In ligating the main artery to a limb, the risk of gangrene is diminished by the simultaneous ligation of the corresponding vein. An explanation is now put forward in terms of the equilibrium between the capillaries and tissues. Ligation of an artery may cause the hydrostatic pressure in the capillaries to fall below the osmotic pressure of the plasma proteins; the circulation of nutrient fluid to the tissues is thus impaired. Ligation of the vein will raise the capillary hydrostatic pressure and may thus restore the tissue circulation.