)
_Malignant_: _Sarcoma_--II. Epithelial tumours: (1) _Innocent_:
_Papilloma_, _Adenoma_, _Cystic Adenoma_; (2) _Malignant_:
_Epithelioma_, _Glandular Cancer_, _Rodent Cancer_, _Melanotic
Cancer_--III. Dermoids--IV. Teratoma. Cysts: _Retention_,
_Exudation_, _Implantation_, _Parasitic_, _Lymphatic or Serous_.
Ganglion.

[2] For the histology of tumours the reader is referred to a text-book
of pathology.

A tumour or neoplasm is a localised swelling composed of newly formed
tissue which fulfils no physiological function. Tumours increase in size
quite independently of the growth of the body, and there is no natural
termination to their growth. They are to be distinguished from such
over-growths as are of the nature of simple hypertrophy or local
giantism, and also from inflammatory swellings, which usually develop
under the influence of a definite cause, have a natural termination, and
tend to disappear when the cause ceases to act.

The _etiology of tumours_ is imperfectly understood. Various factors,
acting either singly or in combination, may be concerned in their
development. Certain tumours, for example, are the result of some
congenital malformation of the particular tissue from which they take
origin. This would appear to be the case in many tumours of blood
vessels (angioma), of cartilage (chondroma), of bone (osteoma), and of
secreting gland tissue (adenoma). The theory that tumours originate from
foetal residues or "rests," is associated with the name of Cohnheim.
These rests are supposed to be undifferentiated embryonic cells which
remain embedded amongst fully formed tissue elements, and lie dormant
until they are excited into active growth and give rise to a tumour.
This mode of origin is illustrated by the development of dermoids from
sequestrated portions of epidermis.

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 gr