A plunger (4) having engaging means and a lever (5) are arranged in a body (1). The lever has a first pivot that is stationary with respect to the body in the direction of an axis, a second pivot, and a third pivot formed by an engaging element of the lever. The pivots are arranged in such a manner that a rotation of the lever with respect to the first pivot moves the second pivot and the third pivot in the same direction with respect to the axis. The engaging element and the engaging means are shaped such that the lever is disengaged from the plunger when the second pivot is moved relatively to the plunger in the proximal direction (30), and the plunger is engaged with the lever when the second pivot is moved in the distal direction (20).