Dogs possess three eyelids, a key anatomical difference from humans. The third, often unseen structure is formally known as the nictitating membrane. This membrane plays a specialized role in maintaining eye health that goes beyond the capabilities of the familiar upper and lower eyelids. Understanding this unique biological feature provides insight into a dog’s ocular anatomy and potential health concerns.
The Familiar Upper and Lower Eyelids
Like humans, dogs have a mobile upper eyelid and a less mobile lower eyelid that frame the eye. These two outer eyelids are composed of skin, muscle, and a supporting fibrous plate, performing the immediate, visible functions of protection. Rapid closure, or blinking, acts as a reflex to shield the eye from foreign objects and potential trauma. The upper and lower lids also work to distribute the tear film across the ocular surface for lubrication and debris removal.
Understanding the Third Eyelid and Its Function
The third eyelid, or nictitating membrane, is a pale pink or whitish structure located at the inner corner of the eye, near the nose. It is typically retracted and not visible when a dog is awake, which is why many owners are unaware of its existence. This membrane is a fold of conjunctival tissue supported by a T-shaped piece of cartilage.
The functions of the third eyelid focus on protection, lubrication, and immune defense. When a dog blinks, the third eyelid sweeps horizontally across the cornea, clearing away dust and debris. The base of this membrane houses the gland of the third eyelid, which produces a significant portion of the tear film. This gland is responsible for between 30% and 60% of the total tear production, keeping the eye moist and healthy.
Common Health Conditions Involving the Third Eyelid
The gland within the third eyelid is susceptible to a condition commonly referred to as “Cherry Eye.” This occurs when the fibrous attachments holding the tear gland in place weaken or rupture. When this happens, the gland prolapses, or pops out, from its normal position.
The condition manifests as a smooth, round, pink or red mass protruding from the inner corner of the eye, giving the condition its name. While the prolapse is not typically painful, the exposed gland can become inflamed and swollen. This leads to secondary issues like irritation, discharge, and reduced tear production. Because the gland is important for tear production, its displacement increases the risk of developing dry eye. Cherry Eye is most often seen in young dogs, particularly in breeds like Bulldogs, Beagles, and Cocker Spaniels, suggesting a genetic predisposition.