Lacrimal sac

The lacrimal sac is the dilated upper part of the nasolacrimal duct that receives tears from the superior and inferior canaliculi and drains them into the nasolacrimal duct. It lies in a bony groove called the lacrimal sac fossa, situated in the medial wall of the orbit, and is formed by the lacrimal bone (behind) and frontal process of the maxilla (in front).

The lacrimal sac is oval in shape and measures about 12 to 15 mm. Its upper end is closed and rounded, while the lower continues downwards as the nasolacrimal duct.

Relations

Anteriorly, its superficial surface is covered by a mucous membrane and is related anteriorly to the medial palpebral ligament,

An extension of the orbicularis oculi muscle, the lacrimal part (a.k.a. the Horner's muscle), extends behind the lacrimal sac and attaches to the posterior lacrimal crest.

Its main significance is in the lacrimal pump mechanism. During blinking, contraction of Horner's muscle compresses and pulls the lacrimal sac laterally, creating negative pressure that helps draw tears from the canaliculi into the lacrimal sac. Relaxation of the muscle then allows tears to pass inferiorly into the nasolacrimal duct. Dysfunction of this mechanism can impair tear drainage and contribute to epiphora.