Anatomy of The Eye�

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INTRODUCTION

• The Eye is the organ of vision.
• Composed of :
1. Eyeball.
2. The adnexa.

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THE POSITION

• In the Predatory species: have set well forward.
• In Herbivores, ruminant and rabbits: have eyes more laterally to have wide area of vision .

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The Eyeball

• Cornea : the transparent part of the eyeball
• Anterior pole: the highest point on cornea
• Posterior pole : the highest point on posterior surface
• Optic axis: the straight line passing through both poles

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The Eyeball

• Equator: an imaginary line about the eyeball, which is the equidistant from the poles.
• Meridian: is one of many lines passing from pole to pole that intersects the equator at right angles.
• Optic nerve :leaves the eyeball slightly ventral to the posterior pole

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Eyeball

The three tunics are:

I. An external fibrous tunic: that gives form to and protects the eyeball; it’s the only complete tunic.

II. A middle vascular tunic: that consist largely of:

• Blood vessels and smooth muscle
• Concerned with the nutrition of the eyeball and the regulation of the shape of the lens and size of pupil.

 III. An internal tunic: that consists largely of:

• Nervous tissue
• Concerned with vision and translation of visual stimuli into nerve impulses for interpretation by the brain.

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The Fibrous Tunic

• It consists of the sclera and the cornea, which meet at the limbus.

1. The sclera is the opaque posterior part of the fibrous tunic and consists of a dense felt work of collagenous and elastic fibers and is generally white but in some species it contain pigment cells

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The fibrous tunic

• The cornea forms about one quarter of the fibrous tunic and bulges forward. It is composed off dense connective tissue arranged in lamellar form .
• The cornea doesn't’contain blood vessels;
• nutrients for its cells permeate the bulk substania propria from vessels in the limbus or are carried to it its surface in the lacrimal fluid and aqueous humor .

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The vascular Tunic (uvea)

• Deep to the sclera, which it composed of three zones .

1) The choroids:

• lies on the sclera from the optic nerve to the limbus and contains a dense network of blood vessels embedded in heavily pigmented connective tissue
• they are branches from the ciliary arteries and drained by the vorticose veins

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The vascular Tunic (uvea)

• In the dorsal part of the fundus the choroids forms colored, light-reflecting area known as tapetum lucidum
• is avascular layer (cellular in the carnivores. fibrous in ruminants and horses) between the capillaries and the vessels.
• The tapetum makes the eyes of animals shine when they look toward a light.
• Our eyes and those of the pig don’t have a tapetum so they don’t reflect the light.
• This reflecting of light is a night vision adaptation because of stimulation of the light sensitive receptors in the retina.

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The vascular Tunic (uvea)

2) The ciliary body :

toward the limbus the choroids thickness to form it.

3) The Iris: the smallest part of the vascular tunic, which extends from the cornea to the lens.

• It attached to sclera and ciliary body by pectinate ligament.
• the opening in the center is the pupil

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The vascular Tunic (uvea)

• The color of the iris determines the color of the eye and depends on the number of the pigmented cells present in its stroma and on the type of the pigment in the cells.

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The vascular Tunic (uvea)

• The iris divided the space between the lens and cornea into anterior and posterior chambers that communicate through pupil and filled with, aqueous humor, and a clear watery fluid.

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The internal tunic

• The internal tunic of the eyeball contains the light-sensitive receptor cells and known as retina.
• It’s an extension of the brain to which it remains connected by the optic nerve.

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The internal tunic

The layers in retina are:

• A single layer of pigmented cells.
• A neuroepithelialm layer containing the receptor cells, rods and cones and their nuclei.
• the rods for black and white
• the cones for the color vision.
• A layer of bipolar ganglion cells.
• A layer of multipolar ganglion cells nonmyelinated axons lying internal to the cells and pass to the optic disc where they form the optic nerve.
• The optic disc is a blind area because there is no receptor cell.

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❖  The adnexa of the eye

1. The orbital fasciae :

a. The periorbital: is attached near the optic foramen at the apex of the cone .

b.The superficial muscular fascia: lies within the periorbital. It’s loose and fatty. And envelops in the levator palpebrae superioris and the lacrimal gland.

c. The deep muscular fascia: is more fibrous and arises from the eyelids and from the limbus of the eyeball.

