Unidirectional valves that prevent the blood from flowing backward are found in the

The answer is veins.  These venous valves prevent the backflow of blood  in these vessels. This is because blood in the veins has lower pressure,  because it is flowing back to the heart, and therefore can be overcome by forces such as gravity and pull. The valves are similar in structure to semilunar valves of the heart.  

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you did not attach the discourse, but i assume it to be the discourse on the origin and basis of inequality among men (the original titile in french is: "discours sur l'origine et les fondements de l'inégalité parmi les hommes"), also know as "second discourse". in this discourse, jean-jacques rousseau emphasizes that private property is the basis of all inequalities.

therefore, what enabled the old regime to maintain as long as it did was private property.