The Eye and its Appendages

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The Eyeball

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- the peripheral organ of vision, is situated in a skeletal cavity, the orbit

- 1 inch in diameter, 8g. in weight

- The eyeball is embedded in orbital fat, separated from it by a thin fascial sheath (capsule of Tenon)

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Cavities of the Eyeball

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1. Anterior Cavity

- further divided into anterior chamber and posterior chamber due to the position of the iris.

- contains the acqueous humor produced by the

ciliary process.

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2. Posterior Cavity

- located behind the lens

- contains the vitreous humor enclosed in hyaloid membrane

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Layers of the Eyeball

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The Layers of the Eyeball

1. Fibrous coat - sclera and cornea

2. Vascular coat - choroid, ciliary body and the iris

3. Neural coat - retina

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1. The Fibrous CoatSclera Cornea

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A. The Sclera

- relatively hard consistency, is a dense layer which, when distended by intraocular pressure, maintains the shape of the eyeball

- external surface is white and smooth and is in contact with the inner surface of the fascial sheath of the eyeball

- provides passage for nerves of the cornea and vascular autonomic nerves and attachment for extrinsic eye muscles

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B. The Cornea

- is the anterior, projecting transparent part of the external tunic, and

- its tear film cover is the major site of refraction of light entering the eye.

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The Vascular CoatChoroidCiliary Body

Iris

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a. The Choroid- The choroid is a thin, highly vascular, dark brown tissue which lines almost the posterior five-sixths of the eye;

- it is pierced behind by the optic nerve and is firmly adherent to the sclera;

- it is thicker posteriorly;

- internally it is firmly attached to the retinal pigmented layer.

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b. The Ciliary Body

- The ciliary body is directly continuous with the choroid behind and with the iris in front

- Internally it shows a scalloped periphery where it is continuous with the choroid and retina, termed the ora serrata.

- The ciliary body is concerned with the suspension of the lens and with accommodation

- It is also a major source of aqueous fluid for the anterior segment of the eye

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c. The Iris

- is an adjustable diaphragm around a central aperture

- the pupil, which controls the amount of light entering the eye

- The concentration of melanocytes is the main factor determining the hue of the iris.

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Muscles of the Iris

1. Sphincter Pupillae

- This is a flat annulus of smooth muscle about 0.75 mm wide and 0.15 mm thick.

- Its densely packed fusiform myocytes are often arranged in small bundles, as in the ciliary muscle, and pass circumferentially around the pupil.

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Muscles of the Iris

2. Dilator Pupillae

- A thin stratum lying immediately anterior to the epithelium of the posterior iridial surface,

- its fibers are in fact the muscular processes of the anterior layer of this epithelium,

- their apical processes form the epithelium itself.

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The Neural CoatRetina

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The Retina - is the neural, sensory stratum of the eyeball

- It is thin, being thickest near the optic disc

- contains a variety of cell types. They include the photoreceptors (rod and cone cells),

- Near the centre of the retina is an oval, yellowish area, the macula lutea which has a central depression, the fovea centralis, where visual resolution is highest.

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Optic Disc

- This region, where retinal tissues meet the neural elements of the optic nerve

- Besides being the exit point for the optic nerve fibres, it also provides a point of entry and exit for the retinal circulation.

- superomedial to the posterior pole of the eye

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Rods and ConesCone cells

- chiefly responsible for high spatial resolution and colour vision in good lighting conditions (photopic vision),

- highly concentrated at the fovea centralis, but they populate the whole retina, intermingled with rods

Three types according to their maximum spectral sensitivities: red, green and blue

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Rods and Cones

Rods

- provide high monochromatic sensitivity to a much wider range of illumination down to much lower intensities (scotopic vision) although with relatively low spatial discrimination because of their different neural connections.

- Rods are excluded from the fovea.

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The Refractive MediaThe components of the eye which transmit and refract light

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Composed of:

1. Cornea, 2. Acqueous humor, 3. Lens, 4. Vitreous humor.

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Aqueous Humour

- The total quantity of aqueous humour is small, filling the anterior and posterior chambers.

- The ciliary processes are responsible for its production

- is an avenue for nutrients, and metabolic exchange for the

avascular tissues of the cornea and lens.

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Vitreous Body

- fills the vitreous chamber, occupying about four-fifths of the eyeball.

