Anatomy of Eyelids – 7 Layers of Eyelids You Need to Know.

Gross Anatomy of Eyelids:

What is the definition of Eyelids?

• The eyelids are protective, mobile tissue curtains in front of the eyeballs.
• They act as shutters to shield the eyes from injuries, light, and help spread the tear film over the eye’s surface.
• Eyelids play a role in facial expressions and provide cues about a person’s wakefulness and attention.

Extend of Eyelids  (Anatomy of Eyelids):

• Extent of Upper Eyelid:
  • Extends downward from the eyebrow and forms the top boundary of the eye opening (palpebral fissure).
• Extent of Lower Eyelid:
  • Extends upward to meet the skin of the cheek, creating the lower boundary of the eye opening.

Folds of Eyelid:

• Upper Eyelid Fold:
  • Divides the eyelid into an orbital and a tarsal portion, formed by fibers from the levator muscle.
• Lower Eyelid Fold:
  • Less prominent, marks the boundary between the eyelid and cheek area.
• Nasojugal and Malar Folds:
  • These folds on the lower eyelid limit the spread of blood or fluid into the cheek.

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Canthi/Canthus of Eyelids:

• Medial Canthus of Eyelids:
  • The inner angle where eyelids meet, separated from the eyeball by the tear lake (lacus lacrimalis).
• Lateral Canthus of Eyelids:
  • The outer angle, which lies in contact with the eyeball, forming an angle that changes with the eye’s opening.

Margins of Eyelids:

• Divided into a Medial Lacrimal Portion (rounded, no lashes, or glands) and a Lateral Ciliary Portion.
• The Grey Line marks the division between skin and conjunctiva, where lashes and meibomian gland openings are located.

Eyelashes of Eyelids:

• Arranged in 2-3 rows: Upper lid lashes (100-150) curve upward, lower lid lashes (50-75) curve downward to avoid interlocking when closed.
• Lashes are dark, fine-tapered, and renew every 3-4 months.

Palpebral Fissure of Eyelids:

• The elliptical space between the eyelids, which is approximately 28-30 mm horizontally and 9-11 mm vertically in adults.
• The alignment of the fissure can vary: some have a slight upward tilt (mongoloid slant), while others may tilt slightly downward (antimongoloid slant).

Associated Glands of Eyelids:

Glands of Zeis:

• Sebaceous glands that empty into the lash follicles.

Glands of Moll:

• Apocrine glands also associated with lash follicles, providing lubrication.

Note: These glands and lash follicles are highly sensitive, responding to touch for eye protection.

Anatomy/Structures of Eyelids:

What are the Layers of Eyelids (Structures of Eyelids)?

• Eyelid has 7 layers:
  • 1. Skin 
  • 2. Subcutaneous areolar tissue 
  • 3. Striated muscle
  • 4. Submuscular areolar tissue 
  • 5. Fibrous layer
  • 6. Non-striated muscle
  • 7. Conjunctiva

Note: The skin on our eyelids is the thinnest in the entire body

• You can easily remember those layers by the mnemonic

“Smart Students Study Subjects For New Concepts”

• Smart = Skin,
• Students = Subcutaneous areolar tissue,
• Study = Striated muscle,
• Subjects = Submuscular areolar tissue,
• For = Fibrous layer,
• New = Non-striated muscle, and
• Concepts = Conjunctiva.

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1. Skin of Eyelids (Structures of Eyelids):

• The eyelid skin is the thinnest on the body, very elastic, and allows the eyelids to move quickly.
• The nasal skin (near the nose) is smoother and oilier than the skin on the outer part of the eyelid.
• Microscopic Structure: It consists of several layers of cells (epidermis), including a protective outer layer, and a dermis layer that has connective tissue, blood vessels, lymphatics, and nerves.
• The skin’s pigment can darken with chronic swelling or irritation.

