Eyelid
The eyelid, also known as the palpebra, is a crucial anatomical structure that serves multiple functions for the protection and maintenance of the eye. Primarily, it acts as a barrier against injury and foreign particles while regulating the amount of light that enters the eye. In humans, there are two eyelids per eye—an upper and a lower—each composed of thin skin and movable muscle that enable voluntary and reflexive movements. Eyelids play a vital role in distributing moisture across the cornea through blinking, which is essential for eye health.
Additionally, many animals possess a third eyelid, known as the nictitating membrane, which offers extra protection and cleansing, particularly in species that require clear vision for hunting or that spend time in water. The eyelids are supported by structures such as tarsi and are lined by the conjunctiva, which helps lubricate the eye and prevent microbial invasion. Human eyelids are open at birth, contrasting with many other animals, which may not open their eyelids until days after birth, reflecting different developmental strategies. Understanding the anatomy and functionality of eyelids can enhance appreciation for their role in both human and animal health.
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Subject Terms
Eyelid
The eyelid (also known as the palpebra) is a fold of skin that primarily serves to protect the eye from injury and foreign matter. However, eyelids also have several other important functions. They help to control the amount of light received by the eyes; they distribute fluid over the eyes during blinking to help moisturize the cornea; and they help with the flow of tears. Humans have two eyelids on each eye: an upper (palpebra superior) and a lower (palpebra inferior) eyelid. Other species of animals, such as dogs, cats, reptiles, birds, and fish, have a third transparent eyelid called a haw or nictitating (meaning blinking) membrane. This third eyelid serves many of the same functions as the first two eyelids, but it offers an additional form of cleansing for predators that require clear vision to track prey or as a means of protection for animals that dive or move underwater.
Anatomy
As the human eye is highly sensitive to damage, the eyelid is one of several anatomical structures that help protect the eye and preserve vision. Among these is the orbit, which is a bony cavity in the skull that protrudes slightly past the surface of the eye. It is composed of seven hard bones that ensure the safety of the eye, while still freely allowing movement. Inside the orbit are the eyeball, muscles, nerves, and blood vessels that comprise the optical organs. The eyelid is also found within this eye socket.
The eyelid forms the protective case for the eyeball. It is composed of movable tissue and muscle. It is covered in some of the thinnest skin found on any part of the body. The eyelid is capable of either voluntary or reflexive movement. Different sets of muscles are responsible for this ability of the eyelid to open and close. One of the muscles that controls this movement in the upper eyelid is called the levator palpebrae superioris muscle. It allows the eyelid to retract or lift. On the underside of this muscle is the superior tarsal muscle (or Müller's muscle) that further aids in opening the upper eyelid. Another set of muscles, called the orbicularis oculi muscles, enables both the upper and lower eyelids to close.
On the edges of each end of the human eyelids are a set of eyelashes. These are strong hairs that are grown by the eyelid to prevent any wind, dirt, or other foreign particles from entering the eye when open. The lashes on the upper eyelid are longer than those on the bottom, thereby creating a protective case over the eye. The eyelashes have the reflexive capability of causing the eye to blink when placed into contact with debris or when threatened with other external stimuli.
Tears are a salty mixture composed of three elements: mucous, oil, and water. While most tissues in the body have blood vessels that provide nutrients, oxygen, and moisture, the cornea does not. Instead, it must rely on the external transfer of these materials found in the fluid of tears. The lacrimal (tear) glands are responsible for creating the water in tears, which forms the greatest portion of their content. These two glands are located above the outside portion of each eye. They produce tears that run over the eyes. These tears then drain into small canals called puncta that are found at the inner corners of each eye near the outside of the nose. From here, the fluid passes into the lacrimal ducts, which then goes into the tear sacs found between the nose and eye. Finally, it moves into the nasolacrimal ducts and then into the nasal cavity.
On the inside of both of the upper and lower eyelids is a collection of cells called the conjunctiva, which creates a cover for the sclera (the white area of the eyeball). It provides the mucus fluids that lubricate the eye during blinking, while preventing foreign microbes from entering. The lipid (or sebaceous) glands in the tips of the eyelids emit the oil found in tears. In addition to lubricating the eyes and providing materials the corneas need, tears are clear, allowing the eye to see during this process of cleansing.
The structure of the eyelids is held together by tarsi. These plates are formed from dense fibrous tissues. These structures are called the superior and inferior tarsal plates and are found in the upper and lower eyelids, respectively. These plates are thicker in the middle and narrower around the edges.
Overview
In humans, the eyelids close as part of the normal blinking process. This enables the cornea of the eye to receive the moisture and nutrients it continuously needs to function properly. However, for animals with the third nictitating eyelid, the blinking motion of upper and lower eyelids can be dangerous. For other species, these eyelids are not enough to protect the eye. For example, as birds are moving constantly, they are more prone to getting debris in their eyes. As a result, animals use the nictitating eyelid when they need to do activities during which blinking is potentially hazardous, such as while feeding, flying, or hunting. For fish and other animals that spend time underwater, the nictitating eyelid functions as a form of goggles that allow them to see underwater without damaging the eye. The nictitating eyelid uses cartilage to move horizontally across the eye unlike the upper and lower eyelids, which move vertically. Humans have a non-functional, vestigial form of the nictitating eyelid called the plica semilunaris found in the corner of the eye.
Unlike some species of mammals, the eyelids in humans are open at birth. For dogs, cats, and other animals, it takes a few days after birth before they open. Animals that are born comparatively functional at birth such as deer and cattle are called precocial, while animals born functionally immature are called altricial. Altricial animals, such as dogs, tend to have shorter pregnancies, meaning that some aspects of development—like brain development, which controls the opening of the eyelids—take place outside the womb. Shorter pregnancies occur in dogs and other animals because it is harder for predator species to hunt while pregnant. On the other hand, it is more beneficial for prey species to be able to fend for themselves quickly, so they are born with the ability to walk and see shortly after birth. Human parents are able to defend their young better than prey species, so they have the luxury of longer gestations that allow the brain to develop enough to allow the eye and eyelid to function at birth.
Bibliography
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