The eyelids (Latin: palpebrae) are structures working as a barrier to protect the eyes from injury and too much light. The protection is acquired by closing the eyelids. Blinking allows the eyelids to distribute the tears over the eyeball's anterior surface.
Structure of the eyelids
Humans have upper and lower eyelids. Between the eyelids is a palpebral fissure - the space between the lateral and medial canthi. Canthi are the corners in the eye where the upper and lower eyelids meet. The palpebral fissure is an opening to the conjunctival sac.
The eyelid has two parts: an orbital and a tarsal part. The division is made by a horizontal furrow called the superior palpebral sulcus. The sulcus of the upper eyelid is more visible than the one of the lower eyelid. The sulcus in the upper eyelid is formed by the aponeurotic fibers of the superior levator palpebrae muscle. The sulcus in the lower eyelid is formed by some connections between the skin and the orbicularis oculi muscle.
The tarsal part is made of dense connective tissue plates reminding of cartilage - tarsus. There is superior and inferior tarsus. One margin of the tarsus connects to the orbital septa, while laterally and medially, both tarsal plates are connected to the lateral and medial orbital septa by the lateral and medial palpebral ligaments. Most of the lateral part of the margin of the eyelid, also known as the ciliary portion, has squared edges, while the smallest medial part of the margin, the lacrimal portion, has rounded edges.
The upper eyelid is bigger than the lower. The cornea is completely covered by the upper eyelid when the eye is closed. The lower eyelid does not have significant changes when the eye is closed or open. When the eye is closed, the lower eyelid raises only a little. The lateral corner or canthus has direct contact with the eyeball, but the medial corner is medial to the eyeball, approximately 6 mm. Within the medial corner, the eyelids have a triangular space separating them.
This triangular space is known as the lacus lacrimalis. Within the lacus lacrimalis is a small elevation called the lacrimal caruncule. Laterally from the lacrimal caruncule is a semilunar fold of the conjunctiva - the plica semilunaris. The lacrimal caruncule is a pinkish structure. The lacrimal caruncule has modified skin with colourless hairs and sebaceous and sweat glands.
Its difference from the skin is having the accessory lacrimal glands and nonkeratinized stratified squamous epithelium. The lacrimal caruncule is supplied by the superior medial palpebral arteries. The lymph drains into the submandibular nodes. The innervation is received from the infratrochlear branch of the nasociliary nerve.
Near the medial corner is a small elevation - the papilla lacrimalis. On the top of the papilla lacrimalis is the punctum lacrimale that leads into the canaliculus lacrimalis.
In front of the margin's posterior edge are small openings of the tarsal glands. The tarsal glands can be seen on the inner surface of the everted eyelid as yellowish lines. These openings can be considered as a junction site between the skin and the conjunctiva.
The upper eyelid is an insertion site for a powerful muscle - the superior levator palpebrae muscle. This characteristic is what differentiates the upper eyelid from the lower eyelid. The muscle is innervated by the superior branch of the oculomotor nerve. It raises the upper eyelid.
From the outside, the eyelids are covered by the skin, under which is the palpebral part of the orbicular oculi muscle. The inside of the eyelid is covered by the conjunctiva.
Layers of the eyelids
The eyelids have six layers: the skin, subcutaneous tissue, muscle fibers of the orbicularis oculi, orbital septum and tarsal plate, smooth muscle, and conjunctiva.
Skin and subcutaneous tissue
The skin of the eyelids is thin and folds easily. The skin has tiny hair with sebaceous glands and sweat glands. The small tarsal gland openings are where the skin changes into the conjunctiva. The eyelashes are short curved hair present on the margins of the eyelids. The eyelashes extend from the lateral corner of the eye to the lacrimal papilla. The eyelashes on the upper eyelid are longer and more in numbers and curved up. The lower eyelid has eyelashes curing down and arranged in two to three rows. The eyelashes change every 100-150 days.
The subcutaneous tissue is loose and contains many elastic fibers.
Orbicularis oculi muscle
The orbicularis oculi muscle is an elliptical muscle surrounding the orbital margin and extending onto the temporal region. Apart from that, the muscle extends into the eyelids and even further, behind the lacrimal sac. The orbicularis oculi muscle is made of striated muscle. The palpebral part of the muscle is made of thin fiber bundles originating from the medial palpebral ligament. The fibers attach to the superficial and deep part of the ligament. The fibers go laterally and concentrically over the eyelids and in front of the orbital septa. The orbicularis oculi muscle receives its innervation from the temporal and zygomatic branches of the facial nerve (CN VII).
