_Causes._ 1st, Mechanical obstruction to the onward flow of blood, as in the case of disease of the lungs hindering the flow of blood from the right heart; disease of the right heart allowing a reflux of blood into the veins; or pressure by tumors or otherwise on the great or small venous trunks. If in the heart or lungs the whole systemic venous system becomes the seat of passive congestion; if in a single venous trunk then only the parts the venous radicles of which are tributary to this. We find examples of this in phlebitis, in compression by the swellings of strangles, in the result of a bandage or ligature tied round a limb at some distance from its extremity, and in the compression of the iliac veins by a gravid womb. 2d. Diminished force of the blood current in the veins, as from old age or great debility and especially from weakness of the heart’s action. Also from disease of the arterial coats which impairs their tonicity. The force being too weak to force the blood actively through the capillaries and veins, it becomes unduly charged with carbon dioxide and other products of tissue waste, so that nutrition suffers and the walls of the capillaries lose their vital force. This condition is aggravated in the hind limbs by the distance from the heart, and the dependent position, and in decubitus by the compression of the vessels of the limbs. Also by injuries to the vaso-motor nerve supply as œdema appeared in the hind limb after tying of the femoral vein in animals the abdominal sympathetic of which had been cut, but not in animals in which this nerve was left in its normal condition (Ranvier). 3d. Gravitation in weak states of the circulation must be looked upon as a cause of venous congestion. This is seen in the examples of hypostatic congestion and œdema seen in the lungs and other internal organs in low conditions and in the advanced stages of debilitating diseases, and in certain cases of stocking of the limbs in horses. 4th. Valvular insufficiency of the left heart and tumors or aneurisms interfering with circulation through the aorta, cause passive congestion of the pulmonary veins and œdema of the lung. 5th. Tumors and diseases of the liver determine passive congestion of the portal system and ascites. 6th. Passive congestion is very liable to take place in an organ the functions of which are impaired as in a paralyzed part. In this the hyperæmia may start in the capillaries and extend to the veins or even to the arteries.