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Chapter: Human Nervous System and Sensory Organs - The Eye

Eyelids - Structure of the Eye

Eyelids - Structure of the Eye
The eyeball is embedded in the orbit and is covered by the eyelids. The upper eyelid (A1) and the lower eyelid (A2) demarcate the palpebral fissure.


Eyelids


The eyeball is embedded in the orbit and is covered by the eyelids. The upper eyelid (A1) and the lower eyelid (A2) demarcate the palpebral fissure. The latter ends in themedialangle of the eye (A3) with a recess enclosingthe lacrimal caruncle (A4).

In Oriental People, the upper eyelid con-tinues medially onto the side of the nose as a vertical fold of skin, the palpebronasal fold. The fold can also be observed as a transitory formation in infants and is known as epicanthus.



The eyelids are reinforced by firm plates of connective tissue consisting of collagenous fibers, tarsus superior (B5) and tarsus infe-rior(B6); the tarsal plates, or palpebral car-tilages, are attached to the margin of the orbit by the lateral palpebral ligament (B7) and the medial palpebral ligament. The tar-sal plates contain elongated tarsal glands, the meibomian glands (C8), which spread over the entire height of the eyelids. Their secretion prevents the tears from flowing over the edge of the eyelids. They open at the posterior margin of the eyelid. Several rows of eyelashes, the cilia (C10), emerge from the anterior margin (AC9). The inner wall of the eyelids is lined by the conjunctiva (C11), which extends to the anterior aspect of the eyeball at the conjunctival fornix (C12). The smooth superior tarsal muscle (C13) and inferior tarsal muscle (C14) (inner-vated by the sympathetic nervous system), which control the size of the palpebral fis-sure, attach at the tarsus. The eyelids are closed by the orbicular muscle of the eye(C15) (facial nerve, p. 122). The upper lid is lifted by the levator muscle of the upper eye-lid (BC16) (oculomotor nerve), which at-taches at the upper margin of the optic canal. Its superficial tendinous membrane (C17) penetrates into the subcutaneous con-nective tissue of the upper lid, while the deep tendinous membrane (C18) attaches at the upper margin of the tarsus.





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