The small, pink structure in the inner corner of the eye is a familiar feature that often sparks curiosity. This article explains its identity and roles, and when changes might warrant medical attention.
Identifying the Eye’s Inner Corner Structures
The innermost corner of the human eye contains two distinct pinkish structures. One is the plica semilunaris, a small, crescent-shaped fold of conjunctiva. This clear membrane covers the white part of the eye and the inside of the eyelids. It is located at the medial canthus, where the upper and lower eyelids meet nearest the nose. This fold is typically pink or slightly translucent.
Adjacent to the plica semilunaris is the lacrimal caruncle. This small, fleshy, and often reddish nodule sits within the lacrimal lake, a triangular space where tears collect before draining. Its surface may feature fine hairs, distinguishing it from the smoother plica.
Understanding Their Role
The plica semilunaris is considered a vestigial remnant of the nictitating membrane, or “third eyelid,” found in many animal species. While a fully functional third eyelid is common in birds, reptiles, and some mammals, humans and most other primates have evolved away from needing this protective structure. Despite its reduced function, the plica semilunaris may still play a minor role in facilitating tear drainage and allowing for greater eyeball rotation by preventing conjunctiva restriction.
The lacrimal caruncle, however, has more active functions. It is composed of skin containing sebaceous (oil) glands and sweat glands, along with hair follicles. These glands contribute to the tear film, which lubricates and protects the eye. The caruncle’s primary role involves collecting debris and foreign particles that accumulate in the tear film.
Common Concerns and When to Seek Advice
It is common to wake up with whitish or cream-colored discharge, often called “sleep,” in the inner corner of the eye. This accumulation consists of mucus, oils, dead skin cells, and tears, normally flushed away by blinking. During sleep, when blinking ceases, these substances can collect and dry, forming a crusty or sticky residue.
While “sleep” in the eyes is generally harmless, certain symptoms warrant medical attention. Persistent redness, swelling, or pain in the inner corner of the eye should be evaluated. Any significant increase in eye discharge, especially if it is yellow or green, or if it causes the eyelids to stick together, could indicate an infection. Changes in vision, such as blurriness, or the appearance of new, unusual growths or lumps that are painful or changing in size should prompt a visit to an eye care professional.