Eyelids

It is vitally important that the front surface of the eyeball,
the cornea, remain moist. This is achieved by the eyelids,
which during waking hours sweep the secretions of the
lacrimal apparatus and other glands over the surface at
regular intervals and which during sleep cover the eyes
and prevent evaporation. Maintaining moisture levels is
vital for the normal functioning of the conjunctiva and
cornea. The conjunctiva is the mucous membrane that
lines the eyelid and covers the visible portion of the eyeball
except the cornea.




The lids are essentially folds of tissue covering the
front of the orbit and, when the eye is open, leaving an
almond-shaped aperture. The points of the almond are
called canthi; that nearest the nose is the inner canthus,
and the other is the outer canthus. The lid may be divided
into four layers: (1) the skin, containing glands that open
onto the surface of the lid margin, and the eyelashes; (2) a
muscular layer containing principally the orbicularis oculimuscle, responsible for lid closure; (3) a fibrous layer that
gives the lid its mechanical stability, its principal portions
being the tarsal plates, which border directly upon the
opening between the lids, called the palpebral aperture;
and (4) the innermost layer of the lid, a portion of the conjunctiva.
The conjunctiva is a mucous membrane that
serves to attach the eyeball to the orbit and lids but permits
a considerable degree of rotation of the eyeball in the
orbit. The lid borders are kept lubricated by an oily secretion
(called sebum) of the meibomian glands. This
secretion forms part of the tear film and reduces evaporative
tear loss.