4. A non-malignant growth never recurs after operation. The boundaries of the growth are so well defined that complete removal is usually easy, and the operation is a simple and satisfactory pro ceeding. Apparent exceptions to this statement as in adenoma of the thyroid are due to later growth of an adjacent but independent minute neoplasm set free by removal of the pressure exerted by the larger one.
Malignant Tumours, or Cancers. (See CANCER.)-There are three main varieties of malignant tumour : the Sarcomata, arising from the connective tissues; the Carcinomata, arising from epi thelial tissues; the Endotheliomata, arising from endothelium. It is customary to describe them as cancers. The main features of these tumours are as follows : I. Infiltrating Nature.—A cancer follows a course very different from that of an innocent tumour. Its growth has no appointed termination, but continues until death ; moreover, it is more rapid than that of the innocent tumours, and does not permit of the formation of a capsule by the neighbouring tissues. In conse quence such a tumour shows no well-defined boundary, but from its margin fine tendrils of cancer cells, travelling by the lymphatic channels in all directions invade the surrounding parts and induce pressure atrophy of the essential tissue cells in the process. Thus a cancer of the breast will attack both the skin covering it and the underlying muscle and bone ; a cancer of the womb will eat its way into the rectum, bladder and even sacrum, until these organs become to a great extent replaced by cancer cells, and can no longer perform their proper functions.
2. Formation of Secondary Growths.—In addition to this spread of growth by direct extension, another characteristic of malignant tumours is their tendency to reproduce themselves in parts of the body far removed from the original site. These secondary de posits, or metastases, are due to the tumour cells making their way through the walls of the small lymph and blood vessels and becoming detached in small groups by the force of the circulation, by which they are carried to some distant part of the body, there to continue their career of uncontrolled growth.
The sarcomata and carcinomata differ somewhat as regards the path of dissemination. The sarcomata are tumours with blood
containing spaces between the cells ; consequently dissemination usually occurs by way of the blood-stream, and the commonest site for the secondary deposits of sarcoma is the lung. In carcinomata the blood vessels are well formed, and the growth invades the small lymph channels, and the first metastases are in the lym phatic glands ; later, these deposits may occur throughout the body, particularly in the liver and other abdominal organs, the lungs and the bones.
The formation of metastases is of the utmost importance from a clinical point of view, as the success of an operation for cancer depends on the removal of all cells of the growth. For instance, a few months after the first appearance of a cancer of the breast the axillary lymph glands will be found to be hard and enlarged.
This means that some of the cells of the primary growth have beer' carried in the lymph stream to these glands, and have multiplied there ; consequently any opera tion for the removal of the can cer of the breast must include the removal of these glands. If the breast tumour only be taken away the growth will continue unchecked in the glands. In many cases where there is no evident enlargement the micro scope reveals cancer cells ; and a certain opinion can only be given after a microscopical exam ination.
In operations for cancer of the breast or tongue the modern practice is to regard the lymphatic glands of the axilla or neck respectively as infected in every case, however early it be, and to remove them accordingly. In other parts of the body where the glands are inaccessible, the only solution of the difficulty is to urge the removal of the tumour at the earliest possible moment.
The frequency and rapidity of metastasis formation varies greatly. As a general rule cancer of the breast is more liable than other forms of growth to be followed by widespread secondary deposits. On the other hand, in cancer of the skin secondary in fection is usually confined to the neighbouring lymphatic glands, and in a large proportion of cases affecting the neck of the womb, the cancer extends locally but forms no metastases whatever.