(medial canthus). The puncta are openings approximately 0.5 mm in diameter in each eyelid. Fluid drains through them into a canaliculus which moves perpendicular to the eyelid for 2 mm, then follows the eyelid contour for 8–10 mm until the upper and lower portions fuse to form the common canaliculus. The valve of Rosenmuller separates the common canaliculus from the lacrimal sac, preventing reflux of tears. The lacrimal sac is approximately 10 mm long, located within the lacrimal sac fossa at the level of the middle meatus in the nose. The fundus of the sac extends only 3–5 mm above the medial canthus. Tears pass through the nasolacrimal duct, which lies within the maxillary bone. The duct courses laterally and posterior to empty into the nose under the inferior turbinate. The valve of Hasner is a mucosal fold that lies at the distal end of the nasolacrimal duct to prevent the nasal contents from entering the nasolacrimal sac. It is the most common site of blockage in congenital nasolacrimal duct obstruction (3).

Extraocular muscles

Six extraocular muscles are responsible for the motility of the eye (Table 1). The seventh extraocular muscle is the levator palpebrae, which has already been discussed. All the muscles originate in a circular arrangement at the apex of the bone surrounding the optic canal, called the annulus of Zinn, except the inferior oblique muscle.3 The optic nerve, cranial nerves III and VI, and the ophthalmic artery also pass through the annulus of Zinn to enter the orbit. The four rectus muscles course anteriorly to insert on their respective quadrant of the eye: medial rectus, lateral rectus, superior and inferior rectus. The medial rectus inserts closest to the limbus (5.5 mm), followed by the inferior rectus (6.0 mm), then the lateral rectus (7.0 mm), and finally the superior rectus (7.7 mm). The imaginary line connecting these insertions is called the spiral of Tillaux (4). The width of the insertions measures approximately 9–10 mm. The rectus muscles are 37 mm in length with tendons ranging from

3 mm (medial rectus) to 7 mm (lateral rectus). A portion of each rectus muscle also inserts onto connective tissue anchored to the bony orbit called a pulley. These pulleys play an important role in stabilizing the rectus muscles and the globe relative to the orbit during contractions.4 They also prevent slippage of the muscles in extreme positions of gaze. The pulleys contain smooth muscle which contracts to change the location of the pulley. Diseases of the pulleys may contribute to incomitant deviations such as A and V patterns.5

Two muscles insert obliquely on the eye. In the superior quadrant, the superior oblique originates at the annulus of Zinn and is reflected back to the eye through its pulley the trochlea, which is attached to the frontal bone. The superior oblique inserts under the superior rectus muscle posterior to the equator. It is approximately 40 mm in length with a 20 mm tendon. Its insertion measures between 7 and 18 mm in width. The inferior oblique muscle originates from the anterior margin of the maxillary bone lateral to the nasolacrimal

4 Spiral ofTillaux.

T

9 . 2 – 9 . 7

10.6–11.0

7.0–7.7

6.6–6.9

8 . 10 – 3 . 10

4

N

groove. It heads posteriorly, laterally, and superiorly to insert posterior to the equator in the inferotemporal quadrant. The inferior oblique is the shortest of the extraocular muscles, measuring 37 mm, with almost no tendon. Its insertion is 5–14 mm wide (5).

Cranial nerve III innervates the majority of the extraocular muscles, and all the intraocular muscles. A single nucleus of the III nerve innervates both levator palpebrae muscles. This is the only muscle with bilateral innervations from one nucleus. The fibers of the III nerve divide into a superior division, which supplies the levator palpebrae and superior rectus and

the inferior division, which supplies the medial rectus, inferior rectus, and the inferior oblique. The parasympathetic innervation of the iris sphincter responsible for miosis of the pupil also travels with the inferior division of the III nerve, with the nerve to the inferior oblique. The IV cranial nerve supplies the ipsilateral superior oblique muscle, and the VI cranial nerve supplies the ipsilateral lateral rectus. The blood supply to the anterior eye comes from the lateral and medial branches of the ophthalmic artery. These vessels divide into anterior ciliary arteries. Each rectus muscle carries two anterior ciliary arteries, except the

lateral rectus, which has only one ciliary artery. Disinserting more than two rectus muscles carries the risk of compromising the anterior circulation of the eye.

