Lacrimal apparatus

Apparatus lacrimalis

Also known as: lacrimal gland, tear duct, nasolacrimal duct, tears, tear film

The system that produces tears (lacrimal gland) and drains them (puncta → canaliculi → lacrimal sac → nasolacrimal duct → inferior nasal meatus).

Overview

Basal tear secretion≈1–2 µL/min

Tear volume on the eye≈7 µL

Nasolacrimal duct length≈12–18 mm

The lacrimal gland is an almond-sized serous gland in the lacrimal fossa of the frontal bone at the superolateral orbit. The aponeurosis of levator palpebrae superioris divides it into a larger orbital and a smaller palpebral part; ≈6–12 ductules open into the superior conjunctival fornix. Tears wash medially across the eye, helped by blinking, to collect in the lacus lacrimalis at the medial canthus.

They drain through the superior and inferior lacrimal puncta (on the lacrimal papillae) into the lacrimal canaliculi, which usually join to enter the lacrimal sac, lying in its fossa behind the medial palpebral ligament. The sac continues as the nasolacrimal duct (≈12–18 mm) through the nasolacrimal canal to open into the inferior nasal meatus, guarded by a mucosal fold (valve of Hasner). Blinking pumps tears through the system (lacrimal part of orbicularis oculi).

The tear film (a few µm thick) has three layers: an outer lipid layer (meibomian glands — reduces evaporation), a middle aqueous layer (lacrimal and accessory glands — contains lysozyme, lactoferrin, secretory IgA, electrolytes and growth factors) and an inner mucin layer (conjunctival goblet cells — wets the corneal epithelium). Modern models describe the aqueous and mucin layers as a mixed muco-aqueous gel.

Location

Gland: superolateral orbit (lacrimal fossa of the frontal bone). Drainage: medial canthus → lacrimal fossa (lacrimal bone and maxilla) → nasolacrimal canal → inferior nasal meatus.

Parts & subdivisions

Lacrimal gland

Serous tubuloacinar gland; orbital and palpebral lobes separated by the levator aponeurosis. Accessory lacrimal glands (Krause, Wolfring) provide basal secretion.

Lacrimal puncta and canaliculi

Puncta ≈0.3 mm openings on the medial lid margins; canaliculi run vertically ≈2 mm then horizontally ≈8 mm to the sac.

Lacrimal sac

Upper dilated end of the nasolacrimal duct, in the lacrimal fossa.

Nasolacrimal duct

Runs downward, slightly backward and laterally to the inferior meatus; explains a runny nose when crying.

Tear film

Lipid (meibomian), aqueous (lacrimal) and mucin (goblet cell) components; antimicrobial lysozyme, lactoferrin and IgA.

Anatomy

Innervation

Secretomotor (parasympathetic): superior salivatory nucleus → facial nerve (nervus intermedius) → greater petrosal nerve → nerve of the pterygoid canal (Vidian nerve) → pterygopalatine ganglion → postganglionic fibres via the zygomatic nerve (V2) and a communicating branch to the lacrimal nerve (V1) → gland. Sympathetic: from the superior cervical ganglion via the deep petrosal nerve (vasomotor). Sensory: lacrimal nerve (V1).

Arterial supply

Lacrimal artery (branch of the ophthalmic artery).

Venous drainage

Superior ophthalmic vein.

Function

Keep the cornea and conjunctiva moist and optically smooth (the air–tear interface is the eye's main refracting surface)

Supply oxygen and nutrients to the avascular cornea and wash away debris

Antimicrobial defence (lysozyme, lactoferrin, secretory IgA)

Reflex and emotional tearing

Secretions

Basal tear secretion ≈1–2 µL/min; total tear volume ≈7 µL. Reflex tearing is triggered by corneal/conjunctival irritation (afferent V1, efferent VII parasympathetic).

Clinical significance

Dry eye and Sjögren syndrome

Reduced aqueous production (e.g. Sjögren syndrome, ageing, anticholinergic drugs) or excess evaporation (meibomian gland dysfunction) causes a gritty, burning sensation. Schirmer test measures tear production. A facial nerve lesion proximal to the geniculate ganglion (origin of the greater petrosal nerve) abolishes lacrimation on that side.

Nasolacrimal duct obstruction

Congenital obstruction (imperforate valve of Hasner) affects ≈5–10% of newborns and causes a watery, sticky eye; most resolve spontaneously or with sac massage. In adults, obstruction may cause dacryocystitis (painful swelling below the medial canthal tendon) and is treated by dacryocystorhinostomy.

Crocodile tears

After facial nerve injury, regenerating salivary parasympathetic fibres may be misdirected to the lacrimal gland, causing tearing while eating (gustatory lacrimation).