There can scarcely be any independent circulatory disturbances in the pleura. Hyperemia exists in cases of severe dyspnoea, and this may result even .in subpleural haemorrhages, producing Petechiae, which are regarded as of diagnostic significance in death from suffocation (see p. 719).

Haemorrhage into the pleural sac arises as a result of wounds, rupture of aneurysms, and sometimes from tubercular or cancerous new formations in the pleura, more particularly the latter.

Hydrotborax is a dropsy of the pleural sac. Hydrothorax of local origin is brought about by conditions which obstruct the veins or lymphatics. It is induced not infrequently by malignant tumours of the mediastinum or of the pleura itself. It is frequently part of a general dropsy, as in Bright's disease, cardiac disease, or anaemia. The exudation consists of clear watery fluid. It is often limited by adhesions which latter may also be dropsical. The lung is compressed in proportion to the amount of fluid.

Chylous hydrothorax is a rare form, arising from rupture of the thoracic duct (see p. 117).