capsule, but transgressing their proper boundaries and invading the underlying muscles. Figs. 3 and 4 show semi-diagrammatically analogous changes in the formation of an innocent and a malignant tumour of the intestinal mucosa and the skin respectively.
Speaking generally it may be said that the cells of an epithelial non-malignant growth are fully differentiated and typical of the normal, whereas the cells of a carcinoma show less perfect dif ferentiation, are in some degree atypical and resemble rather the actively growing cells found at an early stage 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 con nective tissue, but the resemblance to adult connective tissue is almost non-existent and the cells are essentially of an embryonic type. These differences between the innocent and the malignant cell bear out the well-established physiological rule that the less the functional development of a cell or tissue the greater its power of growth.
In theory it is always possible to distinguish with certainty be tween an innocent tumour and a cancer by means of the micro scope. In practice this is, unfortunately, not the case. There are some tumours whose histological appearances are on the border line between the two conditions, and often these are the very cases in which the clinical features give no direct clue to their nature. In such circumstances it is only by taking into consideration every detail, that an opinion can be formed.