- It is colourless, consisting of approximately 99% water, and although apparently structureless, a sparse but organized cellular and fibrous content is present.

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Lens- is a transparent, encapsulated,

biconvex body, placed between the iris and the vitreous body.

- encircled by the ciliary processes, to which it is attached by the zonular fibres collectively forming the zonule holding the lens in place and transmitting the forces stretching the lens except in visual accommodation.

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CONDITIONS OF THE EYE

Emmetropia

• Ideal state of the eye in which no refractive error is present

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Myopia

• Also known as nearsightedness, is a condition in which a person can see near objects more clearly than distant objects.

• Light focusses in front of the retina

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Hyperopia

• Also known as farsightedness, is a condition in which a person can see distant objects more clearly than near objects.

• light focusses behind the retina

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Presbyopia

• Normal part of aging.

• It develops as the lens of the eye becomes less flexible and loses its ability to focus on near objects.

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Lens become thicker when viewing near objects

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The lens in the presbyopic eye cannot change shape to focus on near objects

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Astigmatism

• A condition in which objects, both near and far, appear blurred.

• Often occurs in combination with myopia nad hyperopia.

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Cataract

• Is a cloudiness or opacificationin the normally clear and transparent lens of the eye.

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THE ACCESSORY STRUCTURES OF THE EYE

• 1. The Eyelids

• 2. The Lacrimal Apparatus

• 3. The Eye Muscles

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Functions:

1. provide protection,2. lubrication, and3. support

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The Eyelids

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- 2 movable folds, with the upper being more larger and movable, due to the presence of the levator palpebra superioris.

- covered by skin, externally, and mucous membrane (palpebral conjunctiva) in the deep aspect.

Functions :

1. provide protection when firmly closed2. keep the surface of the eyeball lubricated with its continual blinking.

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Layers of the Eyelid

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Layers of the Eyelid

1. Skin2. Loose subcutaneous tissue3. Muscular layer4. Submuscular layer5. Tarsal plates6. Conjunctiva

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1. Skin- eyelashes- sebaceous glands (glands of Zeis)

Sty - infection of the glands of Zeis

- sweat glands (glands of Moll)

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2. Loose subcutaneous tissue- extremely loose and easily distended by blood and fluid

3. Muscular layer- orbicularis oculi muscle

4. Submuscular layer- contains sensory nerves

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5. Tarsal plates- 2 thin plates of dense connective tissue- forms and supports the eyelid- presence of lateral and medial tarsal ligaments- receives the levator palpebra superioris

6. Conjunctiva- mucous membrane that attaches the eyeball to the eyelid ( palpebral and bulbar)

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STRUCTURES PRESENT IN THE EYELIDS

1. Palpebral fissure

2. Palpebral commissures (canthi)a. lateral - more acute, placed directly against the globeb. medial - lacrimal lake, lacrimal caruncle, lacrimal papilla, lacrimal punctum

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The Lacrimal Apparatus

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Composed of the following structures:

1. lacrimal gland, which secretes a complex fluid (tears) and whose excretory ducts convey fluid to the surface of the eye,

2. the paired lacrimal canaliculi,

3. the lacrimal sac and

4. the nasolacrimal duct, by which the fluid is collected and conveyed into the nasal cavity.

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1. Lacrimal Gland

- located superolaterally along the orbit

- secretion empties in the pocket between the eyeball and the eyelid

- secretes tears which are alkalinic, watery, and contain lysozyme

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2. Lacrimal Canaliculi

- superior and inferior

- conducts the tears to the lacrimal sac

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3. Lacrimal Sac

- located in the

lacrimal fossa

- presents a fundus

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4. Nasolacrimal Duct

- 3/4 of an inch in length

- drains into the inferior nasal meatus

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The Orbit

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The Orbit

- contains the eyeball in its anterior part and the posterior part is filled with fat, fascia, muscles, blood vessels and nerves

- Capsule of Tenon separates the anterior and posterior

- Periorbita lines the orbit and is continuous with the dura mater of the brain

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The Eye Muscles

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The Eye Muscles

- Composed of the 7 muscles of the eye

- Lateral rectus is supplied by the Abducent N.

- Superior oblique is supplied by the Trochlear N.

- The rest is supplied by the Oculomotor N.

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