Among the local factors concerned in the development of tumours, reference must be made to the influence of irritation.  This is probably an important agent in the causation of many of the tumours met with in the skin and in mucous membranes—­for example, cancer of the skin, of the lip, and of the tongue.  The part played by injury is doubtful.  It not infrequently happens that the development of a tumour is preceded by an injury of the part in which it grows, but it does not necessarily follow that the injury and the tumour are related as cause and effect.  It is possible that an injury may stimulate into active growth undifferentiated tissue elements or “rests,” and so determine the growth of a tumour, or that it may alter the characters of a tumour which already exists, causing it to grow more rapidly.

The popular belief that there is some constitutional peculiarity concerned in the causation of tumours is largely based on the fact that certain forms of new growth—­for example, cancer—­are known to occur with undue frequency in certain families.  The same influence is more striking in the case of certain innocent tumours—­particularly multiple osteomas and lipomas—­which are hereditary in the same sense as supernumerary or webbed fingers, and appear in members of the same family through several generations.

INNOCENT AND MALIGNANT TUMOURS

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.