Final answer:
The medial meniscal cartilage, attached to the articular capsule and tibial collateral ligament, separates from the fibrous capsule in the center of the knee and provides padding and stability to the joint.
Step-by-step explanation:
The structure that separates from the fibrous capsule in the center of the knee is the meniscal cartilage. Specifically, it is the medial meniscus that is attached at its outer margin to the articular capsule and tibial collateral ligament, allowing less movement compared to the lateral meniscus. The meniscal cartilage's primary purpose is to provide padding between the bones of the knee joint, help to fill the gap between the round femoral condyles and flattened tibial condyles, and improve the fit between these bones. However, portions of the menisci that lack an arterial blood supply tend to heal poorly if damaged.