Most people notice a dog’s familiar upper and lower eyelids that protect the eye’s surface. Canine eye anatomy includes a surprising addition rarely seen during normal activity: the third eyelid. This hidden protective structure means dogs have more complex ocular defenses than humans.
The Third Eyelid Structure and Location
The anatomical name for this hidden shield is the nictitating membrane. It is positioned neatly tucked away in the inner corner of the eye, closest to the nose. This membrane is supported by a specialized, translucent, T-shaped piece of cartilage that gives it rigidity and form. Its location allows it to sweep horizontally across the entire surface of the eye when deployed.
Unlike the upper and lower eyelids, which are controlled by voluntary muscles, the third eyelid’s movement is largely involuntary. It is passively deployed when the eyeball retracts slightly into the socket, a process called enophthalmos. When the eye sinks back, pressure pushes the membrane out and over the cornea for a fraction of a second.
Essential Functions of the Nictitating Membrane
The primary purpose of this structure is to provide rapid mechanical protection for the delicate cornea. It acts like a biological windshield wiper, quickly sweeping away dust, debris, and foreign particles. This protective action is so swift that it is often missed by the casual observer.
Beyond physical defense, the nictitating membrane plays a major role in maintaining the eye’s health through lubrication. It is instrumental in distributing the tear film evenly across the entire surface of the eye with every deployment. This ensures the cornea remains moist and nourished, which is necessary for clear vision.
The third eyelid also houses a specialized gland that produces a significant amount of the aqueous portion of the tear film. This gland contributes up to 50% of the total tear volume required to keep the eye properly hydrated. Removing this gland can severely compromise a dog’s ability to produce tears, leading to chronic dryness.
When the Third Eyelid Becomes Visible
Under normal circumstances, the membrane is only visible as a quick flash during a blink or when the eye is deliberately rolled back. However, the third eyelid might be seen resting in the corner of the eye for extended periods due to two primary reasons.
One common cause is a state of deep relaxation, fatigue, or illness. During sedation or when a dog is unwell, the muscles that hold the eye forward can relax, causing the eyeball to sink slightly into the socket. This passive retraction pushes the nictitating membrane out and keeps it visible, often giving the dog a sleepy or vacant appearance. It typically returns to its hidden position once the dog is fully alert.
The second, more concerning reason for permanent visibility is a condition known as “Cherry Eye,” which is the common term for a prolapsed gland of the third eyelid. This occurs when the tear gland pops out of its normal position, appearing as a smooth, pink or reddish mass protruding from the corner of the eye.