4. Spontaneous Tumours (Neoplasm, New Growth).—The following definition of a spontaneous tumour, or tumour proper, suggested by Ziegler is perhaps the most satisfactory : "A neo plasm or tumour is a new formation of tissue, which is atypical in structure, serves no useful purpose to the whole economy, and the growth of which has no typical termination." In this definition the words "new formation of tissue" exclude the cystic swellings; the attribute "atypical in structure" excludes hypertrophies; and the final clause "the growth of which has no typical termination" excludes all inflammatory swellings which progress, however slowly, towards either suppuration or resolution and recovery.
These tumours arise by the exaggerated and abnormal prolifera tion of a single cell, or a group of cells. They increase in size solely by the multiplication of their own cells, and the only contribution which the surrounding tissues sometimes make to the process is the formation of a "stroma," or supporting framework of fibrous tissue ; and even that is wanting in many cases. Inasmuch as the newly-formed cells of the tumour resemble the parent cells the minute structure of a tumour, whatever its situation, recalls that of the tissue whence it originated. A tumour growing from skin therefore imitates in its essential features the cell-structure of normal skin ; the resemblance of some uterine tumours to normal endometrium is often so close as to make it a hard task to dis tinguish the one from the other ; whilst the similarity of bony and cartilaginous tumours to true bone and cartilage is evident to all.
This imitation of the parent type by the spontaneous tumours is one of their most remarkable characteristics, and provides a reliable criterion by which they may be separated from the in flammatory swellings, which, as mentioned above, are all built up on the same general plan. Consequently it is almost always pos sible to determine the origin of a tumour from an examination of its histological appearances ; and conversely we know that an epi thelial tumour will never spring from a connective tissue nor a connective tissue tumour from an epithelium.
The spontaneous tumours are seen in every tissue and organ of the body, though in some they are relatively infrequent. Nor are they confined to man, for they have been found throughout the vertebrate kingdom. It is often stated that a higher state of
civilization brings with it a greater susceptibility to tumour forma tion. As to this, reliable evidence is hard to obtain, and the appar ent immunity of certain native races is, at least in part due to earlier death and lack of sufficient observations.
It is usual to divide the new-growths into two groups : The Non malignant, Innocent or Benign, and the Malignant or Cancerous. Of these two groups the malignant have attracted most attention and study, on account of the danger to life which they involve, but in point of numbers they are greatly outweighed by the non malignant group. Two or more non-malignant tumours, of dif ferent varieties, are often found in the same individual ; but with the cancers this is rare.
Non-malignant Tumours.—These are usually rounded in shape. In size they vary enormously ; a fibroid tumour of the uterus or a fatty tumour may be as small as a pea or may weigh forty pounds. Often they cease growing after attaining a certain size, but there are very many exceptions to this, and it is seldom possible to predict the subsequent course of one of these growths. They possess, however, four constant characteristics by which they may be distinguished from the malignant variety.
I. A non-malignant tumour, whatever its size, remains localized to the part from which it originates. It is not an "infiltrating" growth, i.e., does not eat its way into the surrounding tissues, but rather pushes them aside, and so may be called "expansive." More over, it is separated from them by a thin but usually well-marked layer of fibrous tissue known as the "capsule" of the tumour and formed by the surrounding healthy tissues and representing their reaction to its presence.
2. Non-malignant tumours are not of themselves dangerous to life but may be so from secondary conditions to which they give rise. They may, however, cause a great deal of pain and even death, when situated in special positions. Thus, a small tumour may cause intense pain by pressing on a nerve, or dropsical swell ing of a limb by obstructing a vein, or death from suffocation by blocking the larynx.
3. Non-malignant tumours never reproduce themselves in dis tant parts of the body. More than one may be present in the same individual, but each arises independently.