Anterior segment

The cornea is the transparent, outermost layer of the eye and is responsible for two-thirds of the eye’s refractive power. The average corneal diameter of a child’s eye is 12 mm vertically and 11 mm horizontally. There is a linear increase in corneal diameter during the prenatal period to result in an average diameter of 9.7–10.0 mm horizontally at 40 weeks gestation.6 During the first year of life, there continues to be a rapid rate of growth of approximately 0.14 mm per month. The growth rate then slows or stops, with no further growth detected after 6 years of age.7

The central cornea thickness is an average of 512 µm and increases in the periphery to 1.0 mm.8 The cornea contains the highest concentration of nerve endings per area, but remains avascular for transparency. There are

three layers in the cornea: the endothelium, stroma, and epithelium. The inner endothelial cells pump fluid from the stroma to maintain the transparency of the cornea. Underlying the endothelial cells is a basement membrane called Descemet’s membrane. Tightly arranged lamellae of collagen with minimal keratocytes make up the stroma. The regular arrangement allows for transparency. The outer layer of the cornea provides a barrier function (6).

The sclera is the nontransparent, more rigid outer layer of the eye. Changes in the arrangement and type of collagen give it the characteristic white color. The posterior sclera is almost twice as thick as the anterior sclera, but has only 60% of the stiffness of the anterior sclera.9 The sclera is thinnest just posterior to the insertions of the rectus muscles (0.3 mm). Transport of drugs across the sclera is dependent on its hydration level. The degree of hydration of the sclera differs with age, varying with the crosslinking and interweave of the collagen fibers.10 The axons of the ganglion cells of the retina exit through a sievelike opening in the sclera called the lamina cribrosa. These fibers will form the optic nerve.

Posterior chamber

Pupil

Anterior chamber

Lens

Iris

Schlemm’s canal

Ciliary body

The transition between the cornea and the sclera is called the limbus (7). This region is the source of the stem cells for both the corneal and conjunctival epithelium. The cornea is reliant on the conjunctiva and fluid from within the eye (anterior chamber) for moisture, nutrition, and immune protection. The conjunctiva is a mucus membrane that covers the sclera (bulbar conjunctiva) and the inner eyelids (palpebral conjunctiva). The tight fold between the bulbar conjunctiva and the palpebral conjunctiva is called the fornix. Foreign bodies are limited in their migration into the orbit by the conjunctiva. The conjunctiva is well vascularized and contains numerous lymphoid cells11 in addition to goblet cells to provide mucin to the tear film. The conjunctiva also secretes electrolytes and water into the aqueous layer. Growth factors,

androgens, and direct innervations regulate the conjunctival secretions into the tear film.12,13

Underlying the cornea is the anterior chamber of the eye. This aqueous-filled cavity measures approximately 3 mm in depth. New devices allow more precise quantification and better imaging into the iridocorneal angle within the anterior chamber. These include anterior segment optical coherence tomography

(OCT) and classic ultrasound biomicroscopy (UBM).7,14 The majority of the aqueous fluid is resorbed at the anterior chamber angle, although 10% exits through scleral absorption. The anterior chamber angle is made up of the trabecular meshwork, which extends from the sclera spur to the termination of Descemet’s membrane called Schwalbe’s line. Fluid egresses through this meshwork of thin, fibrocellular sheets to gather in the canal of Schlemm.

The anterior chamber ends at the iris. The iris sphincter and dilator muscles control the amount of light that can enter the eye through the pupil. The dilator muscle is innervated by the sympathetic system, and the sphincter muscle is innervated by the parasympathetic system.15 The color of the iris is dependent on the amount of melanin within the stroma. Absence of melanin makes the iris appear whitish blue. The area between the iris and the anterior surface of the lens and zonules is called the posterior chamber. Its volume is approximately 0.06 mL.16 The ciliary body is a ring of tissue that extends from the sclera at the limbus to the anterior extent of the retina called the ora serrata. This ciliary body is further divided into the pars plicata, which is a 2 mm