A Treatise on Painting/Chapter 97
Chap. XCVII.—Of turning the Leg without the Thigh.
It is impossible to turn the leg inwards or outwards without turning the thigh by the same motion, because the setting in of the bones at the knee is such, that they have no motion but backwards and forwards, and no more than is necessary for walking or kneeling; never sideways, because the form of the bones at the joint of the knee does not allow it. If this joint had been made pliable on all sides, as that of the shoulder, or that of the thigh bone with the hip, a man would have had his legs bent on each side as often as backwards and forwards, and seldom or never straight with the thigh. Besides, this joint can bend only one way, so that in walking it can never go beyond the straight line of the leg; it bends only forwards, for if it could bend backwards, a man could never get up again upon his feet, if once he were kneeling; as when he means to get up from the kneeling posture (on both knees), he gives the whole weight of his body to one of the knees to support, unloading the other, which at that time feels no other weight than its own, and therefore is lifted up with ease, and rests his foot flat upon the ground; then returning the whole weight upon that foot, and leaning his hand upon his knee, he at once extends the other arm, raises his head, and straightening the thigh with the body, he springs up, and rests upon the same foot, while he brings up the other.