The Lacrimal System
ANATOMY OF THE LACRIMAL SYSTEM
A) Secretory system:-
This is formed of:
1. The main lacrimal gland:
* It is an almond shaped gland. It is incompletely divided by the levator aponeurosis into two parts:
a. Main orbital portion present in the lacrimal fossa which is present in the upper outer part of roof of the orbit.
b. Small palpebral portion present in the upper outer part of the upper lid just above the lateral aspect of superior conjunctival fornix.
* The duct system consists of 10-12 ducts arise from the orbital portion of the gland and after traversing the palpebral portion, open in the superior fornix. Removal of the palpebral part of the gland will stop all the secretion of the gland because all ducts will be injured. Dryness, however, will not occur because the accessory lacrimal glands and goblet cells are sufficient to moisten the conjunctiva.
* Microscopically it is formed of compound tubulo-acinar gland.
* The lacrimal gland is supplied by;-
- The lacrimal artery which is a branch of ophthalmic artery.
- The lacrimal vein ---> superior ophthalmic vein.
- Nerve supply:
a. Sensory from lacrimal nerve (ophthalmic division of V).
b. Secretomotor: Facial n. ---> greater superficial petrosal nerve ---> sphenopalatine ganglion ---> postganglionic fibers join the infraorbital n. to the orbit, then join the lacrimal nerve to the lacrimal gland.
c. Sympathetic vasoconstrictor fibers.
2. Accessory Lacrimal glands:
- Krause's glands: They are microscopic in size and about 40 in the superior fornix and 10 in inferior fornix.
- Wolfring glands.
3. Goblet cells of the conjunctiva:
They secrete the mucus layer of tears.
B) Drainage system:-
1. Lacrimal punctum:
It is a small opening (0.25 mm. in diameter) at the summit of the lacrimal papilla in each lid. It is directed posteriorly to face the conjunctival sac, so that it is seen only when the lid is everted or in cases of ectropion.
2. Lacrimal canaliculus:
Arising from each lacrimal punctum a canaliculus which runs vertically for 2 mm., then horizontally for 6 mm.. Then both upper and lower canaliculi open in the lacrimal sac by a common canaliculus.
3. lacrimal sac:
- It is present in the lacrimal fossa which is the most anterior part of the medial wall of the orbit.
- When it is distended it is 6 x12 mm.
- The medial palpebral ligament crosses in front of its upper part.
- The lacrimal sac is surrounded by a lacrimal fascia in which Horner’s muscle is inserted (see orbicularis oculi).
4. Naso-lacrimal duct:
It is 15 mm. long. It is related laterally to the maxillary sinus. It passes from the lower end of the sac to open in the inferior meatus of the nose. The nasal opening of the duct is guarded by a mucosal fold called Hasner's valve to prevent passage of air and nasal secretions into the sac.
The tear film:-
It is formed of the following 3 layers:-
1. The superficial oily layer: It is derived from Meibomian glands. It prevents evaporation of tears and overflow of tears over the lid margin.
2. The watery layer: It is secreted from the main and accessory lacrimal glands. It forms about 90% of the tear film thickness. Aqueous tears flow from outer canthus to inner canthus along the upper and lower lid margins forming the upper and lower tear strips to reach the punctal openings.
3. The innermost mucine layer: It is formed by the goblet cells of the conjunctiva. It makes the corneal and conjunctival surfaces hydrophilic, so that tears can spread homogeneously over their surfaces.
Functions of tears are:-
1. Optical function: It fills the minute irregularities of the corneal surface, so that it becomes a perfectly smooth refracting surface.
2. Wetting of the cornea and conjunctiva to prevent their dryness.
3. The cornea takes 85 % of its O2 requirement from air through the tears.
4. Tears contain lysozyme that has antibacterial action.
5. Mechanical wash of foreign bodies and irritants.