Healthcare and Medicine Reference
General Appearance of the Ocular Surface
In the normal ocular surface, the cornea occupies the approximate centre
of the exposed surface. The outer limit of the cornea adjacent to the bulbar
conjunctiva is the limbus. At both sides of the limbus are two triangular
sclero-conjunctival areas, visualised as the white part of the sclera. When
a person is looking straight ahead, the lower eyelid height is normally 1-2
mm higher or lower than the lower corneal limbus (see section on Cornea),
whereas the superior eyelid is normally 1-2 mm higher than the visual axis
but just lower than the superior corneal limbus (Lens et al. 2007).
The upper and lower eyelids meet at the medial (inner) and lateral
(outer) canthi which are the angles of the palpebral fi ssures. The medial
canthus is near to the nose, whereas the lateral canthus is located temporally.
The medial canthus is usually positioned slightly lower than the lateral
canthus. The positions of the canthi have signifi cance in oculoplastic surgery.
They facilitate the fl ow of the tear fi lm from the lateral to medial direction,
and are youthful-looking aesthetically.
The eyelid or palpebrae refers to a movable fold of skin, muscle and cartilage
that can be closed or opened over the eyeball. The upper and lower eyelids
form a covering over the globe protecting against excessive light or injury.
When the eyelids are open, the margins or the palpebral fi ssures form an
almond-shaped structure (Lens et al. 2007).
The eyelid structure consists of four layers. The fi rst or outermost
layer includes the skin, eyelashes and associated glands. The second layer
comprises the muscular layer, namely the orbicularis oculi, the circular
sphincter-like muscles responsible for closing the eyelids. The third fi brous
layer, important for mechanical stability of the eyelid, consists mainly of the
tarsal plate. The innermost layer of the eyelid is the palpebral conjunctiva.
The fi rst two layers are sometimes termed anterior lamella of the eyelid,
whereas the last two layers are termed posterior lamella of the eyelid. In
oculoplastic surgery, a substantial full- thickness eyelid defect may need to
have the anterior and posterior lamellae reconstructed separately, as these
lamellae have different mechanical requirements.
The orbicularis oculi are innervated by cranial nerve VII. When there
is pathology of cranial nerve VII, such as in Bell's palsy, the eyelids may
not be able to close. On the other hand, some conditions irritate the ocular
surface, resulting in secondary blepharospasm and tonic contraction of
orbicularis muscle. The levator palpebrae muscles are innervated by a
branch of the cranial nerve III. In the event that the cranial nerve III function
is compromised, there may be drooping of the upper eyelid or, in more
extreme cases, inability to open the lid.