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The lacrimal sac is the upper widened part of the nasolacrimal duct, which is known as the tear duct in layman's terms. In turn, the tear duct is one of the main parts of the lacrimal apparatus. The lacrimal sac plays a role in the production and drainage of tears in the eyes.
With an oval shape, the lacrimal sac has a length of around 0.47 to 0.59 inches (12 to 15 mm). The upper end of the sac has a closed and rounded structure. Its lower end runs into the tear duct. Coverin the sac's outer surface is a fibrous part of the medial palpebral ligament, or tendo oculi, which is responsible for attaching the plate of the eyelids' edges with the eye socket. The muscle responsible for closing the eyelids — orbicularis oculi — crosses the sac's inner surface.
Placed in a deep furrow, the lacrimal sac is created by the lacrimal bone and the frontal process of the maxilla. The tinniest and least sturdy of the face's bones, the lacrimal bone houses the sac from its upper half via an opening called the lacrimal fossa, or fossa for the lacrimal sac. This opening's lower part is where the tear duct is located. The frontal process of the maxilla, or the upper jaw, is involved with the sac's formation by uniting with the inner margin of the lacrimal bone's longitudinal groove called the lacrimal sulcus, or sulcus lacrimalis.
The lacrimal sac connects two parts of the lacrimal apparatus: the lacrimal canaliculi and the nasolacrimal duct. It drains tears from the surface of the eye via the lacrimal canaliculi, which is just as commonly called the lacrimal canals and lacrimal ducts. These are connected to the sac by the superior duct, which travels to it in a middle-downward fashion, and the longer and bigger inferior duct beneath the superior duct, which runs in a horizontal course. The tear duct then transports the tears from the sac to the nasal cavity, thus the "naso" prefix.
In some cases, the nasolacrimal duct may collect excess tears from the lacrimal sac. The excess fluid is then deposited into the inferior nasal meatus, which is the largest of the nose's three openings. This explains why the nose begins to run when a person cries or has watery eyes due to an allergic reaction. Also, excess tears can be facilitated by the orbicularis oculi. This especially happens during blinking as the lacrimal sac pumps in an inwardly and outwardly fashion.