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Sagittal Section of the Eyelids and Anterior Orbital Structures
The conjunctiva
The conjunctiva is a mucous membrane that performs the task of
attaching the eyeball to the orbit and lids. It permits a certain
degree of rotation of the eyeball in the orbit (the hole in the
skull intended for the eyeball). The conjunctiva lines the lids and
also covers the surface of the eyeball. The part of the conjuctiva
that lines the lids is called the palpebral portion and the part
that covers the white of the eyeball is called the bulbar
conjunctiva.
The other parts of the conjuctiva include
two loose recesses, known as the upper and lower fornices. They are
also known as conjunctival sacs and are located between the bulbar
and the palpebral conjunctiva. It is possible for the eyelids and
eyeballs to move only because of the looseness of the conjunctiva at
these points. If you’ve been to an ophthalmologist or had the
experience of having eye drops put in your eyes. The eye drops are
put inside the lower fornice. This is done by pulling the outer lid
away from the globe. The drops are retained in this cavity for a
short while, which is enough for the liquid to diffuse through the
cornea and act on the internal structure of the eye.
The functions of the eyelids
It is extremely important that the front surface of the eyeball
and cornea remain moist. Our eyelids perform this all-important task
for us. When we are awake, the eyelids sweep the secretions of the
lacrimal (tear gland) apparatus and other glands over the surface at
regular intervals. And when we are sleeping, they cover our eyes
help prevent evaporation.
Normally the
eyelids of human eyes close by reflex action about every six
seconds, but if dust reaches the surface of the eye and is not
washed away, the eyelids blink more often and more tears are
produced. The Meibomian glands, a set of small glands, located on
the edges of the eyelids, produce a fatty secretion that lubricates
the eyelids and the eyelashes.
The eyelids also
protect our eyes from injuries caused by foreign bodies through the
process of blinking.
The eyelashes, a
fringe of short hairs growing on the edge of either eyelid, act as a
screen to keep dust particles and insects out of the eyes when the
eyelids are partly closed. |
