The captivating gaze of a cat occasionally reveals a structure foreign to the human eye, suggesting an anatomy more complex than our own. This seemingly “second eyelid” is actually the third eyelid, known scientifically as the nictitating membrane. This adaptation represents an evolutionary advantage that supports the cat’s remarkable visual acuity and overall eye health.
Anatomy of the Feline Nictitating Membrane
The third eyelid is a thin, whitish-pink or translucent fold of tissue located in the inner corner of the eye, closest to the nose (the medial canthus). Unlike the cat’s upper and lower eyelids, which move vertically, the nictitating membrane moves horizontally across the surface of the eye. It is generally retracted and hidden from view when the cat is alert and awake.
This membrane is lined with the conjunctiva, the mucous membrane that covers the white part of the eye and the inner surfaces of the other eyelids. Embedded within the third eyelid is a specialized glandular tissue called the gland of the third eyelid. This gland is a significant contributor to the production of the tear film that keeps the eye lubricated.
The movement of the third eyelid is passive, meaning the cat cannot consciously control its protrusion. The membrane extends across the eye when the eyeball retracts slightly into the socket. This reflex action is triggered by muscle relaxation or protection, allowing for rapid deployment when needed.
Essential Roles of the Third Eyelid
The primary function of the nictitating membrane is to provide rapid protection for the delicate corneal surface. It acts as a biological shield, sweeping across the eye to guard it from dust, debris, and potential injury during hunting or rough play. This swift protective action is an evolutionary benefit for a predator that often moves through dense vegetation.
The third eyelid is also an integral part of the ocular lubrication system. The gland of the third eyelid produces a substantial portion of the total tear volume, estimated to be around 30% of the tear film. This fluid is distributed evenly across the eye surface by the membrane’s sweeping motion, keeping the cornea moist and healthy.
Maintaining this consistent layer of moisture is crucial for clear vision and preventing damage to the cornea. The tear film helps to wash away minor irritants and provides oxygen and nutrients to the eye’s outer layer.
When Visibility Indicates a Health Concern
While the third eyelid is normally retracted, a brief glimpse can be seen when a cat is deeply relaxed, just waking up from sleep, or recovering from anesthesia. In these instances, the muscles around the eye are relaxed, allowing the membrane to protrude temporarily. This brief visibility is considered normal and is not a cause for alarm.
However, persistent or prolonged visibility of the nictitating membrane when the cat is awake often signals an underlying health issue. This protrusion can occur if the eyeball sinks slightly into the socket, which may be caused by severe dehydration or significant weight loss. Systemic illnesses, such as a high fever or an upper respiratory infection, can also cause the membrane to remain elevated.
In some cases, the protrusion is related to a specific neurological or eye disorder. A condition known as Haw Syndrome involves the protrusion of both third eyelids and is frequently associated with gastrointestinal issues like diarrhea. Protrusion in only one eye may indicate local trauma, a foreign object, or a neurological issue like Horner’s Syndrome.
If a cat’s third eyelid remains visible for more than a few hours, especially if accompanied by signs like redness, discharge, squinting, or a change in behavior, veterinary consultation is necessary. Persistent elevation can also be a sign of a primary eye problem, such as conjunctivitis or corneal ulcers. Another potential issue is a prolapsed gland of the third eyelid, sometimes called “cherry eye,” which requires professional diagnosis and treatment.