The local dropsies are often characterized by special terms. Hydrops ex vacuo is applied to the collections of fluid found in closed cavities with unyielding walls, as the cranium and thorax, or to the recurrence of fluid in cavities from which the same has been rapidly removed, in the absence of inflammatory disturbances. Collateral oedema is usually applied to the association of oedema with inflammatory disturbances, and represents an extension of the inflammatory process to the region concerned. Oedema of the glottis and circumscribed oedema of the lung are instances. The term hypostatic oedema is often used to designate the association of oedema and inflammation, the former caused by the latter, and to indicate the effect of gravitation in the localization of oedema from the general causes already mentioned. Another localized oedema of interest, from its frequent occurrence and {72} importance, is oedema of the lungs, often taking place toward the end of life, at times quite suddenly. This form has usually been attributed to increased transudation from arterial congestion or venous stagnation. The former view is directly refuted by the experiments of Welch,[28] who offers the explanation now accepted. With the obliteration of three-fourths of the arterial supply to the lungs of the animals experimented upon, no oedema resulted from the assumed collateral fluxion into the branches of the pulmonary artery which were left open. The obliteration of the same area of venous distribution was necessary before the occurrence of oedema. Oedema of the lungs was further found to result from a ligature of the aorta near the heart. The comparative frequency of oedema of the lungs in man, and the rarity of such extreme mechanical disturbances as those produced experimentally, led Welch to paralyze the left ventricle. The conditions as regards the pulmonary circulation then corresponded with those mentioned as causes for oedema from venous obstruction. The continued action of the right ventricle forced blood into the pulmonary capillaries, where it was compelled to accumulate in consequence of the inability of the left ventricle to receive and expel it. Welch consequently regards the immediate cause of this form of pulmonary oedema as a predominant weakness of the left ventricle. A weak heart does not suffice for the production of the oedema, since this condition is not found when both ventricles are alike enfeebled. [Footnote 28: _Virchow's Archiv_, 1878, lxxii. 375.] Degenerations.