Surgical device for stabilising or immobilising moving tissue, for example a part of a beating heart, or for positioning organs or for positioning and holding surgical instruments and apparatuses during a surgical intervention, comprising: a base member which can be fastened via a fastening portion to a retaining device, in particular a sternum spreader, situated in or at an operation site; a flexible arm, in particular an articulated arm, which can be fastened to the base member, which arm can be brought into different positions and/or states and at the free end of which at least one retaining element is arranged; a manually controllable power source; and a tightening mechanism (44, 48) by which the arm can be fixed in a desired position and which has: a pull rope which is guided through a plurality of articulated elements (82; 82'; 82") of the arm that are movable relative to one another and in particular formed in a complementary manner, and by which the articulated elements (82; 82'; 82") can be frictionally braced against each other; and a cylinder-piston mechanism which can be actuated via the power source for actuating the pull rope, characterised in that the device has a modular design and the arm is mechanically, in particular positively, connectable to the base member, the tightening mechanism (44, 48) is automatically tightened by means of spring force, and the tightening mechanism (44, 48) is released by means of the power source, and a slider of the cylinder-piston mechanism can be brought into an extended position by means of the power source, so that, when the mechanical connection between the arm and the base member is made, at the same time the pull rope is coupled to the slider of the cylinder-piston mechanism. a base member which can be fastened via a fastening portion to a retaining device, in particular a sternum spreader, situated in or at an operation site; a flexible arm, in particular an articulated arm, which can be fastened to the base member, which arm can be brought into different positions and/or states and at the free end of which at least one retaining element is arranged; a manually controllable power source; and a tightening mechanism (44, 48) by which the arm can be fixed in a desired position and which has: a pull rope which is guided through a plurality of articulated elements (82; 82'; 82") of the arm that are movable relative to one another and in particular formed in a complementary manner, and by which the articulated elements (82; 82'; 82") can be frictionally braced against each other; and a cylinder-piston mechanism which can be actuated via the power source for actuating the pull rope, characterised in that the device has a modular design and the arm is mechanically, in particular positively, connectable to the base member, the tightening mechanism (44, 48) is automatically tightened by means of spring force, and the tightening mechanism (44, 48) is released by means of the power source, and a slider of the cylinder-piston mechanism can be brought into an extended position by means of the power source, so that, when the mechanical connection between the arm and the base member is made, at the same time the pull rope is coupled to the slider of the cylinder-piston mechanism. a pull rope which is guided through a plurality of articulated elements (82; 82'; 82") of the arm that are movable relative to one another and in particular formed in a complementary manner, and by which the articulated elements (82; 82'; 82") can be frictionally braced against each other; and a cylinder-piston mechanism which can be actuated via the power source for actuating the pull rope, characterised in that the device has a modular design and the arm is mechanically, in particular positively, connectable to the base member, the tightening mechanism (44, 48) is automatically tightened by means of spring force, and the tightening mechanism (44, 48) is released by means of the power source, and a slider of the cylinder-piston mechanism can be brought into an extended position by means of the power source, so that, when the mechanical connection between the arm and the base member is made, at the same time the pull rope is coupled to the slider of the cylinder-piston mechanism.