The eyebrows are folds of thickened skin covered with hair. The skin fold is supported by underlying muscle fibers. The glabella is the hairless prominence between the eyebrows.

The upper and lower eyelids (palpebrae) are modified folds of skin that can close to protect the anterior eyeball. Blinking helps spread the tear film, which protects the cornea and conjunctiva from dehydration.

The eyelids consist of five principal planes of tissues. From superficial to deep, they are the skin layer, a layer of striated muscle (orbicularis oculi), areolar tissue, fibrous tissue (tarsal plates), and a layer of mucous membrane (palpebral conjunctiva).

Structures of the Eyelids

Skin Layer

The skin of the eyelids differs from skin on most other areas of the body in that it is thin, loose, and elastic and possesses few hair follicles and no subcutaneous fat.

Orbicularis Oculi Muscle

The function of the orbicularis oculi muscle is to close the lids. Its muscle fibers surround the palpebral fissure in concentric fashion and spread for a short distance around the orbital margin. Some fibers run onto the cheek and the forehead. The portion of the muscle that is in the lids is known as its pretarsal portion; the portion over the orbital septum is the preseptal portion. The segment outside the lid is called the orbital portion. The orbicularis oculi is supplied by the facial nerve.

Areolar Tissue

The submuscular loose areolar tissue that lies deep to the orbicularis oculi muscle communicates with the subaponeurotic layer of the scalp.

Tarsal Plates

The main supporting structure of the eyelids is a dense fibrous tissue layer that—along with a small amount of elastic tissue—is called the tarsal plate. The lateral and medial angles and extensions of the tarsal plates are attached to the orbital margin by the lateral and medial palpebral ligaments. The upper and lower tarsal plates are also attached by a condensed, thin fascia to the upper and lower orbital margins. This thin fascia forms the orbital septum.

Palpebral Conjunctiva

The lids are lined posteriorly by a layer of mucous membrane, the palpebral conjunctiva, which adheres firmly to the tarsal plates. A surgical incision through the gray line of the lid margin (see the next section) splits the lid into an anterior lamella (margin) of the skin and the orbicularis muscle and a posterior lamella (margin) of the tarsal plate and the palpebral conjunctiva.

Lid Margins

The free lid margin is 25–30-mm long and about 2-mm wide. It is divided by the gray line (mucocutaneous junction) into anterior and posterior margins.

Anterior Margin

1. Eyelashes—The eyelashes project from the margins of the eyelids and are arranged irregularly. The upper lashes are longer and more numerous than the lower lashes and turn upward; the lower lashes turn downward.

   Glands of Zeis—These are small, modified sebaceous glands that open into the hair follicles at the base of the eyelashes.

   Glands of Moll—These are modified sweat glands that open in a row near the base of the eyelashes.

Posterior Margin

The posterior lid margin is in close contact with the globe, and along this margin are the small orifices of modified sebaceous glands (meibomian, or tarsal, glands).

Lacrimal Punctum

At the medial end of the posterior margin of the lid, a small elevation with a central small opening can be seen on the upper and lower lids. The puncta serve to carry the tears down through the corresponding canaliculus to the lacrimal sac.

Palpebral Fissure

The palpebral fissure is the elliptic space between the two open lids. The fissure terminates at the medial and lateral canthi. The lateral canthus is about 0.5 cm from the lateral orbital rim and forms an acute angle. The medial canthus is more elliptic than the lateral canthus and surrounds the lacrimal lake.

Two structures are identified in the lacrimal lake: the lacrimal caruncle, a yellowish elevation of modified skin containing large modified sweat glands and sebaceous glands that open into follicles that contain fine hair, and the plica semilunaris, a vestigial remnant of the third eyelid of lower animal species.

In the Asian population, a skin fold known as epicanthus passes from the medial termination of the upper lid to the medial termination of the lower lid, hiding the caruncle. Epicanthus may be present normally in young infants of all races and disappears with the development of the nasal bridge but persists throughout life in Asians.

Orbital Septum

The orbital septum is the fascia behind that portion of the orbicularis muscle that lies between the orbital rim and the tarsus and serves as a barrier between the lid and the orbit.

The orbital septum is pierced by the lacrimal vessels and nerves, the supratrochlear artery and nerve, the supraorbital vessels and nerves, the infratrochlear nerve, the anastomosis between the angular and ophthalmic veins, and the levator palpebrae superioris muscle.

The superior orbital septum blends with the tendon of the levator palpebrae superioris and the superior tarsus; the inferior orbital septum blends with the inferior tarsus.

Lid Retractors

The lid retractors are responsible for opening the eyelids. They are formed by a musculofascial complex, with both striated and smooth muscle components, known as the levator complex in the upper lid and the capsulopalpebral fascia in the lower lid.

In the upper lid, the striated muscle portion is the levator palpebrae superioris, which arises from the apex of the orbit and passes forward to divide into an aponeurosis and a deeper portion that contains the smooth muscle fibers of Müller's (superior tarsal) muscle. The aponeurosis elevates the anterior lamella of the lid, inserting into the posterior surface of the orbicularis oculi and through this into the overlying skin to form the upper lid skin crease. Müller's muscle inserts into the upper border of the tarsal plate and the superior fornix of the conjunctiva, thus elevating the posterior lamella.

In the lower lid, the main retractor is the inferior rectus muscle, from which fibrous tissue extends to enclose the inferior oblique muscle and insert into the lower border of the tarsal plate and the orbicularis oculi. Associated with this aponeurosis are the smooth muscle fibers of the inferior tarsal muscle.

