Structure and Function of the Tear Film

We used a high-speed camera system to study in detail the eyelid motion dynamics of the human eyeblink. Films were made from a hidden location through a one-way mirror. In this manner, normal, unforced blinks were recorded. The descent of the upper eyelid reached its maximum speed at about the time that it crossed the visual axis, generally in the range of 17 to 20 cm/sec, but occasionally reaching a speed of over 40 cm/sec. The motion of the lower eyelid was mostly horizontal, in a nasally directed movement, with a total displacement in the range of 20 to 5 min. A distinction must be made between normal, unforced blinks, and voluntary, forced eyelid motion. In a normal blink, no appreciable upward rotation of the globe is observed. A forced blink or a restraint of motion of the upper eyelid results in a significant demonstration of Bell's movement. The globe moves posteriorly up to 1 to 6 mm as the upper eyelid descends, probably caused by eyelid pressure during the closing phase of the blink.