Eyelids anatomy

The eyelids are mobile tissue curtains placed in front
of the eyeballs (Fig. 14.1). These act as shutters
protecting the eyes from injuries and excessive light.
These also perform an important function of spreading
the tear film over the cornea and conjunctiva and
also help in drainage of tears by lacrimal pump system.
Parts of eyelid. Each eyelid is divided by a horizontal
furrow (sulcus) into an orbital and tarsal part.
Position of lids. When the eye is open, the upper lid
covers about one-sixth of the cornea and the lower
lid just touches the limbus.
Canthi. The two lids meet each other at medial and
lateral angles (or outer and inner canthi). The medial
canthus is about 2 mm higher than the lateral canthus.
Palpebral aperture. It is the elliptical space between
the upper and the lower lid. When the eyes are open
it measures about 10-11 mm vertically in the centre
and about 28-30 mm horizontally.
The lid margin. It is about 2-mm broad and is divided
into two parts by the punctum. The medial, lacrimal
portion is rounded and devoid of lashes or glands.
The lateral, ciliary portion consists of a rounded
anterior border, a sharp posterior border (placed
against the globe) and an intermarginal strip (between
the two borders). The grey line (which marks junction
of skin and conjunctiva) divides the intermarginal strip
into an anterior strip bearing 2-3 rows of lashes and a
posterior strip on which openings of meibomian
glands are arranged in a row. The splitting of the
eyelids when required in operations is done at the
level of grey line.
STRUCTURE
Each eyelid consists (from anterior to posterior) of
the following layers (Fig. 14.2):
1. The skin. It is elastic having a fine texture and is
the thinnest in the body.
2. The subcutaneous areolar tissue. It is very loose
and contains no fat. It is thus readily distended by
oedema or blood.
3. The layer of striated muscle. It consists of
orbicularis muscle which forms an oval sheet across
the eyelids. It comprises three portions: the orbital,
palpebral and lacrimal. It closes the eyelids and is
supplied by zygomatic branch of the facial nerve.
Therefore, in paralysis of facial nerve there occurs
lagophthalmos which may be complicated by
exposure keratitis.
In addition, the upper lid also contains levator
palpebrae superioris muscle (LPS). It arises from
the apex of the orbit and is inserted by three parts
on the skin of lid, anterior surface of the tarsal plate
and conjunctiva of superior fornix. It raises the
upper lid. It is supplied by a branch of oculomotor
nerve.
4. Submuscular areolar tissue. It is a layer of loose
connective tissue. The nerves and vessels lie in this
layer. Therefore, to anaesthetise lids, injection is
given in this plane.
5. Fibrous layer. It is the framework of the lids and
consists of two parts: the central tarsal plate and the
peripheral septum orbitale (Fig. 14.3).
i. Tarsal plate. There are two plates of dense
connective tissue, one for each lid, which give
shape and firmness to the lids. The upper and
lower tarsal plates join with each other at medial
and lateral canthi; and are attached to the orbital
margins through medial and lateral palpebral
ligaments. In the substance of the tarsal plates lie
meibomian glands in parallel rows.
ii. Septum orbitale (palpebral fascia). It is a thin
membrane of connective tissue attached centrally
to the tarsal plates and peripherally to periosteum
of the orbital margin. It is perforated by nerves,
vessels and levator palpebrae superioris (LPS)
muscle, which enter the lids from the orbit.
6. Layer of non-striated muscle fibres. It consists
of the palpebral muscle of Muller which lies deep to
the septum orbitale in both the lids. In the upper lid
it arises from the fibres of LPS muscle and in the
lower lid from prolongation of the inferior rectus
muscle; and is inserted on the peripheral margins of
the tarsal plate. It is supplied by sympathetic fibres.
7. Conjunctiva. The part which lines the lids is
called palpebral conjunctiva. It consists of three
parts: marginal, tarsal and orbital.
GLANDS OF EYELIDS (Fig. 14.4)
1. Meibomian glands. These are also known as
tarsal glands and are present in the stroma of
tarsal plate arranged vertically. They are about
30-40 in the upper lid and about 20-30 in the
lower lid. They are modified sebaceous glands.
Their ducts open at the lid margin. Their secretion
constitutes the oily layer of tear film.
2. Glands of Zeis. These are also sebaceous glands
which open into the follicles of eyelashes.
3. Glands of Moll. These are modified sweat glands
situated near the hair follicle. They open into the
hair follicles or into the ducts of Zeis glands.
They do not open directly onto the skin surface
as elsewhere.
4. Accessory lacrimal glands of Wolfring. These
are present near the upper border of the tarsal
plate.
BLOOD SUPPLY
The arteries of the lids (medial and lateral palpebral)
form marginal arterial arcades which lie in the
submuscular plane in front of the tarsal plate, 2 mm
away from the lid margin, in each lid. In the upper lid
another arcade (superior arterial arcade) is formed
which lies near the upper border of the tarsal plate.
Branches go forward and backward from these arches
to supply various structures.
Veins. These are arranged in two plexuses: a posttarsal
which drains into ophthalmic veins and a pretarsal
opening into subcutaneous veins.
Lymphatics. These are also arranged in two sets: the
pre-tarsal and the post-tarsal. Those from lateral half
of the lids drain into preauricular lymph nodes and
those from the medial half of the eyelids drain into
submandibular lymph nodes.
NERVES OF LIDS
Motor nerves are facial (which supplies orbicularis
muscle), oculomotor (which supplies LPS muscle) and
sympathetic fibres (which supply the Muller’s
muscle). Sensory nerve supply is derived from
branches of the trigeminal nerve