Lacrimal Excretory System
The lacrimal drainage system includes the upper and lower puncta, the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct (Fig 1-40). The lacrimal puncta are small (roughly 0.3 mm in diameter) openings on the eyelid margin, located at the extreme nasal border of the eyelids at their junction with the inner canthus (see Fig 1-27A). The inferior punctum is approximately 6.5 mm from the medial canthus; the superior punctum is 6.0 mm from it. The lower eyelid punctum sits closer to the corneal limbus because of the growth of the maxillary sinus, which draws the lower eyelid punctum laterally. The puncta are directed posteriorly into the tear lake at the inner canthus. The ampulla is a slight dilation at the angle of the canaliculus, just beyond the punctum.
These openings lead to the lacrimal canaliculi, the lacrimal sac, and finally the nasolacrimal duct, which, in turn, leads to the nose. In 90% of people, the canaliculi join to form a common canaliculus prior to entering the lacrimal sac. Fibers of the tarsal orbicularis oculi muscles surround the canalicular system and lacrimal sac, driving the tears into the system and down the duct with blinking (see Fig 1-29). A persistent membrane over the valve of Hasner is often associated with tearing and discharge in infants with nasolacrimal duct obstruction.
Figure 1-40 Lacrimal excretory system. The measurements given are for adults.
(Illustration by Christine Gralapp.)
The lacrimal puncta and the canaliculi are lined with nonkeratinized stratified squamous epithelium that merges with the epithelium of the eyelid margins. Near the lacrimal sac, the epithelium differentiates into 2 layers:
Goblet cells and occasionally cilia are present. In the canaliculi, the substantia propria consists of collagenous connective tissue and elastic fibers. The wall of the lacrimal sac resembles adenoid tissue and has a rich venous plexus and many elastic fibers.
For further discussion, see BCSC Section 7, Oculofacial Plastic and Orbital Surgery.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.