The simple tumour is mere enlargement of a part, from the infiltration of solid matter deposited by its bloodvessels. There can be little doubt that the action which lays the foundation of such enlargement is inflammatory: in consequence of inflammation of the tissue, lymph is effused into the cellular substance during the progress of the incited action; and after it has subsided, the dilated and debilitated vessels probably do not regain their condition, as to size and vigour, but remain somewhat dilated, and continue to free themselves from portions of their contents; thus the cellular tissue is opened out in proportion as the infiltration advances, and the process may be occasionally accelerated by fresh attacks of subacute inflammatory action. The patient at first feels pain, heat, &c., as in an inflammatory tumour; these afterwards abate, and ultimately go off entirely; and during the increase of the swelling, little or no pain is experienced, unless when these subacute inflammatory attacks supervene, and then it is but slight; or unless the enlargement be resisted by an unyielding structure, and then it is acute and troublesome. The size and rapidity of increase in such tumours will vary according to circumstances; the vessels of the part soon regain their size and action, either by the efforts of nature or of art, so that the tumour will have attained no great size, and be stationary in its progress, being denied the materials necessary for its increase. If the morbid action be thus stopped, the absorbents will remove the newly-formed matter, and restore the parts to their healthy condition. But when the deposition proceeds in a superior ratio to absorption, the new matter becomes organised, and by means of its own vessels, secretes a substance similar to itself, whereby the increase of the tumour becomes more rapid, and the new structure may attain an enormous bulk. Thus the tumour is formed, not merely by dilatation of capillary vessels, and extension of its original tissue, but by the formation of new matter, which, becoming organised, assumes a secreting power. At first the former circumstance is the chief cause of the enlargement; but after the latter process has existed for some time, the tumour loses much of its resemblance to the primary tissue, assumes a more dense structure and a different action, and therefore cannot be designated a simple enlargement.