The Eye

The EyeMovements of the EyeballStructure of the EyeContents of the EyeballClinical notesMovements of the Eyeball

Adduction is the rotation of the eye medially.Elevation is the rotation of the eye upward.Depression is the rotation of the eye downward.Abduction is the rotation of the eye laterally.The eye rotates either medially or laterally.Rotatory movements ofthe eyeball use the pupil anterior pole as the marker.Extrinsic Muscles Producing Movement of the Eye

Medial rectusInsertion:eyeball just posterior tocorneoscleral junction.

Origin:Tendinous ring on posterior wall of orbital cavity.

Lateral rectusSuperior rectusInferioroblique muscleInferior rectusSuperior oblique muscle

Extrinsic Muscles Producing Movement of the EyeThe tendon of the superior oblique muscle passes through a fibrocarti-laginous pulley (trochlea) attached to the frontal bone.The tendon now turns backward and laterally and is inserted into the sclera beneath the superior rectus muscle.Superior oblique muscle

Extrinsic Muscles Producing Movement of the Eye

Inferioroblique muscleMedial Superior oblique: Depresses the eye when looking medially Inferior oblique: Elevates the eye when looking medially Superior rectus: Elevates the eye when looking laterally Inferior rectus: Depresses the eye when looking laterally Medial rectus: Adduction when pupil moving along horizontal plane Lateral rectus: Abduction when pupil moving along horizontal plane

Cranial 4 (Trochlear): innervates the superior obliqueCranial Nerve 6 (Abducens): innervates the lateral rectusCranial Nerve 3 (Oculomotor): innervates all the remaining muscles (ie medial rectus, inferior oblique, superior rectus and inferior rectus).When you ask a patient to lookmedially and downward at the tip of his or her nose, you are testing the superior oblique at its best position.Conversely, by asking the patient to look medially and upward, you are testing the inferior oblique at its best position.Testing Extraocular Movements

1. Stand in front of the patient. 2. Ask them to follow your finger with their eyes while keeping their head in one position 3. Using your finger, trace an imaginary "H" or rectangular shape in front of them, making sure that your finger moves far enough out and up/down so that you're able to see all appropriate eye movements (ie lateral and up, lateral down, medial down, medial up). 4. At the end, bring your finger directly in towards the patient's nose. This will cause the patient to look cross-eyed and the pupils should constrict, a response referred to as accommodation.

Left CN 6 PalsyPatient was asked to look left. Note that left eye will not abduct.Right CN 3 Palsy - Note: Right eye is deviated laterally, there is ptosis of right lid, and the right pupil is dilated.

PathologyParalysis of the 4th Cranial Nerve

Fascial Sheath of the Eyeball

The fascial sheath surrounds the eyeball from the optic nerve to the corneoscleral junction.It separates the eyeball from the orbital fat and provides it with a socket for free movement. The medial and lateral check ligaments attach the socket to the orbital wall.It is perforated by the tendons of the orbital muscles and is reflected onto each of them as a tubular sheath.The lower part of the fascial sheath, is thickened and serves to suspend the eyeball; it is called the suspensory ligament of the eye.Structure of the EyeThe eyeball consists of three coats:

fibrous coat, vascular pigmented coat, nervous coat.The cornea (anterior transparent part) The fibrous coat

The sclera is directly continuous in front with the cornea at the corneoscleral junction, or limbus.

The sclera (posterior opaque part)The Sclera

The opaque sclera is composed of dense fibrous tissue andis white.Posteriorly, it is pierced by the optic nerve (lamina cribrosa) and is fused with the dural sheath of that nerve.The sclera is also pierced by the ciliary arteries and nerves and their associated veins, the venae vorticosae. The scleral stroma is predominantly composed of collagen fibers that vary in diameter and are in haphazard array. Scattered fibroblasts occur between the collagen bundles. This figure illustrates a blood vessel that penetrates the sclera and extends to the prominently vascularized choroid through an emissarial canal ( single arrows ). Pigmented melanocytes are also present. The fibers of the inferior oblique muscle are present at the site of insertion upon the outer sclera ( double arrows ).

The Cornea

The transparent cornea is largely responsible for the refraction of the light entering the eye. It is in contact posteriorly with the aqueous humor.Blood Supply The cornea is avascular and devoid of lymphatic drainage. It is nourished by diffusion from the aqueous humor and from the capillaries at its edge.Nerve Supply Long ciliary nerves from the ophthalmic division of the trigeminal nerve.Function: The cornea is the most important refractive medium of the eye. This refractive power occurs on the anterior surface of the cornea, where the refractive index of the cornea (1.38) differs greatly from that of the air. The importance of the tear film in maintaining the normal environment for the corneal epithelial cells should be stressed.HISTOLOGY: The stratified squamous epithelium of the cornea overlies the basal lamina and Bowman's layer. The clefts within the collagenous stroma represent artifacts of tissue processing. No blood vessels or lymphatics are normally present within the cornea.