The smooth muscle components of the lid retractors are innervated by sympathetic nerves. The levator and inferior rectus muscles are supplied by the third cranial (oculomotor) nerve. Ptosis is thus a feature of both Horner's syndrome and third nerve palsy.

Levator Palpebrae Superioris Muscle

The levator palpebrae muscle arises with a short tendon from the undersurface of the lesser wing of the sphenoid above and ahead of the optic foramen. The tendon blends with the underlying origin of the superior rectus muscle. The levator belly passes forward, forms an aponeurosis, and spreads like a fan. The muscle, including its smooth muscle component (Müller's muscle), and its aponeurosis form an important part of the upper lid retractor (see previous section). The palpebral segment of the orbicularis oculi muscle acts as its antagonist.

The two extremities of the levator aponeurosis are called its medial and lateral horns. The medial horn is thin and is attached below the frontolacrimal suture and into the medial palpebral ligament. The lateral horn passes between the orbital and palpebral portions of the lacrimal gland and inserts into the orbital tubercle and the lateral palpebral ligament.

The sheath of the levator palpebrae superioris is attached to the superior rectus muscle inferiorly. The superior surface, at the junction of the muscle belly and the aponeurosis, forms a thickened band (Whitnall's ligament) that is attached medially to the trochlea and laterally to the lateral orbital wall, the band forming the check ligaments of the muscle.

The levator is supplied by the superior branch of the oculomotor nerve (III). Blood supply to the levator palpebrae superioris is derived from the lateral muscular branch of the ophthalmic artery.

Sensory Nerve Supply

The sensory nerve supply to the eyelids is derived from the first and second divisions of the trigeminal nerve (V). The lacrimal, supraorbital, supratrochlear, infratrochlear, and external nasal nerves are branches of the ophthalmic division of the fifth nerve. The infraorbital, zygomaticofacial, and zygomaticotemporal nerves are branches of the maxillary (second) division of the trigeminal nerve.

Blood Supply & Lymphatics

The blood supply to the lids is derived from the lacrimal and ophthalmic arteries by their lateral and medial palpebral branches. Anastomoses between the lateral and medial palpebral arteries form the tarsal arcades that lie in the submuscular areolar tissue.

Venous drainage from the lids empties into the ophthalmic vein and the veins that drain the forehead and temple. The veins are arranged in pretarsal and posttarsal plexuses.

Lymphatics from the lateral segment of the lids run into the preauricular and parotid nodes. Lymphatics draining the medial side of the lids empty into the submandibular lymph nodes.

The

The lacrimal complex consists of the lacrimal gland, accessory lacrimal glands, lacrimal puncta, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct (Figure 1–24).

View Large Figure 1-24 Add to 'My Saved Images' The lacrimal drainage system.

The lacrimal gland consists of the following structures:

1. The almond-shaped orbital portion, located in the lacrimal fossa in the anterior upper temporal segment of the orbit, is separated from the palpebral portion by the lateral horn of the levator palpebrae muscle. To reach this portion of the gland surgically, one must incise the skin, the orbicularis oculi muscle, and the orbital septum.

2. The smaller palpebral portion is located just above the temporal segment of the superior conjunctival fornix. Lacrimal secretory ducts, which open by approximately 10 fine orifices, connect the orbital and palpebral portions of the lacrimal gland to the superior conjunctival fornix. Removal of the palpebral portion of the gland cuts off all of the connecting ducts, and thus prevents secretion by the entire gland.

The accessory lacrimal glands (glands of Krause and Wolfring) are located in the substantia propria of the palpebral conjunctiva.

Tears drain from the lacrimal lake via the upper and lower puncta and canaliculi to the lacrimal sac, which lies in the lacrimal fossa. The nasolacrimal duct continues downward from the sac and opens into the inferior meatus of the nasal cavity, lateral to the inferior turbinate. Tears are directed into the puncta by capillary attraction and gravity and by the blinking action of the eyelids. The combined forces of capillary attraction in the canaliculi, gravity, and the pumping action of Horner's muscle, which is an extension of the orbicularis oculi muscle to a point behind the lacrimal sac, all tend to continue the flow of tears down the nasolacrimal duct into the nose.

Blood Supply & Lymphatics

The blood supply of the lacrimal gland is derived from the lacrimal artery. The vein that drains the gland joins the ophthalmic vein. The lymphatic drainage joins with the conjunctival lymphatics to drain into the preauricular lymph nodes.

Nerve Supply

The nerve supply to the lacrimal gland is by (1) the lacrimal nerve (sensory), a branch of the trigeminal first division; (2) the great petrosal nerve (parasympathetic secretory), which comes from the superior salivary nucleus and is a branch of the facial nerve; and (3) sympathetic nerves in the deep petrosal nerve and accompanying the lacrimal artery and the lacrimal nerve. The greater and deep petrosal nerves form the nerve of the pterygoid canal (Vidian nerve).

Related Structures

The medial palpebral ligament connects the upper and lower tarsal plates to the frontal process at the inner canthus anterior to the lacrimal sac. The portion of the lacrimal sac below the ligament is covered by a few fibers of the orbicularis oculi muscle. These fibers offer little resistance to swelling and distention of the lacrimal sac. The area below the medial palpebral ligament becomes swollen in acute dacryocystitis, and fistulas commonly open in the area.

The angular vein and artery lie just deep to the skin, 8 mm to the nasal side of the inner canthus. Skin incisions made in surgical procedures on the lacrimal sac should always be placed 2–3 mm to the nasal side of the inner canthus to avoid these vessels.