The falcon’s stoop

Ferocious speed in a predator is an admired, envied and, certainly, feared strategy. But, pure acceleration and speed have their own perils—problems which must be either solved or avoided for success. For example, the ocular surface will rapidly dry as the rush of wind evaporates the aqueous tear film, especially with the high speeds generated during the spectacular stoop of a falcon which approaches 300 km/hour. The disturbance of the ocular surface will challenge tear physiology, and the smooth surface essential for clarity of the image.

The nictitating membrane, or third eyelid, is unique to vertebrates, although not found in all groups. In many species, it represents the principal mechanism of ocular cleansing. In birds, the external eyelids possess smooth muscle, and may close only during sleep. The nictitans, on the other hand, is operated by two striated muscles and is capable of extremely rapid sweeps across the ocular surface to clear the cornea of debris. Ocular surface lubrication originates from two secretory glands. The lacrimal gland is situated in the inferior temporal quadrant …