cornea. The cornea accounts for two-thirds of the

refractive power of the eye, focusing incident light on to

the retina.

• Middle vascular layer (uveal tract): anteriorly this consists

of the ciliary body and the iris, and posteriorly the choroid.

• Inner neurosensory layer (retina): the retina is the structure

responsible for converting light to neurological signals.

Extraocular muscles

The six extraocular muscles are responsible for eye movements

(Fig. 8.2). Cranial nerve III innervates the superior rectus, medial

rectus, inferior oblique and inferior rectus muscles. Cranial nerve

IV innervates the superior oblique muscle and cranial nerve VI

innervates the lateral rectus muscle. The cranial nerves originate

Anatomy and physiology

The eye is a complex structure situated in the bony orbit. It is

protected by the eyelid, which affords protection against injury

as well as helping to maintain the tear film. The upper lid is

elevated by the levator palpebrae superioris, innervated by cranial

nerve III, and Müller’s muscle, innervated by the sympathetic

autonomic system. Eyelid closure is mediated by the orbicularis

oculi muscle, innervated by cranial nerve VII.

The orbit also contains six extraocular muscles: the superior

rectus, medial rectus, lateral rectus, inferior rectus, superior

oblique and inferior oblique. In addition, the orbit houses the

lacrimal gland, blood vessels, autonomic nerve fibres and cranial

nerves II, III, IV and VI. The contents are cushioned by orbital

fat, which is enclosed anteriorly by the orbital septum and the

eyelids (Fig. 8.1).

The conjunctiva is a thin mucous membrane lining the posterior

aspects of the eyelids. It is reflected at the superior and inferior

fornices on to the surface of the globe. The conjunctiva is coated

in a tear film that protects and nourishes the ocular surface.

Retina

Fovea centralis

Central retina vein

Central retina artery

Optic nerve

Optic disc

Inferior rectus muscle

Superior rectus muscle

Levator palpebrae superioris muscle

Cornea

Orbicularis oculi muscle

Septum

Skin

Frontalis muscle

Orbital fat

Müller’s muscle Frontal sinus

Iris

Pupil

Sclera

Inferior oblique

Lens

Anterior chamber

Meibomian glands

Tarsal plate

Ciliary body

Vitreous body

Zonules

Ora serrata

Hyaloid canal

Fig. 8.1 Cross-section of the eye and orbit (sagittal view).

Right eye

Superior

rectus

Lateral

rectus

Lateral

rectus

Inferior

rectus

Inferior

rectus

Superior

oblique

Superior

oblique

Inferior

oblique

Inferior

oblique

Medial

rectus

Superior

rectus

Left eye

Fig. 8.2 Control of eye movements. The direction of

displacement of the pupil by normal contraction of a particular

muscle can be used to work out which eye muscle is paretic. For

example, a patient whose diplopia is maximal on looking down and

to the right has either a weak right inferior rectus or a weak left

superior oblique muscle.

Anatomy and physiology • 153

8

of Budge at the level of T1. Fibres then pass to, and synapse

in, the superior cervical ganglion before joining the surface of

the internal carotid artery and passing to the pupil along the

nasociliary and the long ciliary nerves (Fig. 8.6B).

in the midbrain and pons and then pass through the cavernous

sinus (Fig. 8.3).

Refractive elements of the eye

The major refracting elements of the eye are the tear film, the

cornea and the crystalline lens. The cornea possesses the greatest

refractive power and is the main refracting element of the eye;

the lens provides additional controllable refraction, causing the

light to focus on to the retina. When light is precisely focused on

to the retina, refraction is called emmetropia (Fig. 8.4A). When

the focus point falls behind the retina, the result is hypermetropia

(Fig. 8.4B, long-sightedness). When rays focus in front of the

retina, the result is myopia (Fig. 8.4C, short-sightedness). These

refractive errors can be corrected with lenses or with a pinhole

(Fig. 8.4D).

Visual pathway

The visual pathway consists of the retina, optic nerve, optic

chiasm, optic tracts, lateral geniculate bodies, optic radiations

and visual cortex (Fig. 8.5). Deficits in the visual pathway lead

to specific field defects.

Pupillary pathways

The pupil controls the amount of light entering the eye. The

intensity of light determines the pupillary aperture via autonomic

reflexes. Pupillary constriction is controlled by parasympathetic

nerves, and pupillary dilatation is controlled by sympathetic

nerves.

For pupillary constriction, the afferent pathway is the optic

nerve, synapsing in the pretectal nucleus of the midbrain. Axons

synapse in both cranial nerve III (Edinger–Westphal) nuclei, before

passing along the inferior division of the oculomotor nerve to

synapse in the ciliary ganglion. The efferent postganglionic

fibres pass to the pupil via the short ciliary nerves, resulting in

constriction (Fig. 8.6A).

For pupillary dilatation, the sympathetic pathway originates

in the hypothalamus, passing down to the ciliospinal centre

Cavernous sinus

Arachnoid mater

Subarachnoid space

Third ventricle Optic tracts

Anterior cerebral arteries

Oculomotor nerve

Trochlear nerve

Ophthalmic division

of trigeminal nerve

Maxillary division

of trigeminal nerve

Temporal

lobe

Anterior pituitary

Sphenoidal air sinuses

Internal carotid arteries Abducens nerve

Dura mater

Fig. 8.3 Cavernous sinus (coronal view). Neuroanatomy of cranial nerves III, IV and VI.

B

A

C

D