Position structure and nerve supply of the lacrimal gland The lacrimal

gland is about the size of a walnut; it lies beneath the superior temporal margin of the orbital bone in the lacrimal fossa of the frontal bone and is neither visible nor palpable. A palpable lacrimal gland is usually a sign of a pathologic change such as dacryoadenitis. The tendon of the levator palpebrae muscle divides the lacrimal gland into a larger orbital part (two-thirds) and a smaller palpebral part (one-third). Several tiny accessory lacrimal glands (glands of Krause and Wolfring) located in the superior fornix secrete additional serous tear fluid.

The lacrimal gland receives its sensory supply from the lacrimal nerve. Its parasympathetic secretomotor nerve supply comes from the nervus interme-dius. The sympathetic fibers arise from the superior cervical sympathetic ganglion and follow the course of the blood vessels to the gland.

Tear film: The tear film (Fig. 3.2) that moistens the conjunctiva and cornea is composed of three layers:

1. The outer oily layer (approximately 0.1 ^m thick) is a product of the meibomian glands and the sebaceous glands and sweat glands of the margin of

— Structure of the tear film.

- cholesteryl esters

- cholesterol

- triglyceride

- phospholipids

Water layer (approx. 8 |am)

- approx. 1% inorganic salts

- approx. 0.2-0.6% proteins, globulins, and albumin

- Rest: glucose, urea, neutral mucopolysaccharides (mucin), and acidic mucopolysaccharides

Epithelium with microvilli and folds

Krause Gland

m

Conjunctival goblet cells

Fig. 3.2 The tear film is composed of three layers:

❖ An oily layer (prevents rapid desiccation).

❖ A watery layer (ensures that the cornea remains clean and smooth for optimal transparency).

❖ A mucin layer (like the oily outer layer, it stabilizes the tear film).

the eyelid. The primary function of this layer is to stabilize the tear film. With its hydrophobic properties, it prevents rapid evaporation like a layer of wax.

2. The middle watery layer (approximately 8 ^m thick) is produced by the lacrimal gland and the accessory lacrimal glands (glands of Krause and Wolfring). Its task is to clean the surface of the cornea and ensure mobility of the palpebral conjunctiva over the cornea and a smooth corneal surface for high-quality optical images.

3. The inner mucin layer (approximately 0.8 ^m thick) is secreted by the goblet cells of the conjunctiva and the lacrimal gland. It is hydrophilic with respect to the microvilli of the corneal epithelium, which also helps to stabilize the tear film. This layer prevents the watery layer from forming beads on the cornea and ensures that the watery layer moistens the entire surface of the cornea and conjunctiva.

Lysozyme, beta-lysin, lactoferrin, and gamma globulin (IgA) are tear-specific proteins that give the tear fluid antimicrobial characteristics.

Tear drainage: The shingle-like arrangement of the fibers of the orbicularis oculi muscle (supplied by the facial nerve) causes the eye to close progressively from lateral to medial instead of the eyelids simultaneously closing along their entire length. This windshield wiper motion moves the tear fluid medially across the eye toward the medial canthus (Figs. 3.3a-c).

The superior and inferior puncta lacrimales collect the tears, which then drain through the superior and inferior lacrimal canaliculi into the lacrimal sac. From there they pass through the nasolacrimal duct into the inferior concha (see Fig. 3.1).

— Combined function of the orbicularis oculi muscle and the lower lacrimal system.

Opening the eye Closing the eye Levator palpebrae Orbicularis oculi superioris muscle muscle (facial (oculomotor nerve) nerve)

Opening the eye Closing the eye Levator palpebrae Orbicularis oculi superioris muscle muscle (facial (oculomotor nerve) nerve)

Levator Muscle
Figs. 3.3 a-c As the eyelids close, they act like a windshield wiper to move the tear fluid medially across the eye toward the puncta and lacrimal canaliculi.

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