A mixing device for mixing first and second substances to form a mixed substance includes a housing having first and second inlets for first and second substances and an outlet for the mixed substance. A flow channel between the first inlet and the outlet and has a flow axis therebetween. The flow channel has a Venturi section having a side wall. A lateral feed opening in the side wall in a first lateral direction is connected to the second inlet, and the opening has a feed axis in said first lateral direction substantially perpendicular to the flow axis of the flow channel. Upstream from the lateral feed opening the side wall of the Venturi tube section forms a first constricted part having a first width in the first lateral direction and a second width perpendicular to the first lateral direction. The second width is larger than the first width.