Fig. 5. Ball and membrane model of the eye of a supine observer gazing upward. A. Restless position of the “eye,” when it is in the primary position. The foveal axis (see stalk) is perpendicular to the membrane and is oriented in the primary direction of regard. B. Rotational displacement of the “eye” showing three possible degrees of rotational freedom; horizontal, vertical, and torsional. The stalk can be displaced as well as twisted between the fingers. C. Rotational displacement of the “eye” allowed the capability of only 2 degrees of rotational freedom. Note that the smooth rod can displace the eye in only two dimensions. It has no control of the third dimension, torsion. This model obeys both Donder's law and Listing's law (see text).