For clinical purposes, tumours are arbitrarily divided into two classes—the innocent and the malignant. The outstanding difference between them is, that while the evil effects of innocent tumours are entirely local and depend for their severity on the environment of the growth, malignant tumours wherever situated, in addition to producing similar local effects, injure the general health and ultimately cause death.
Innocent, benign, or simple tumours present a close structural resemblance to the normal tissues of the body. They grow slowly, and are usually definitely circumscribed by a fibrous capsule, from which they are easily enucleated, and they do not tend to recur after removal. In their growth they merely push aside and compress adjacent parts, and they present no tendency to ulcerate and bleed unless the overlying skin or mucous membrane is injured. Although usually solitary, some are multiple from the outset—for example, fatty, fibrous, and bony tumours, warts, and fibroid tumours of the uterus. They produce no constitutional disturbance. They only threaten life when growing in the vicinity of vital organs, and then only in virtue of their situation—for example, death may result from an innocent tumour in the air-passage causing suffocation, in the intestine causing obstruction of the bowels, or in the vertebral canal causing pressure on the spinal medulla.
Malignant tumours usually show a marked departure from the structure and arrangement of the normal tissues of the body. Although the cells of which they are composed are derived from normal tissue cells, they tend to take on a lower, more vegetative form; they may be regarded as parasites living at the expense of the organism, multiplying indefinitely and destroying everything with which they come in contact.
Malignant tumours grow more rapidly than innocent tumours, and tend to infiltrate their surroundings by sending out prolongations or ; they are therefore liable to recur after an operation which is restricted to the removal of the main tumour. They are not encapsulated, although they may appear to be circumscribed by condensation of the surrounding tissues; they are rarely multiple at the outset, but show a marked tendency to spread to other parts of the body. Fragments of the parent tumour may become separated and be carried off in the lymph or blood-stream and deposited in other parts of the body, where they give rise to secondary growths. Malignant tumours tend to invade and destroy the overlying skin or mucous membrane, and thus give rise to bleeding ulcers; if the tumour tissue protrudes through the gap in the skin, it is said to fungate. In course of time they give rise to a condition of ill-health or cachexia, the patient becoming pale, sallow, feverish, and emaciated, probably as a result of chronic poisoning from the absorption of toxic products from the tumour. They ultimately destroy life, it may be by their local effects, such as ulceration and hæmorrhage, by favouring the entrance of septic infection, by interfering with the function of organs which are essential to life, by cachexia, or by a combination of these effects.
The situation of a malignant tumour exercises considerable influence on the rapidity, as well as on the mode, in which it causes death. Some cancers, such as that known as “rodent,” show malignant features which are entirely local, while others, such as melanotic cancer, exhibit a malignancy characterised by rapid generalisation of growths throughout the body. Tumours that are structurally alike may show variations in malignancy, according to their situation and to the age of the patient, as well as to other factors which are as yet unknown.
In attempting to arrive at a conclusion as to the innocence or malignancy of any tumour, too much reliance must not be placed on its histological features; its situation, rate of growth, and other clinical features must also be taken into consideration. It cannot be too emphatically stated that there is no hard-and-fast line between innocent and malignant growths; there is an indefinite transition from one to the other. The possibility of the transformation of a benign into a malignant tumour must be admitted. Such a transformation implies a change in the structure of the growth, and has been observed especially in fibrous and cartilaginous tumours, in tumours of the thyreoid gland, and in uterine fibroids. The alteration in character may take place under the influence of injury, prolonged or repeated irritation, incomplete removal of the benign tumour by operation, or the altered physiological conditions of the tissues which attend upon advancing years.
After a tumour has been removed by operation it should as a routine measure be subjected to microscopical examination; the results are often instructive and sometimes other than what was expected.
Varieties of Tumours.—In the following description, tumours are classified on an anatomical basis, taking in order first the connective-tissue group and subsequently those that originate in epithelium.