2. Subcutaneous Areolar Tissue layer of Eyelids (Structures of Eyelids):

• A layer of loose connective tissue just below the skin with no fat, allowing easy movement of the skin.
• This layer can swell with fluid or blood, and it’s also helpful for eyelid surgeries.
• Not present near the lash line or where skin attaches to ligaments at the eyelid edges.

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3. Striated Muscles layer of Eyelids (Structures of Eyelids):

• This layer consists of orbicularis muscle which forms an oval sheet across the eyelids. In addition, the upper eyelid also contains levator muscle.

Orbicularis Oculi Muscle:

• This muscle encircles the eyelid and has two main parts:

Orbital Part of Orbicularis Oculi Muscle:

• Helps close the eyelid tightly and supports eyebrow movement.

Palpebral Part of Orbicularis Oculi Muscle:

• Helps in gentle eyelid closure, such as blinking or during sleep.
• This part is further divided into:

Preseptal and Pretarsal Portions of Orbicularis Oculi Muscle:

• Important for tear drainage and keeping the lids close to the eye.

Levator Palpebrae Superioris Muscle (LPS):

• Only in the upper lid, this muscle lifts the upper eyelid.
• Its tendon, called the aponeurosis, helps form the upper lid fold and connects to various points in the eyelid structure.

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4. Submuscular Areolar Tissue layer of Eyelids (Structures of Eyelids):

• A loose tissue layer between the muscles and deeper structures, containing blood vessels and nerves.
• It splits the eyelid into front and back sections, allowing for easy surgical access.
• In the upper eyelid, it has two spaces (pretarsal and preseptal), separated by the levator muscle.
• These spaces allow movement and house blood supply.

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 5. Fibrous Layer of Eyelids (Structures of Eyelids):

• The fibrous layer forms the structural support of the eyelids and includes:

a. Tarsal Plate:

• This is the dense, fibrous tissue that gives the eyelid its shape and rigidity.
• It’s about 29 mm in length, with the upper tarsus being larger than the lower.

Surfaces of Tarsal Plate:

• The front (anterior) surface is convex and separates from the orbicularis muscle, allowing smooth movement.
• The back (posterior) surface curves to align with the eye.

Borders of Tarsal Plate:

• The edges provide attachment points for key structures like the orbital septum and muscles.

b. Septum Orbitale (Palpebral Fascia):

• A thin membrane that moves with the eyelids, linking the tarsal plates with the orbital rim.

Attachments and Relations of Septum Orbitale:

• Attaches to the tarsal plate centrally and to the orbital bone peripherally, allowing it to act as a barrier for orbital fat and connect with the surrounding muscle structures.

c. Medial and Lateral Palpebral Ligaments:

Medial Ligament:

• Attaches to the frontal bone, providing support and anchoring the eyelids at the inner corner of the eye.

Lateral Ligament:

• Anchors to the outer corner, connecting the tarsal plates with the orbital margin, ensuring stability during movement.

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 6. Non-Striated Muscle Fibers layer of Eyelids (Structures of Eyelids):

• This layer consists of Müller’s muscles (smooth muscle fibers), which are controlled by the sympathetic nervous system:

Location and Function of Non-Striated Muscle Fibers layer of Eyelids:

• Found in both upper and lower eyelids, just beneath the septum orbitale.
• They help elevate the upper eyelid and slightly lower the lower eyelid.

Sympathetic Control of Non-Striated Muscle Fibers layer of Eyelids:

• Stimulates lid retraction (pulls eyelids up).
• If damaged, it can lead to conditions like Horner’s syndrome, where the lid droops due to reduced muscle tone.

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 7. Conjunctiva of Eyelids (Structures of Eyelids):

• The conjunctiva is the innermost layer of the eyelid:

Structure and Location Conjunctiva of Eyelids:

• A thin mucous membrane that extends from the inner lid margin to the conjunctival fornix, covering the back of the tarsal plate and Müller’s muscle.

Function Conjunctiva of Eyelids:

• Provides a protective barrier and helps produce mucus to keep the eye moist.

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