1
Anatomy

Learning Objective

• To learn the anatomy of the eye, the orbit and the third, fourth and sixth cranial nerves, as a background to the medical conditions affecting them.

Introduction

Knowledge of ocular anatomy and function is important to the understanding of eye diseases. A brief outline is given below.

Surface Anatomy of the Face

The eyes are disposed symmetrically about the face and their forward-looking arrangement permits a large overlap in visual fields, the basis of stereopsis. Lying within the bony orbits, they are protected from trauma by the orbital walls and rims and by the eyelids, by blinking and eye closure. With the eyes open and looking straight ahead, all but the upper and lower corneal margins are exposed in the palpebral aperture, together with two small white triangles of bulbar conjunctiva, overlying the sclera. The medial and lateral ends of the fissure are known as the medial and lateral canthi (Figure 1.1).

Figure 1.1 The eye, looking straight ahead.

The lids and the upper and lower orbital rims are overlain by the orbicularis muscle which sweeps over these structures in an ellipse, from a region just medial to the medial orbital rim. It acts as the palpebral sphincter (Figure 1.2). Like all other muscles of the face, it is supplied by the seventh cranial nerve. Contraction of its orbital part results in protective, forced eye closure, while contraction of its palpebral part is employed in the downstroke of the upper lid during a blink. The levator palpebrae muscle, the elevator of the upper lid (see below), is concerned with the upstroke of the blink (third cranial nerve). These synchronized contractions are completed within just 300 ms. The contents of the orbit are separated from those of the lid by a connective tissue sheet, or orbital septum, which extends from the orbital rim to the tarsal plate, deep to orbicularis.

Figure 1.2 Disposition of the orbicularis is muscle.

Sensory Innervation of the Face: The Fifth Cranial Nerve

The sensory innervation of each half of the face is provided by the trigeminal nerve (Figure 1.3). The eye, upper lid, eyebrow, forehead and nose are supplied by its ophthalmic division (V1), via its lacrimal, frontal and nasociliary branches, which enter the orbit through the superior orbital fissure. The maxillary division (V2), lying inferolaterally to V1 in the cavernous sinus, exits the cranial cavity via the foramen rotundum and, at the inferior orbital fissure, gives rise to the infraorbital and zygomatic nerves. These supply, chiefly, the lower lid and the upper lip and cheek. The mandibular division (V3), exiting the skull via the foramen ovale, supplies the lower lip, chin and jaw and the preauricular skin and temporal region. It is also motor to the muscles of mastication.

Figure 1.3 Sensory innervation of the face by the trigeminal nerve.

The neurons of the three divisions of the trigeminal nerve converge upon the trigeminal ganglion, whose sensory roots enter the pons to be distributed to the trigeminal nuclei in the brainstem. The mesencephalic nucleus is concerned with proprioception, the main sensory nucleus with touch and the medullary nucleus of the spinal tract with pain and temperature sensibility. Fibres from the ophthalmic division go to the lowest part of this nucleus, those from the mandibular division to its highest part.

Gross Anatomy of the Eye

The eye comprises (Figure 1.4):

• A tough, collagenous outer coat which is transparent anteriorly (the cornea) and opaque posteriorly (the sclera). The junction between them is called the limbus. The extraocular muscles attach to the outer sclera, while the optic nerve leaves the globe posteriorly.
• A rich vascular coat (the uvea) forms the choroid posteriorly and the ciliary body and iris anteriorly. Internal to the choroid lies the retina, to which it is firmly attached and whose outer two-thirds it nourishes.
• The ciliary body contains the smooth ciliary muscle, whose contraction controls focusing by altering lens shape. The lens lies behind the iris, supported by the zonules, whose fine fibres run from the lens equator to the ciliary body. When the eye is focused for distance, tension in the zonule maintains a flattened profile of the lens. When the ciliary body contracts, tension is relaxed, the lens takes up a more curved shape and focusing for near objects is achieved.
• The ciliary body also provides attachment for the iris, which forms the pupillary diaphragm. The ciliary epithelium secretes aqueous humour and maintains the ocular pressure.
• The space between the cornea anteriorly and the iris and central lens posteriorly, filled with aqueous humour, is the anterior chamber, whose periphery is the iridocorneal angle or drainage angle. The angle gives access to a meshwork of cells and collagen beams called the trabecular meshwork, through which aqueous drains into Schlemm's canal and thence into the venous system via the aqueous veins. This is the basis of aqueous drainage.
• Between the iris, lens and ciliary body lies the posterior chamber, a narrow space distinct from the vitreous body behind. Both the anterior and posterior chambers are filled with aqueous humour. Between the lens and the retina lies the vitreous body, occupying most of the posterior segment of the eye. The posterior segment refers to the posterior two-thirds of the eye, lying behind the anterior vitreous face. The anterior segment comprises all those structure lying anterior to the vitreous.