Vascular Pigmented CoatThe choroidThe ciliary bodyThe irisouter pigmented layer;Inner vascular layer.

13The ciliary body

3. The ciliary body contains smooth muscle fibers called ciliary muscles that is composed of meridianal and circular fibers of smooth muscle and help to control the shape of the lens.1. The ciliary ring is the posterior part of the body, that encircles the lens.2. Towards the posterior surface of the lens there are ciliary processes which are connected with the suspensory ligaments to the lens and contain capillaries. The capillaries secrete the fluid (aqueous humor) into the anterior segment of the eyeball. The ciliary muscle

Suspensory ligaments connect the ciliary muscle to the lens. Action: When the ciliary muscle is relaxed the ligaments are taut, and the lens is stretched thin enabling it to focus on distant objects.

When the ciliary muscle is contracted the suspensory ligaments become less taut, and the lens becomes rounder so that it can focus on objects that are nearby (accommodation).Ligaments are tautLigaments are looseThe ciliary muscle

Nerve supply:

Sympathetic nerve fiber stimulation causes relaxation of the muscle, whereas parasympathetic stimulation causes muscle contraction. The parasympathetic fibers come to the ciliary muscle from the oculomotor nerve. After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves.The Iris and Pupil

PupilIrisThe periphery of the iris is attached to the anterior surface of the ciliary body.It divides the space between the lens and the cornea into an anterior and a posterior chamber filled with the aqueous humor.The iris contains blood vessels, pigment cells and smooth muscles.

Chambers of the Eye

The aqueous humor passes through the pupil into the anterior chamber and drains into the canal of Schlemm (scleral venous sinus) at the limbus of the cornea.

The anterior and posterior chambers of the anterior cavity of the eye are filled with aqueous humor. Aqueous humor is continually produced at the ciliary processes of the ciliary body. Sphincter and dilator pupillaeThe radial fibers form the dilator pupillae and consist of a thin sheet of radial fibers that lie close to the posterior surface.The smooth muscle fibers of the iris control the diameter of the central opening (pupil) and are involuntary. It consists of circular and radiating fibers. The circular fibers form the sphincter pupillae and are arranged around the margin of the pupil.

The sphincter pupillae constricts the pupil in the presence of bright light and during accommodation.The dilator pupillae dilates the pupil in the presence of light of low intensity or in the presence of excessive sympathetic activity such as occurs in fright.

Sphincter and dilator pupillaeThe sphincter pupillae is supplied by parasympathetic fibers from the oculomotor nerve.The dilator pupillae is supplied by sympathetic fibers, which pass forward to the eyeball in the long ciliary nerves.After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves.

Nervous Coat: The Retina1. Pigmented layer 2. Neural layer

The pigmented layer is a single-cell layer of pigmented cells that absorb light after it passes through the retina. The neural layer consists of three layers of cells that include the photoreceptors, cells that initiate the processing of visual information and blood vessels that supply the neural layer.Visual receptors Rods are more sensitive to light but there is only one type of rod and color discrimination is not possible with rods.

There are two types of photoreceptors: Cones are less sensitive to light but there are three types of cones with sensitivities in different regions of the light spectrum. Cones provide color discrimination and greater detail. The photoreceptors synapse with bipolar cells. The transmission of visual information through the bipolar cells is modulated by horizontal cells found in this layer.This is a high power image of the retina, showing its characteristic multiple layers.

Ocular fundus

The axons of the ganglion cells leave the retina by converging on the optic disc and penetrating through the wall of the eye.

The left ocular fundus as seen with an ophthalmoscope.The cornea and lens focuses light on a spot of the retina that contains only cones and is called the macula lutea.

In the center of the macula lutea there is a depression called the fovea which provides the greatest visual acuity.The Lens

The lens consists of:

an elastic capsule, which envelops the structure; a cuboidal peithelium, which is confined to the anterior surface of the lens; lens fibers, which are formed from the cuboidal epithelium at the equator of the lens. The lens is attached to the ciliary processes of the ciliary body by the suspensory ligament.

Vitreous Body

The vitreous body maintains the shape of the eye, supports the lens, and presses the neural layer against the pigmented layer. The hyaloid canal is a narrow channel that runs through the vitreous body from the optic disc to the posterior surface of the lens; in the fetus, it is filled by the hyaloid artery, which disappears before birth.The posterior cavity of the eye is behind the lens and is filled with a gelatinous body called the vitreous body.

A comparison of the information coming from each eye in the overlapping visual fields gives us depth perception. retinalateral geniculate thalamusvisual cortex(partial decussation) Visual Pathways