All the processes already described, especially the exudative and exanthematous, are attended with oedema; and so also is the ulcerative, in various degrees and to various distances from the actual seat of disease; but oedema may originate also in the submucous cellular tissue, in consequence of many primary and secondary congestions and inflammations which occur in it and in its neighborhood.

The usual seat of the infiltration is the submucous tissue. It generally forms a pale yellowish or grayish, translucent and tense, or a flabby and movable swelling, not distinctly circumscribed, over which the upper layer of the mucous membrane is stretched: when it is considerable, it extends through the whole thickness of the mucous membrane also, which then loses the character of its structure, and may be torn with the slightest effort.

The oedema reaches its higher degrees chiefly on membranes and du-plicatures of membrane which are extended loosely and movably over thick strata of cellular tissue, especially upon the mucous membrane of the intestines and their valves and folds, and the duplicatures at the orifice of the larynx.

Its importance depends upon the processes which occasion it: it is only at certain parts that it becomes at all serious, as at the glottis, where it contracts and at last closes the orifice.