Cherry eye is an eye condition known primarily in dogs and cats, presenting as a noticeable red mass in the inner corner of the eye. Humans cannot develop this disorder. The reason humans are immune to this specific condition lies in the anatomical differences between the human eye and the eyes of the animals that contract it.
The Anatomy of Cherry Eye
Cherry eye is the common name for the prolapse of the gland of the nictitating membrane, also known as the third eyelid. This third eyelid is a protective structure found in many animal species, including dogs and cats, situated within the inner corner of the lower eyelid. The gland at the base of this membrane produces a significant portion of the eye’s tear film, often ranging from 30 to 60 percent.
The condition occurs when the connective tissue anchoring the gland weakens, allowing the tear gland to flip out of its normal position. Once prolapsed, the gland protrudes as a round, fleshy, reddish-pink mass, earning the name “cherry eye.” When left exposed, this sensitive tissue can become inflamed, dry, and susceptible to secondary infection, potentially leading to dry eye syndrome.
Why Humans Lack the Third Eyelid
The anatomical structure required for cherry eye does not exist in the human eye. Humans possess only a vestigial remnant of the nictitating membrane, a small fold of tissue called the plica semilunaris. This structure is located in the inner corner of the eye, but it is not a functional third eyelid that sweeps across the cornea.
The plica semilunaris lacks the specialized tear-producing gland structure prone to prolapse in animals. While it plays a role in tear drainage and allows for greater eye rotation, it is primarily an evolutionary leftover. Because this essential glandular tissue is absent, the mechanical failure that defines cherry eye cannot occur in a human eye.
Human Conditions That Mimic Appearance
While true cherry eye is impossible for humans, several conditions can cause the appearance of severe redness, swelling, or protruding tissue in the eye area. Chemosis involves significant fluid accumulation and swelling in the conjunctiva, the clear membrane covering the white of the eye. Severe allergic reactions, such as those triggered by environmental irritants, commonly cause chemosis, making the eye surface look like a large, blister-like bulge.
Another condition is a pterygium, a fleshy, triangular-shaped growth that typically starts in the inner corner and grows across the cornea. This growth is an overgrowth of conjunctival tissue, often caused by chronic exposure to UV light or wind, and can appear pink and inflamed. Any sudden or severe eye redness, irritation, or the appearance of a mass should prompt a consultation with a medical professional, as these symptoms require specific medical diagnosis and treatment.