Injuries, Irritations, and Chronic inflammations play an important part in determining the occurrence of tumours. Thus, fractures of bones are sometimes the starting points of cartilaginous or bony tumours or of sarcomas. Cancers of the mamma are often referred to blows on the breast. The seat of election of cancers is often determined by the local conditions of exposure to irritation by friction or otherwise. For example, cancer of the lip and tongue are referred to irritation by a tobacco pipe or carious tooth; cancer of the stomach is often ascribed to prolonged irritation from alteration in the gastric juice. The frequency of cancer at the narrow parts of mucous canals, such as the pyloric orifice of the stomach, the ileo-ccecal valve, the os uteri, has also been ascribed to their special exposure to irritation.

Parasitic microbes have been suspected as forming the determining causes of tumours, but without any definite proof. We have seen that specific microbes are the exciting causes of the infective tumours, and the analogy between their mode of growth and that of malignant tumours, such as sarcomas and cancers, has led some to suppose a similar causation. An important distinction must be drawn, however, between the infective characters of the tumours due to parasitic microbes and those of tumours proper. In the former case, the infectiveness is due to the extension of a parasite, which, wherever carried, induces an effect comparable to inflammation, but with certain specific characters. The lesion is the result of irritation of the tissue to which the parasite is applied. In the case of a tumour, on the other hand, there is everywhere a proper new-formation of tissue, which of its kind is completely organized, and not merely an effect of irritation. Even when produced at a distance from the primary tumour, the secondary one is, in the details of its structure, an exact reproduction of the tissue of the original growth. Thus, while cancers differ very greatly in structure according to their seat of origin, they always, so to speak, breed true; the secondary tumours, however various their seats, having the same structure as the primary one. A striking illustration of this argument is afforded by a case recorded by the author, in which an adenoma of the thyroid gland had led to the formation of secondary tumours in the bones of the skull. The primary tumour had the structure of the normal thyroid gland, and the tumours in the skull had the same structure.

Whilst ordinary microbes seem incapable of producing such results, it must be acknowledged that facts recently observed point to the possibility of animal parasites belonging to the protozoa having to do with the causation of atypical tumours. An animal cell of a parasitic character may enter info conjunction with the living cells of the tissues, and lead to peculiar stimulations of the latter. (See under Cancer).

Age is a factor in the etiology of tumours. The frequency of tumours increases in proportion to the persons living up to the age of seventy, and is greatly increased from thirty upwards. This latter fact is due to the prevalence of cancer in the more advanced years of life. Cancer is a disease of decadence.

Growth And Extension Of Tumours

Most tumours enlarge by a new-formation of tissue within themselves, just as the normal tissues increase during the period of growth. Their growth is typical. The tissue of the tumour is sometimes continuous with that of surrounding parts, sometimes it is separated by a capsule composed of connective tissue; sometimes it begins by being continuous and afterwards gets separated. In any case the ordinary tumour simply grows by new-formation of its own tissue. It often produces effects on neighbouring parts, by pressure especially, but apart from these merely mechanical effects, it does not prejudice neighbouring structures. Tumours possessing these characters are usually called Simple or Innocent growths.

But then there are tumours of which this is not true. They have a tendency to grow into and infiltrate neighbouring tissues, presenting characters of what may be designated Local malignancy; their mode of growth is atypical. This is peculiarly the case with sarcomas and cancers. In the case of sarcomas the tumour seems to penetrate into and develop, as it were, on the mould of the existing tissue - apparently very much in the style that granulation-tissue grows into and moulds itself on a thrombus or a piece of catgut in the tissue. In the case of cancers, on the other hand, their tissue, which is epithelial in character, penetrates among the tissues, largely destroying them and producing inflammatory disturbances.

Besides this local malignancy the same classes of tumours frequently show a still more atypical growth by extending beyond their local seat. This character is expressed in the term Metastasis, which is another feature of malignancy. In this case secondary growths spring up in parts removed from the primary tumour, and doubtless something is carried from the original tumour to the remote part. In the case of cancers the epithelial processes penetrating into the tissues readily find their way into the lymphatic spaces, and portions may be carried thence to lymphatic glands.

Besides this metastasis by the lymphatic system, there is sometimes a more distant metastasis by the blood, in which case the tumour is transplanted to various parts of the body, and the term Generalization is used. This occurs usually after the lymphatic glands have been involved, but in sarcomas it is usually direct, the tumour penetrating through the walls of the vessels. It is mostly the veins which are thus opened, into, and the first extension is therefore to the lungs, but the pulmonary capillaries may be in part traversed, and implantation occur by the systemic arteries.

The material which passes from the tumour and is thus transplanted is undoubtedly the living cells of the tissue, together with such parasitic or other agents as may be proved to co-exist. This is clearly demonstrated in secondary cancerous infection of the peritoneum where the secondary tumours are not regularly distributed over the peritoneal surface, but occur here and there, or in groups, just as if solid particles had been carried and produced their effects where they got leave to lie. Then also it must be solid particles which are arrested by the lymphatic glands and give rise to the secondary tumours there.

In this connection it is important to note that the secondary tumour for the most part exactly reproduces the tissue of the original one, even to the smallest details, and it is natural under these circumstances to believe that pieces of the original tumour are actually transported.