The conjunctiva and
cornea are very sensitive, and as well as being a physical protective layer, they also
contain cells and glands which produce part of the tear film which continually washes over
the surface of the eye. A gland (lacrimal) is situated under the upper, outer aspect of
the eyelid near the eyebrow, and this produces the water content of the tear film, it also
produces extra tears when we cry or if the eye has been traumatised.
The surface of the eye also contains
many sensory nerve endings, with a much greater concentration in the cornea. These nerves
act as a protective mechanism for the eye, alerting the person to foreign objects (e.g.
eyelashes, sand/grit), or trauma (e.g. scratches from sharp objects such as fingernails),
which may affect the tissues of the eye, and therefore the ability to see.
Tears
Tears form a thin layer that covers the
cornea and conjunctival epithelium (skin). The functions of this ultrathin layer are:
To wet and protect the delicate
surface of the epithelium.
To inhibit infection by mechanical
flushing with each blink and the action of chemical components within the tear film.
To provide the cornea with necessary
nutrients.
To make the cornea a smooth surface
for light to pass undistorted into the eye.
The tear film is composed of
three layers.
A superficial oily layer derived from
the Meibomian glands situated along the eyelid margins.
The middle aqueous (water) layer
supplied by the main lacrimal gland and minor lacrimal glands (Krause and Wolfring located
in the conjunctiva superiorly).
A deep mucous layer derived from
Goblet cells scattered throughout the conjunctiva.
The oily layer helps prevent rapid
evaporation of the tears, and the mucous layer helps the tear film to adhere to the
surface of the eye.
Changes to the tear film components, and
or the surface of the eye cause instability resulting in what is called dry eye syndrome. |
