(Chapter 32) Footnote 17:

See Galen De usu partium 3.253.9-254.5: “The part called the millstone by some and the kneecap by others is a cartilaginous bone covering the whole front of the articulation; it prevents the femur from slipping forward . . . and it is of considerable assistance in preventing falls, especially on slopes where the whole body is inclined forward. I have seen clear proof of this in a certain youth . . . [whose] kneecap was torn loose from the ligaments and, separating from the knee, moved up along the femur, making it so dangerous for him to bend his knee and walk down inclines that he needed a staff in such places.” (tr. May 1968, pp. 196f.).