Final answer:
A bursa that is wrapped around a tendon is known as a subtendinous bursa, functioning to reduce friction between the tendon and the bone. Tendon sheaths, though similar, are distinct structures that provide smooth movement of tendons across joints.
Step-by-step explanation:
A bursa wrapped around a tendon is called a subtendinous bursa. This bursa serves the purpose of reducing friction between the tendon and the bone, thus protecting the tendon as it moves during muscle contraction and joint movements. It is similar to a tendon sheath, but specifically, a tendon sheath is the connective tissue sac that surrounds a muscle tendon at places where the tendon crosses a joint, allowing for smooth motions of the tendon.
For example, the subacromial bursa protects the tendon of the shoulder muscle as it passes under the acromion of the scapula, and the suprapatellar bursa separates the tendon of the large anterior thigh muscle from the distal femur just above the knee. These bursae contain lubricating fluid vital for the smooth operation of joints and the tendons that move across them.