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❖  The adnexa: Muscle of the eye

2. The muscles of the eyeball:

• The rectus muscles: dorsal, ventral, medial and lateral are inserted anterior to the equator by wide but very thin tendons.
• The ventral and dorsal oblique muscles: attach to the eyeball near the equator.
• A small synovial sheath protects the muscle as it passes around the trochlea.

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❖  The adnexa: Muscles of the eye

2. The muscles of the eyeball:

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• The retractor bulbi arises from the vicinity of the eyeball and inserted on the eyeball posterior to the equator.

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• The levator palpebral superiors: striated muscle within the orbit that doesn't attach to the eyeball but passes over it to enter and elevate the upper eyelid

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❖  The adnexa of the eye

3. The eyelids and conjunctiva :

• The eyelids (palpebrae) are two musculofibrous folds of which the upper is the more extensive and more mobile.

• The free margins of the lids are meet at the medial and lateral angles of the eye and bound an opening known as the palpebral fissure.

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❖  The adnexa of the eye

3. The eyelids and conjunctiva :

They are consist of three layers:

1.The skin: is thin and delicate and is covered with short hairs: it may also carry a few prominent tactile airs.

2.The musculofibrous layer: is formed by the orbicularis oculi, the orbital septum, the aponeurosis of the levator muscle and the smooth tarsal muscle.

3.The mucous (palpebral conjunctiva) a thin, transparent mucous membrane

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❖  The adnexa of the eye

3. The eyelids and conjunctiva :

• The third eyelid is supported by a T-shaped piece of cartilage:
• bar lies in the free edge of the fold

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• stem points backward into the orbit medial to the eyeball.

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• The stem of cartilage is surrounded by lacrimal gland (the gland of the third eyelid).

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❖  The lacrimal apparatus:

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• This consists of:
• lacrimal gland proper
• the glands associated with the third eyelids
• several small accessory glands
• a duct system that conveys the lacrimal fluid after it has washed over the eye into the nasal cavity for evaporation.
• The lacrimal gland is flat and lies between the eyeball and the dorsolateral wall of orbit.

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❖    The blood supply of the eye:

• The external ophthalmic artery carries the principle supply of the blood to the eye, which is a branch of the maxillary artery.

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• The arteries arising from the external ophthalmic and malar arteries (smaller branch of the maxillary) can be divided into three groups:

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❖    The blood supply of the eye:

• 1) The branches of the external ophthalmic for the eyeball penetrate the sclera to reach the vascular tunic and retina.
• -Short posterior ciliary a. / supply the adjacent choroids in addition to branches to the optic nerve.
• -Long posterior ciliary a. /pass close the sclera closer to the equator.
• -The anterior ciliary a. / supply the anterior potion of the choroids, the ciliary body and the iris
• These arteries anastomose to form the greater arterial circle of the iris.

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❖    The blood supply of the eye:

2) The arteries that supply the ocular muscles. Which the absence of the large vessels in distal ends reduces bleeding when the muscles are cut during the enucleating.

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❖    The blood supply of the eye:

3) The arteries that leave the orbit:

• The lacrimal a. / supply the lacrimal gland in route.
• The supraorbital a. / send branches to the upper eyelids

• The malar a. /supply the eyelids and also adjacent area of the face.
• The external ethamoid a. / supply the ethamoid labyrinth of the nasal cavity.

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❖ The nerve supply of the eye:

• The optic nerve II: enters the orbit through the optic foramen and passes to the light receptor cells in the retina.
• It allows the movements of the eye and is covered by meninges that it acquired during its development.

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❖ The nerve supply of the eye:

• The oculomotor nerve III: control the movement of the eyeball. it inters the orbit through the orbital fissure.
• (VO, DR, VR, MR)
• The trochlear nerve IV: innervate the dorsal oblique muscle.
• The trigeminal nerve V: send branches to the eye.
• The abducent nerve VI: enters through the orbital foramen and innervates most of retractor bulbi and lateral rectus muscles.

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❖ The nerve supply of the eye:

• The facial nerve VII: which passes between the eye and the ear and innervates the orbicularis oculi.

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