Humans possess two eyelids per eye: an upper and a lower. These eyelids are integral for protecting the eyes and maintaining their health. The animal kingdom, however, exhibits a broader range, with some species having additional eyelid structures.
The Anatomy and Function of Human Eyelids
Human eyelids are composed of multiple layers, including thin skin, muscles, connective tissue, and glands. The orbicularis oculi muscle controls eyelid movement, enabling both voluntary and involuntary actions like blinking. Eyelashes, growing along the margins of the eyelids, act as a natural barrier, protect the eyes from dust and debris.
The primary function of human eyelids is to protect the eye from foreign objects, irritants, and injury. Blinking, a semi-autonomic and rapid closing of the eyelid, occurs frequently, around 15 to 20 times per minute. This action is crucial for distributing tears across the eye’s surface, keeping it moist and clean.
Tears, composed of water, oil, and mucus, are spread evenly with each blink, preventing dryness and discomfort. Meibomian glands within the eyelids produce an oily substance, reducing evaporation and preventing tears from spilling over. Eyelids also help regulate the amount of light entering the eye, closing reflexively in bright conditions or during sleep.
Beyond Two: Eyelids in the Animal Kingdom
Many animals possess a unique anatomical feature called a nictitating membrane, often referred to as a “third eyelid.” This transparent or translucent membrane sweeps horizontally across the eye, unlike human eyelids which move vertically. It protects and moistens the eye while allowing the animal to maintain vision.
The nictitating membrane is found in a diverse range of species, including birds, reptiles, sharks, and some mammals such as cats, dogs, camels, polar bears, and beavers. It offers specialized protection based on their environment and activities. For instance, aquatic animals like beavers and seals use it to protect their eyes underwater, while birds of prey can deploy it during high-speed flight or hunting. Sharks utilize it for eye protection when attacking prey.
Humans do not have a functional nictitating membrane. A small, pink fold of tissue in the inner corner of the eye, known as the plica semilunaris, is considered a vestigial remnant of this structure. Its presence indicates a shared evolutionary history with animals that possess a fully developed third eyelid. As humans developed other protective mechanisms, such as bony orbital sockets and enhanced hand-eye coordination, the need for a functional third eyelid diminished.