Connective tissue sacs lined with synovial membranes that act as cushions in places where friction develops are called

The answer is the Bursae. A bursa (plural bursae) is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of viscous synovial fluid. It provides a cushion between bones and tendons and/or muscles and also reduces friction. The major bursae are located next to the tendons near the large joints, such as elbows, shoulders, hips, and knees. 

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