The lacrimal sac (latin: saccus lacrimalis) is the upper dilated segment of the nasolacrimal duct that is a part of the tear-drainage system of the lacrimal apparatus.
The lacrimal sac is situated in the inferomedial aspect of the orbit, embedded in a deep fossa located in the beginning of the nasolacrimal canal formed by the lacrimal bone and the frontal process of the maxilla. The lacrimal sac is enclosed by the lacrimal fascia and the lacrimal part of the orbicularis oculi muscle.
The size of the lacrimal sac is about 1.5 cm in length and 0.5 cm in width and it is oval in form. Its dome-shaped upper margin is called the fornix of the lacrimal sac. The lower part of the lacrimal sac continues directly into the nasolacrimal duct.
The lacrimal sac connects the superior and inferior lacrimal canaliculi, which drain tears from the eye-surface, with the nasolacrimal duct, which opens into the nasal cavity. The lacrimal sac drains into the nasolacrimal duct via the valve of Krause, which is a fold of mucous membrane at the junction between the lacrimal sac and the nasolacrimal duct.
The main function of the lacrimal sac is to serve as a reservoir for overflow of tears. The excess tears are pumped inward and outward by the lacrimal sac during blinking with the help of contractions of the lacrimal part of the orbicularis oculi muscle.