Figure 1.4 The basic anatomy of the eye.

Anteriorly, the bulbar conjunctiva of the globe passes from the limbus into the fornices of the conjunctival sac and thence onto the posterior surface of the lids, where it becomes the tarsal conjunctiva. A connective tissue layer (Tenon's capsule) separates the conjunctiva from the sclera and is prolonged backwards as a sheath around the rectus muscles.

The Orbit

The eye, or globe, lies within the bony orbit, which has the shape of a four-sided pyramid (Figure 1.5). At its posterior apex is the optic canal, which transmits the optic nerve to the chiasm, tract and lateral geniculate body. The superior and inferior orbital fissures transmit the blood vessels and cranial nerves that supply the orbital structures. The lacrimal gland lies anteriorly in the superolateral aspect of the orbit. On the anterior part of the medial wall lies the fossa for the lacrimal sac.

Figure 1.5 The anatomy of the orbit.

The Eyelids (The Tarsus)

The eyelids (Figure 1.6):

• offer mechanical protection to the globe;
• spread the tears over the conjunctiva and cornea with each blink.

Figure 1.6 The anatomy of the eyelids.

The levator muscle is the main elevator of the upper lid. It passes forwards from an attachment on the sphenoid bone, above the optic foramen, to an aponeurosis which inserts into the tarsal plate. It is innervated by the third cranial nerve. Damage to the nerve or weakening of the aponeurosis in old age results in drooping of the upper eyelid (ptosis). A flat, smooth muscle, (the superior tarsal, or Müller's muscle) innervated by the sympathetic nervous system, arises from the deep surface of the levator and inserts into the tarsal plate. Müllers muscle also contributes to a lesser extent to elevation of the lid, and if the sympathetic supply is damaged, a slight ptosis results as part of Horner's syndrome.

Each eyelid comprises:

• an anterior layer of skin;
• the palpebral part of the orbicularis muscle;
• a tough collagenous layer (the tarsal plate) which houses the meibomian oil glands;
• an epithelial lining, the tarsal conjunctiva;
• the lash-bearing, lid margins.

The tarsal conjunctiva is reflected, via the fornices, onto the anterior surface of the globe, where it becomes the bulbar conjunctiva. When the eyes are closed, this lining forms the conjunctival sac, which contains the tears. When the eyes open, a tear film is formed which covers and protects the exposed cornea and conjunctiva. At the lid margins, the tear film is bordered by the tear menisci (Figure 1.7).

Figure 1.7 Drawing of the eye: (a) in cross section, (b) in frontal view to illustrate the distribution of the tears. (Source: Gaffney EA et al. Progress in Retinal and Eye Research 2010; 29(1):59–78. Reproduced with permission of Elsevier.)

The lid margins exhibit a narrow, posterior conjunctival zone, continuous with the tarsal conjunctiva and a cutaneous zone anteriorly, which bears the lashes. These zones are separated by the mucocutaneous junction which forms the anterior boundary of each tear meniscus (Figure 1.8). At the medial ends of each lid margin, dipping into a lake of tears at the nasal canthus, are the lacrimal puncta, through which tears drain from the tear menisci into the lacrimal drainage system.

Figure 1.8 Diagram of lid margin to show meibomian...