Lacrimal apparatus

The lacrimal apparatus (Latin: apparatus lacrimalis) is a set of connected anatomical structures located within the orbit that are responsible for the production and drainage of tears.

The lacrimal apparatus include the following structures:

  • Lacrimal gland
  • Lacrimal canaliculi
  • Lacrimal sac
  • Nasolacrimal duct

The lacrimal gland is the tear-producing organ of the lacrimal apparatus. The gland secretes the serous transparent fluid via its excretory ducts into the lateral aspect of the superior fornix, in the space between the eyeball and the eyelids.

The tears wash over the eyeball and drain into the lacrimal canaliculi (superior and inferior) that open in the medial corner of the eye with puncta lacrimalia, the beginning of the tear drainage system. The lacrimal canaliculi convey the tears to the lacrimal sac, a dilated portion of the nasolacrimal duct lodged in a depression between the lacrimal bone and the frontal process of the maxilla.

The fluid drains further into the nasolacrimal duct, which is the final part of the lacrimal apparatus and opens into the nasal cavity, specifically, into the inferior nasal meatus. If there is excess of tears, it drains via the nasolacrimal duct into the nasal cavity, thus causing what is called a runny nose.

The secretory functions of the lacrimal apparatus are controlled by the autonomic nervous system. The sympathetic fibers arise from the carotid plexus, while the parasympathetic fibers originate from neurons of the superior salivatory nucleus in the brainstem.

Tear circulation and drainage

After being produced and secreted, tears enter the conjunctival sac. From the conjunctival sac, tears flow to the lacus lacrimalis due to the blinking. Most of the tears are carried within the fornices of the sac.

Normally, the tears, specifically tear fluid, do not flow over the cornea's surface. If that happened, that would affect the refraction of the eye. The movement of the upper eyelid during blinking allows the tear film to be distributed across the cornea maintaining its environment and preventing the cornea's superficial cells from desiccation. The tarsal glands' secrete is responsible for preventing the overflow of tears. When tears are produced too much during crying, they flow across the cornea and over the margin of the lower eyelid.

When the eyelids are closed during sleeping, the conjunctival sac is closed, so the tears are drained by the lacrimal canaliculi. The tear flow down the nasolacrimal duct is possible due to the gravity and the evaporation of the fluid at the orifice into the nose.