Structure, typical or atypical. Causation, Cohnheim's theory of origin; inheritance; effect of injuries, etc.; parasitic microbes; influence of age. Growth and Extension, typical or atypical; local malignancy; metastasis and generalization of tumours; occasional malignancy of typical tumours. Classification and Nomenclature.

A TUMOUR means literally a swelling, and formerly any swelling was called a tumour. Tumor, for example, is often named as one of the cardinal signs of inflammation, and it is still customary to speak of the inflammatory tumour. But the modern use of the word is limited to a class of new-formations of which it is difficult to give a strict definition.

It may be said that tumours are pieces of tissue which have a life and growth of their own irrespective of the needs of the organism, and of the local conditions around them. In the elements of their structure, tumours do not differ from normal tissues. They are nourished by the same blood, the blood-vessels are continuous with those of surrounding parts, the nerves are connected with neighbouring nerve-stems, and the tissue, if not always precisely the same as that near it, is frequently identical. But it grows independently and without apparent object. Thus a fatty tumour goes on increasing in size irrespective of the adipose tissue in which it has its seat, and the person may be reduced to the greatest emaciation, most of the ordinary fat being absorbed, while little or no impression is made upon the fatty tumour.

Structure Of Tumours

While all tumours in their ultimate structure are analogous to the normal tissues, yet some of them vary considerably from the type of tissue to which they belong. Hence, it is possible to distinguish tumours whose structure is Typical, and others whose structure is Atypical. In the former, the details, including size and shape of cells, abundance of cells, relation to intercellular substance, and so on, are all within the limits of the corresponding normal tissue. In the latter the structure varies from the normal, chiefly in respect that the cells become much more numerous, and that the intercellular substance passes into the background. We shall see afterwards that in their mode of growth, tumours show somewhat similar differences, some being typical and some atypical, and that in general the variations in mode of growth correspond with those in structure.

The terms Homologous and Heterologous were at one time employed to express the idea that the structure of a tumour may, or may not, correspond with that of the tissues of the body, the heterologous tumours being looked upon as altogether different in structure and foreign to the body. In the modern use of the words a homologous tumour is one which exists in tissue of its own kind, as an osseous tumour growing from bone. A heterologous tumour on the other hand is one which is present in a situation where no normal tissue of that kind exists, as, for example, an osseous tumour in the brain.

Causation Of Tumours

It is implied in the definition given above that the causation of tumours is exceedingly obscure. For the most part, without any apparent stimulus, they begin to grow, and go on without control.

An explanation of the origin of tumours has been suggested by Cohnheim, which may apply to some tumours but certainly does not account for all. He suggests that the primordial tumour is a piece of tissue, which in the process of development and growth has been, as it were, left over, has retained its embryonic powers of growth, and is not subject to the general laws which control the growth of the tissues.

In enforcing his argument Cohnheim points out that pieces of living tissue, especially if from young animals, will bear transplantation, and, if placed in a favourable situation, may grow to considerable dimensions. Thus, the spur of the cock, transplanted to the more vascular comb, will sometimes form a considerable tumour. Leopold has transplanted living cartilage into the anterior chamber of the eye, where it acquires vascular connections with the iris. If cartilage from an early foetus be used it will grow so as to form a tumour of some size. The earlier the period of development of the foetus the more likely is there to be a vigorous growth.

Certain facts in the pathology of tumours themselves lend some support to this view. Thus cartilaginous tumours not infrequently originate in bones, as if portions of the original foetal caVtilage had been left over and had afterwards grown. Then there occur in the subcutaneous tissue, tumours composed of mucous tissue, a form of tissue which in the embryo occupies the place of the subcutaneous fat. It is as if prions of this foetal tissue had been left over from the embryo and formed the nucleus of the tumour. Then also tumours not infrequently develop from congenital moles or soft warts. These little outgrowths have commonly a rudimentary or embryonic structure (see section on Skin Affections), but they may lie quiescent throughout life, and only occasionally start into vigorous growth.

While Cohnheim's theory may apply to some tumours, more particularly to the typical ones, it does not explain a large proportion of cases, and especially the cancers. In these the existing normal structures, usually in an increasing area, give rise to the tumour. There must be here some stimulus acting on the epithelial structures especially.

Inheritance is generally believed to play an important part in the causation of tumours. The embryonic tissue presupposed in Cohnheim's theory may, like an ordinary malformation, be transmitted by inheritance, or the predisposition to the abnormal growth in cancer may be so transmitted.

The whole subject of inheritance of tumours stands in need of elucidation. It is asserted on the one hand " we cannot overestimate the importance of inheritance in the origination of cancer" (Paget), while on the other hand it is asserted that when the families of non-cancerous persons are compared with those of cancerous, there is little or no appreciable difference in the number of cancerous relatives (Snow, Cripps). We have already seen that so far as we are able to go on demonstrable facts, inheritance deals with local peculiarities of structure. It is stated by Paget that in the transmission of cancer, the disease often passes from one situation to another, as from the breast to the stomach or uterus. This author would even seem to indicate that in transmission a cancer may become a sarcoma. Such statements run so counter to the known facts of inheritance that they throw considerable doubt on the whole subject. The strongest arguments in favour of inheritance are based on the observation of individual families, such as those recorded by Paget, Broca, and others.