Structure of tear film

Wolff was the first to describe the detailed structure
of the fluid covering the cornea and called it
precorneal film. He described this film to consist of
three layers, which from posterior to anterior are
mucus layer, aqueous layer and lipid or oily layer
(Fig. 15.2).
1. Mucus layer. It is the innermost and thinnest
stratum of the tear film. It consists of mucin secreted
by conjunctival goblet cells and glands of Manz. It
converts the hydrophobic corneal surface into
hydrophilic one.
2. Aqueous layer. The bulk of tear film is formed by
this intermediate layer which consists of tears
secreted by the main and accessory lacrimal glands.
The tears mainly comprise of water and small
quantities of solutes such as sodium chloride, sugar,
urea and proteins. Therefore, it is alkaline and salty
in taste. It also contains antibacterial substances like
lysozyme, betalysin and lactoferrin.
3. Lipid or oily layer. This is the outermost layer of
tear film formed at air-tear interface from the secretions
of Meibomian, Zeis, and Moll glands. This layer
prevents the overflow of tears, retards their
evaporation and lubricates the eyelids as they slide
over the surface of the globe.
Functions of tear film
1. Keeps the cornea and conjunctiva moist.
2. It provides oxygen to the corneal epithelium.
3. Washes away debris and noxious irritants.
4. Prevents infection due to presence of antibacterial
substances.
5. Facilitates movements of the lids over the globe.
Secretion of tears
Tears are continuously secreted throughtout the day
by accessory (basal secretion) and main (reflex
secretion) lacrimal glands. Reflex secretion is in
response to sensations from the cornea and
conjunctiva, probably produced by evaporation and
break-up of tear film. Hyperlacrimation occurs due to
irritative sensations from the cornea and conjunctiva.
Afferent pathway of this secretion is formed by fifth
nerve and efferent by parasympathetic (secretomotor)
supply of lacrimal gland.
Elimination of tears
Tears flow downward and medially across the surface
of eyeball to reach the lower fornix and then via lacus
lacrimalis in the inner canthus. From where they are
drained by lacrimal passages into the nasal cavity
(Fig. 15.3A). This is brought about by an active
lacrimal pump mechanism constituted by fibres of the
orbicularis (especially Horner’s muscle) which are
inserted on the lacrimal sac. When the eye lids close
during blink, contraction of these fibres distends the
fundus of the sac, creates therein a negative pressure
which syphons the tears through punctum and
canaliculi into the sac (Fig. 15.3B). When the eyelids
open, the Horner’s muscle relaxes, the lacrimal sac
collapses and a positive pressure is created which
forces the tears down the nasolacrimal duct into the
nose (Fig. 15.3C). Therefore, in atonia of sac, tears
are not drained through the lacrimal passages, in
spite of anatomical patency; resulting in epiphora.