Orbital septa and tarsal plates
The eyelid forming base is made of a membranous sheet called the orbital septa. The orbital septa attaches to the orbital margin. Here, the septa continues into the periosteum. The eyelids are separated from the contents of the orbital cavity by the septa. The orbital septa is found behind the medial palpebral ligament, while on the lateral side of the orbit, the septa is found behind the orbicularis oculi muscle's lateral raphe and front of the lateral palpebral ligament.
The tarsal plates are made of dense fibrous tissue and are responsible for the shape of the eyelids. The upper eyelid's tarsal plate is larger than the lower's. The orbital septa and the fibers of the superior levator palpebrae muscle attach to the upper margin of the upper eyelid's tarsal plate.
The lower eyelid's tarsal plate is smaller, and its margin attaches to the orbital septa. The medial palpebral ligament connects the tarsal's medial end to the lacrimal crest and the frontal process of the maxilla. The lateral palpebral ligament extends between the tarsal's lateral end and the marginal tubercle on the orbital margin made by the zygomatic bone.
The orbital septa has small openings for nerves and blood vessels and the aponeurotic fibers of the superior levator palpebrae muscle. Within the tarsal plates are the tarsal glands. The tarsal glands are transformed sebaceous glands. The tarsal glands produce oily secrete and prevent the overflow of tears.
The superior and inferior tarsal muscles are made by the smooth muscle. The superior tarsal muscle continues with the superior levator palpebrae muscle above, while below attaching to the upper edge of the upper eyelid's tarsal plate. The superior tarsal muscle raises the upper eyelid and helps the superior levator palpebrae muscle. The inferior tarsal muscle attaches to the lower eyelid's inferior tarsal plate's lower margin. The muscle lowers the lower eyelid. Both muscles are innervated by sympathetic nerves originating from the superior cervical sympathetic ganglion.
The conjunctiva is a mucous membrane covering the inside of the eyelids. The conjunctiva covers part of the sclera. The conjunctiva changes into the skin at the margin of the eyelid, along the tarsal glands' opening. The conjunctiva covering the eyelids is vascularized, and that gives the reddish colour to the inner surface of the eyelids. The conjunctiva is more detailed described under the conjunctiva article.
Movement of the eyelids
During rest, the eyelids depend on the orbicularis oculi and superior levator palpebrae muscles and the position of the eyeball. When a human experiences fear or excitement, the palpebral fissure widens more due to the increased tone of the superior tarsal muscle’s smooth fibers and the inferior tarsal muscle.
Contraction of the orbicularis oculi muscle and relaxation of the superior levator palpebrae muscle closes the eyelids. Contraction of the superior levator palpebrae muscle opens the eyelid. Looking up is permitted by contraction of the superior levator palpebrae muscle. Looking down happens via the conjunctiva pulling down the lower eyelid. The conjunctiva is pulled down by contraction of the inferior rectus muscle.
Vasculature and innervation of the eyelids
Blood supply and venous drainage
The eyelids receive blood from the lateral and medial palpebral arteries. The medial palpebral arteries originate from the ophthalmic artery just below the trochlea of the superior oblique muscle. The medial palpebral arteries travel behind the lacrimal sac and enter the eyelids.
Each medial palpebral artery (superior and inferior) branches into two branches, forming two arches in each eyelid. These arches anastomose with the lateral palpebral arteries as well as with the superficial temporal, transverse facial, and infraorbital arteries. The lateral palpebral arteries originate from the lacrimal artery, a branch from the ophthalmic artery.
Venous drainage of the eyelids happens through the veins of the eyelids that drain into the ophthalmic and angular veins medially and into the superficial temporal vein laterally.
The lymph from the lateral 2/3 of the upper and lower eyelids drains into the superficial parotid nodes, while from the medial corner - into the submandibular nodes.
The upper eyelid receives its innervation from the infratrochlear, supratrochlear, supraorbital, and lacrimal nerves originating from the ophthalmic division of the trigeminal nerve (CN V). The lower eyelid is innervated by the infratrochlear branch of the ophthalmic division of the trigeminal nerve. The structure known as the remainder of the lower eyelid is innervated by branches of the infraorbital nerve, the terminal portion of the maxillary division of the trigeminal nerve.