Patella

The patella (/pəˈtɛlə/), also known as the kneecap or kneepan, is a thick, circular-triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as mice, cats, and birds, but not in whales or most reptiles and amphibians such as snakes or frogs.