Final answer:
The tough fibrous sleeve that encloses a joint is called the articular capsule. This fibrous connective tissue provides stability to the synovial joint while allowing for movement and is different from other structures like tendon sheaths and ligaments.
Step-by-step explanation:
A tough fibrous sleeve that encloses a joint is called the articular capsule. The walls of this space are formed by this fibrous connective tissue structure that is attached to each bone just outside the area of the bone's articulating surface. This capsule includes an outer fibrous membrane that may contain ligaments, and an inner synovial membrane that secretes synovial fluid to lubricate the joint, reducing friction and allowing for smooth movements. It is vital in providing stability, while permitting a degree of movement, to the encapsulated synovial joint. The tendons that cross over these joints may be enclosed in a tendon sheath, which further helps to reduce friction and facilitate the tendon's smooth movement over the joint.
It is important not to confuse the articular capsule with other structures such as the tendon sheath — which surrounds a tendon at the points where it crosses a joint, or ligaments — which are connective tissues that connect bones to bones at a joint. The capsule is a key component in the anatomy of synovial joints, offering both protection and movement abilities.