The nictitating membrane, or third eyelid, is a transparent anatomical feature found in many animal species. This eyelid serves specialized purposes, distinguishing it from human eyelids. Humans have a vestigial remnant of this structure.
Understanding the Nictitating Membrane
The third eyelid appears as a thin membrane in the inner corner of an animal’s eye. Unlike human eyelids, it sweeps horizontally across the eyeball. Many animals, including cats, dogs, birds, reptiles, amphibians, camels, polar bears, and seals, possess a well-developed third eyelid. In humans, this structure is a small fold of tissue, the plica semilunaris, aiding in tear drainage.
Essential Roles of the Third Eyelid
The nictitating membrane performs multiple functions for an animal’s ocular health and environmental adaptation. A primary role is protection, shielding the eye from debris, scratches, and impacts. Raptors use it to protect their eyes from particles during high-speed flight, while sharks use it to safeguard their eyes when attacking prey.
It also plays a role in maintaining eye moisture. It effectively distributes tears across the cornea, preventing dryness. A specialized gland, the nictitans gland, is often associated with this membrane and can produce a substantial portion of tears, sometimes up to 50% in dogs.
The nictitating membrane can also contribute to vision enhancement in certain species. For aquatic animals like beavers and manatees, it acts as a transparent goggle, allowing them to see clearly underwater. Birds, such as peregrine falcons, can rapidly sweep this membrane across their eyes to clear debris and maintain clear vision during dives or flight.
When the Third Eyelid Becomes Prominent
In healthy, alert animals like dogs and cats, the third eyelid is usually not visible or only minimally so. It becomes more noticeable during relaxation, sleep, or when an animal is under sedation or anesthesia, as muscles controlling its retraction relax.
Persistent visibility of the third eyelid often signals an underlying health issue. Conditions such as “cherry eye” occur when the tear-producing gland of the third eyelid prolapses, appearing as a red, swollen mass in the corner of the eye. Other causes for a protruding third eyelid can include dehydration, significant weight loss, eye pain, inflammation, or certain neurological conditions. If an animal’s third eyelid remains visible for an extended period, or appears red, swollen, or unusually prominent, veterinary attention is advisable to determine the cause and appropriate treatment.