Eyelids and Lacrimal Apparatus

EYELIDS

• The eyelids are two movable folds of skin that protect the eyeball and help maintain the moisture of the cornea.
• The palpebral fissure is the opening between the upper and lower eyelids.
• The eyelids meet at the medial and lateral angles of the eye, forming the medial and lateral canthi.

Features of the Medial Canthus

• Lacus lacrimalis: It is a small triangular space at the medial angle of the eye.
• Lacrimal caruncle: It is a small, raised structure within the lacus lacrimalis, consisting of modified skin and glands.
• Plica semilunaris: It is a crescent-shaped fold of conjunctiva lateral to the lacrimal caruncle. It represents the vestigial third eyelid (nictitating membrane).

Margins of the Eyelids

• Each eyelid has an attached margin fixed to the orbital region and a free margin.
• The free margin presents two lips:
  • Outer lip: It is rounded and bears two or more rows of eyelashes (cilia).
  • Inner lip: It is sharp and contains the openings of the tarsal glands.
• The free margin is divided into two parts:
  • Lateral five-sixths: It bears eyelashes.
  • Medial one-sixth: It is devoid of eyelashes and lies adjacent to the lacus lacrimalis.

Lacrimal Papillae and Puncta

• Lacrimal papillae: These are small elevations near the medial ends of the eyelid margins, where the eyelashes terminate.
• Lacrimal puncta: Each papilla bears a small opening called the lacrimal punctum, through which tears enter the lacrimal drainage system.

Structures of Eyelid

Each eyelid consists of five layers arranged from superficial to deep:

1. Skin
2. Superficial fascia
3. Palpebral part of orbicularis oculi muscle
4. Tarsal plate and palpebral fascia
5. Palpebral conjunctiva

1. Skin

• The skin of the eyelid is thin, loose, and nearly devoid of hair except at the eyelid margin.
• Its loose attachment allows fluid accumulation, producing periorbital edema, particularly in renal disorders.

2. Superficial Fascia

– It consists of loose areolar tissue and contains little or no fat.

3. Orbicularis Oculi

– The palpebral part of orbicularis oculi lies beneath the superficial fascia and produces gentle closure of the eyelids.

4. Tarsal Plates and Palpebral Fascia

Tarsal Plates

• The tarsal plates are dense fibrous structures near the free margins of the eyelids. They provide firmness and maintain the shape of the eyelids.
• The superior tarsal plate is larger than the inferior plate.
• The superior tarsal plate receives attachments from the levator palpebrae superioris aponeurosis and superior tarsal muscle (Müller’s muscle).

Tarsal Glands

• The tarsal (Meibomian) glands are embedded within the tarsal plates.
• They secrete an oily substance that reduces evaporation of the tear film.
• Their ducts open along the posterior part of the free margin, behind the eyelashes.

Palpebral Ligaments

• Medial palpebral ligament: It connects the medial ends of the tarsal plates to the medial orbital margin, mainly at the anterior lacrimal crest of the maxilla, anterior to the lacrimal sac.
• Lateral palpebral ligament: It attaches the lateral ends of the tarsal plates to the lateral orbital tubercle (Whitnall’s tubercle) of the zygomatic bone.

Lateral Palpebral Raphe

– It is formed by the interlacing fibers of orbicularis oculi near the lateral angle of the eye.

Orbital Septum

• The orbital septum is a fibrous membrane extending from the orbital margin towards the tarsal plates.
• It forms a protective barrier between the eyelids and orbital contents.
• It blends with the levator aponeurosis in the upper eyelid and is traversed by vessels and nerves.

5. Palpebral Conjunctiva

• The palpebral conjunctiva is a thin, transparent mucous membrane lining the inner surface of the eyelids.
• It reflects onto the anterior surface of the eyeball to become the bulbar conjunctiva, which covers the sclera.
• The reflections between palpebral and bulbar conjunctiva form the superior and inferior conjunctival fornices.

Glands of eyelid

The eyelids contain four types of glands:

1. Meibomian (tarsal) glands:

• These are elongated sebaceous glands embedded vertically within the tarsal plates.
• Their oily secretion reduces evaporation of tears and maintains the stability of the tear film.

2. Glands of Zeis:

• These are small, modified sebaceous glands associated with the hair follicles of eyelashes.
• Their secretion lubricates the eyelashes.

3. Glands of Moll:

• These are modified apocrine sweat glands located near the free margins of the eyelids.
• They are also called ciliary glands.

4. Glands of Wolfring and Krause:

• These are accessory lacrimal glands consisting of serous tubuloacinar secretory units.
• Glands of Wolfring lie near the upper borders of the tarsal plates.
• Glands of Krause are situated mainly within the conjunctival fornices.
• Both contribute to the aqueous component of tears.

Blood Supply, Lymphatic Drainage and Innervation

Arterial supply

– The eyelids receive blood from the superior and inferior palpebral branches of the ophthalmic artery and the lateral palpebral branches of the lacrimal artery.

Venous drainage

– Venous blood from the eyelids drains into the ophthalmic and facial veins.

Lymphatic drainage

·  The medial part of the eyelids drains into the submandibular lymph nodes.

·  The lateral part drains into the preauricular (superficial parotid) lymph nodes.

Innervation

·  Upper eyelid: It is supplied by the lacrimal, supraorbital, supratrochlear and infratrochlear nerves.

·  Lower eyelid: It receives sensory fibres from the infraorbital and infratrochlear nerves.

LACRIMAL APPARATUS

– The lacrimal apparatus consists of structures responsible for the secretion, distribution and drainage of lacrimal fluid (tears).

