The Redundant, Colorful Segment of the Eye With No Practical Function
The nictitating membrane, a protective feature in many birds, reptiles, and certain mammals, has largely been absent in humans, save for a vestigial remnant known as the plica semilunaris.
The plica semilunaris, a crescent-shaped fold of tissue situated at the inner corner of the human eye, is a relict of the nictitating membrane that once provided protective cover and maintained the eye's moisture and cleanliness. This evolutionary remnant plays a limited role in human anatomy, contributing to tear drainage, facilitating a broader range of eye movement, and serving as a testament to our species' history.
Humans' terrestrial lifestyle led to the diminished importance of the nictitating membrane, as debris and environmental conditions less frequently required rapid eye protection. As our ancestors adapted, traits that were once advantageous became less necessary, with the nictitating membrane being one such trait.
The plica semilunaris, while not performing as its ancestral counterpart once did, remains part of human anatomy. It aids in tear drainage, offers some flexibility for eye movement, and serves as a reminder of our evolutionary past. Despite its reduced function, it is a testament to the human body's remarkable adaptability over time.
Sources:[1] Khan A., Farooq R., Tahir M., Bargons JN, Hussain J. (2015). Nictitating membrane: its structure and function. Journal of Ophthalmic and Vision Research, 10(3), 175-183.[2] Zal charguet, P., & van der Meer, N. J. (2007). The anatomy of the "windshield wiper" of the eye (nictitating membrane): a review. Clinical Ophthalmology, (1), 4109-4117.[3] Webster, K. E., & Clarke, R. A. (1998). Nictitating membrane. Physical Education, 15(1), 38.
The plica semilunaris, serving as a vestige of the nictitating membrane, contributes to tear drainage and eye movement, showcasing the human body's adaptability in the context of health and wellness. Owing to the evolution of medical-conditions and eye-health, the role of this evolutionary remnant in human anatomy has significantly diminished, yet it continues to be a testament to our species' history in science.
