Tears pass through the eyelid puncta and enter the superior and
inferior canaliculi oriented at right angles to the lid. The vertical
portions of the canaliculi course for 2 mm before turning at
right angles and moving parallel to the lid for 8 mm. There is
some variability in the way that the superior and inferior canaliculi
unite. They may join the sac separately or converge into a
common canaliculus. At this junction lies the valve of Rosenmüller,
a flap of mucosa that prevents reflux of tears back into
the canaliculi.
The inferior end of the lacrimal sac tapers to form the lacrimal
duct as it enters the nasolacrimal canal formed by the
maxillary, lacrimal, and inferior turbinate bones. The NDL
runs within this osseous canal for a distance of approximately
12 mm. It continues beneath the inferior turbinate as a membranous
duct for an additional 5 mm before opening into the
inferior meatus. The duct orifice is found at the junction of the
middle and anterior third of the meatus, approximately 8 mm
behind the anterior tip of the inferior turbinate and 29 mm from
the anterior nasal spine. It is often covered by a flap of mucosa,
known as Hasner’s valve, which is thought to prevent reflux of
nasal secretions. Gentle pressure over the medial canthal region
will often produce fluid or bubbles at the duct orifice to confirm
its location.
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