Structures (Components)

The lacrimal apparatus includes the following structures:

1. Lacrimal gland and its ducts.
2. Conjunctival sac.
3. Lacrimal puncta.
4. Lacrimal canaliculi.
5. Lacrimal sac.
6. Nasolacrimal duct.

Lacrimal Gland

• The lacrimal gland is a small, J-shaped serous gland located in the superolateral part of the orbit.
• It is divided into two parts by the aponeurosis of the levator palpebrae superioris.

1. Orbital part

– It is the larger, almond-shaped part situated in the lacrimal fossa on the anterolateral aspect of the orbital roof.

2. Palpebral part

• It is the smaller part located in the lateral region of the upper eyelid, beneath the levator palpebrae superioris.
• Both parts are continuous around the lateral margin of the levator palpebrae superioris.

Ducts of Lacrimal Gland

• Approximately 12 excretory ducts open into the lateral part of the superior conjunctival fornix.
• Most ducts from the orbital part pass through the palpebral part before opening into the conjunctival sac.

Histological Features

• The lacrimal gland is a compound tubuloacinar serous gland.
• Its secretory acini are lined by cuboidal to low columnar cells with eosinophilic cytoplasm.
• The acini have relatively wide, irregular lumina.

Functions of Lacrimal Fluid

• It maintains corneal moisture and transparency.
• It supplies oxygen and nutrients to the corneal epithelium.
• It washes away dust and foreign particles from the conjunctival sac.
• It provides antimicrobial protection through lysozyme and other substances.
• Increased tear secretion accompanies emotional responses such as grief and happiness.

Conjunctival Sac

·  It is a potential space between the palpebral and bulbar conjunctivae.

·  The palpebral conjunctiva lines the inner surface of the eyelids, whereas the bulbar conjunctiva covers the anterior eyeball, except the cornea.

·  The superior and inferior conjunctival fornices are formed where the palpebral conjunctiva reflects onto the eyeball.

Lacrimal Puncta

·  These are two minute openings situated on the lacrimal papillae near the medial ends of the upper and lower eyelid margins.

·  Each punctum receives tears and opens into its corresponding lacrimal canaliculus.

Lacrimal Canaliculi

·  Each eyelid contains one lacrimal canaliculus measuring approximately 10 mm in length.

·  Each canaliculus begins at the lacrimal punctum and consists of two parts:

• Vertical part: It measures approximately 2 mm.
• Horizontal part: It measures approximately 8 mm.

·  A small dilatation called the ampulla is present near the junction of these parts.

·  The canaliculi usually unite to form a common canaliculus before entering the lacrimal sac.

Lacrimal Sac

• It is a fibroelastic, membranous sac measuring approximately 12 mm in length and 5 mm in width.
• It occupies the lacrimal fossa at the medial orbital margin.
• It receives tears from the lacrimal canaliculi and continues inferiorly as the nasolacrimal duct.

Relations of Lacrimal Sac

• Anteriorly: Medial palpebral ligament.
• Posteriorly: Lacrimal part of the orbicularis oculi muscle.
• Medially: Bony lacrimal fossa formed by the frontal process of maxilla and lacrimal bone.
• Laterally: Lacrimal fascia and adjacent venous plexus.

Nasolacrimal Duct

·  It is a membranous passage measuring approximately 18 mm in length.

·  It extends from the lacrimal sac to the inferior meatus of the nasal cavity.

·  It passes downward, backward and laterally.

·  Its lower opening lies in the anterior part of the inferior meatus.

·  The valve of Hasner is a mucosal fold at its lower opening that helps prevent reflux of nasal contents into the duct.

Blood Supply and Innervation of Lacrimal Apparatus

Arterial Supply

– The lacrimal gland receives blood from the lacrimal artery, a branch of the ophthalmic artery.

Innervation

• Parasympathetic fibres: Secretomotor fibres arise from the superior salivatory nucleus and relay in the pterygopalatine ganglion.
• Sympathetic fibres: They originate from the upper thoracic spinal cord and relay in the superior cervical ganglion.
• Sensory fibres: They are carried by the lacrimal nerve, a branch of the ophthalmic division of the trigeminal nerve.

Innervation of lacrimal gland

The lacrimal gland receives parasympathetic, sympathetic and sensory nerve fibres.

1. Parasympathetic Innervation

– Parasympathetic fibres are secretomotor and stimulate tear secretion.

Preganglionic fibres

• These fibres arise from the lacrimatory part of the superior salivatory nucleus in the pons.
• They pass through the nervus intermedius of the facial nerve and traverse the geniculate ganglion without synapsing.
• They continue through the greater petrosal nerve and nerve of the pterygoid canal to reach the pterygopalatine ganglion, where they relay.

Postganglionic fibres

• These fibres arise from the pterygopalatine ganglion and join the maxillary nerve.
• They pass successively through the zygomatic nerve, zygomaticotemporal nerve and its communicating branch to the lacrimal nerve.
• They finally reach the lacrimal gland through the lacrimal nerve.

2. Sympathetic Innervation

– Sympathetic fibres are mainly vasomotor and regulate blood flow to the gland.

Preganglionic fibres

• These fibres arise from the lateral horn of the T1 spinal cord segment.
• They enter the sympathetic trunk and ascend to the superior cervical sympathetic ganglion, where they relay.

Postganglionic fibres

• These fibres form a plexus around the internal carotid artery.
• Some fibres form the deep petrosal nerve, which joins the greater petrosal nerve to form the nerve of the pterygoid canal.
• They pass through the pterygopalatine ganglion without synapsing.
• They reach the lacrimal gland through the zygomatic, zygomaticotemporal and lacrimal nerves.

3. Sensory Innervation

– Sensory fibres are carried by the lacrimal nerve, a branch of the ophthalmic division of the trigeminal nerve.