e) Lacrimal System

The lacrimal system generates the tear film that protects the eyeball and assists in the nourishment of the cornea. It is comprised of the lacrimal gland itself, small accessory lacrimal glands of the conjunctiva, and a system of ducts that drains excess tear fluid from the conjunctival space into the nasal cavity.

Structure

The lacrimal gland lies in the lateral orbit. It curves around the lateral border of levator palpebrae superioris, dividing it into a larger orbital (superior to LPS) and smaller palpebral part. Ducts from the orbital portion pierce the LPS and unite with ducts from the palpebral part. These ducts empty into the superior lateral fornix (reflection of the conjunctiva from orbital to bulbar parts).
Accessory glands are situated in the submucosa of the orbital conjunctiva. Other tear components are produced by goblet cells in the conjunctiva and the tarsal glands.
A superior and inferior canaliculus drain tears from the medial eyelids, beginning at the lacrimal puncta located at the lid margin several millimetres from the medial canthus. These small ducts convey fluid to the lacrimal sac. This dilated sac leads to the nasolacrimal duct, which passes inferiorly to empty into the inferior meatus of the nasal cavity. The duct is lined by respiratory epithelium and is contained within a bony canal.

Microscopic Structure

The lacrimal gland is tubuloacinar and similar in structure to salivary glands. The acini contain secretory low columnar cells with visible granules. The ducts are lined by non-secretory cuboidal epithelium. The lacrimal canaliculi and sac are lined by non-keratinising stratified squamous epithelium.

Relations

The lacrimal gland is lateral to the levator palpebrae superioris and superior to the lateral rectus muscle. The eyeball is medial and inferior.

Neurovascular Supply

Blood

The lacrimal gland is predominately supplied by the lacrimal branch of the ophthalmic artery. Venous drainage passes posteriorly to the superior ophthalmic vein.

Lymphatics

Lymphatic vessels pass laterally to the pre-auricular lymph nodes within the parotid gland.

Innervation

The lacrimal gland receives secretomotor supply via a convoluted route. Preganglionic fibres originate in the brainstem and exit with the facial nerve (VII); they then leave via the greater petrosal nerve, re-enter the cranium before exiting again as the nerve of the pterygoid canal. The fibres synapse in the pterygopalatine ganglion. Post-synaptic parasympathetic fibres then join the continuation of the maxillary/infraorbital nerve before branching off with the zygomatic nerve. These nerves branch from the zygomatic nerve as it passes through the orbit to join with the lacrimal nerve, a branch of the ophthalmic nerve. The lacrimal nerve passes through the lateral wall of the orbit and supplies the lacrimal gland.

Potential Routes of Malignant Spread

Lacrimal tumours are very rare. Local invasion can lead to local swelling or proptosis. Lymphatic spread leads to metastases in pre-auricular lymph nodes.


Links