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Anatomy of the Eyeball

 


Anatomy of the Eyeball

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The eyeball is responsible for transforming light energy into electric impulse that reaches the brain through the visual pathway. It is protected inside the orbit which is a pyramidal cavity in the skull. Both eyeballs move together by means of the extra-ocular muscles (EOM). The eyelids are folds that shut and open to protect the eyeball from the front. The conjunctiva is a thin transparent mucus membrane that lines the eyelids and covers the sclera. The lacrimal system is responsible for wetting the surface of cornea and conjunctiva.

 

ANATOMY OF THE EYEBALL

It is approximately spherical in shape with average anteroom-posterior diameter 24 mm. It consists of the following 3 concentric coats surrounding the ocular cavity.

The coats of the eye:-

1. The outer fibrous coat: It is formed of opaque sclera (posterior 5/6 of the eyeball) and clear cornea (anterior 1/6). The transitional zone between them is about 1 mm in width and called "limbus".

2. The middle vascular coat (the uveal tract): It is formed of the iris, ciliary body and choroid.

3. The inner coat: The retina which is formed of 10 layers.

The cavity of the eye is divided into:-

1. The anterior chamber (AC): It is the space between the cornea and iris. Its average depth is 2.5-3.5 mm. It is bounded peripherally by the angle between cornea and iris. This angle is responsible for filtration of aqueous humour from anterior chamber into the canal of Schlemm.

2. The posterior chamber (PC): It is the small space between the iris anteriorly and lens posteriorly. It is communicated with the anterior chamber through the pupil. Both the anterior chamber and posterior chamber are filled with aqueous.

3. The vitreous cavity: It is the space between the lens and retina. It is occupied by the vitreous body.

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The refractive media (the optical apparatus) of the eye:

For vision to occur, the image should be focused at the retina. This is done by an optical apparatus which consists of:

1. The cornea which is the major refracting surface (about + 42 D).

2. The aqueous humour.

3. The lens (about + 18 D).

4. The vitreous body.

NB. The total diopteric power of the eye is about + 60 D.

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Blood Supply of the Eyeball:-

It is derived from the internal carotid artery to ophthalmic artery then blood is distributed to retinal circulation and ciliary circulation.

A. Retinal system:

I. The central retinal artery:-

The central retinal artery is a branch of the ophthalmic artery. It supplies the inner 5 layers of the retina. It is an end artery (without anastomosis), so that occlusion of one of its branches leads to death of the corresponding area of the retina.

II. The central retinal vein:-

B. Ciliary system:

I. Arteries:-

a. Anterior ciliary arteries (7 arteries):-

- They are branches from the muscular arteries of the four rectus muscles. There are 2  anterior ciliary arteries on each muscle, except the lateral rectus (LR) where one anterior ciliary artery.

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- They pierce the sclera about 4 mm from the limbus to reach the ciliary body where they anastomose with each other and with the long posterior ciliary arteries forming "the circulus arteriosus iridis major (CAIM)".

b. Long posterior ciliary arteries (2 arteries):-

They pierce the sclera on either side of the optic nerve to reach the supra-choroidal space. They do not give any branches till they reach ciliary body where they end in the CAIM.

NB. Circulus arteriosus iridis major (CAIM):-

* It is a circle of anastomosis present in the ciliary body near the root of iris.

* Its feeders are: - The 7 anterior ciliary arteries.

                          - The 2 long posterior ciliary arteries.

* It is distributed to:-

    - Ciliary muscle.

    - Ciliary processes.

    - Radial branches to the iris.

    - Recurrent branches to anterior part of choroid.

c. Short posterior ciliary arteries (10 - 20 arteries):

- They pierce the sclera around the optic nerve.

- They supply the part of the choroid posterior to the equator of the eyeball as well as optic nerve head.

 

II. Veins:-

a. Vortex veins:

* They are 2 superior and 2 inferior vortex veins.

- The 2 superior veins drain into superior ophthalmic vein.

- The 2 inferior veins drain into inferior ophthalmic vein.

* They pierce the sclera behind the equator of the eye.

* The run obliquely through long tunnels inside the sclera. So that they are compressed and occluded in cases of acute congestive glaucoma leading to ciliary congestion.

b. Anterior ciliary veins:

They correspond to the anterior ciliary arteries. They originate from the "circulus venous iridis major (CVIM) and drain into the veins of the rectus muscles, then to superior and inferior ophthalmic veins.

NB. Both superior and inferior ophthalmic veins drain into the cavernous sinus.

N.B.: The avascular parts of the eyeball are:

1. Cornea.

2. Lens.

3. Vitreous.

4. The outer layers of the retina and all layers of the fovea.

Blood supply of the conjunctiva:-

a. Posterior conjunctival vessels:  they are branches of palpebral vessels. They supply the conjunctiva except the circumcorneal 4 mm. So that, conjunctivitis causes hyperemia that appears in the fornix more than around the limbus. This is called "conjunctival injection".

b. Anterior conjunctival vessels: They are branches of anterior ciliary vessels before they pierce the sclera, they supply only an area about 4 mm around the limbus. This circumcorneal area of conjunctiva becomes hyperemic with intraocular diseases as corneal ulcers, iridocyclitis and acute congestive glaucoma. This is called "ciliary injection".

Nerve supply of the eyeball:-

1. Vision:

Through the optic nerve.

 

2. Pain supply:

 It reaches the globe through the following routes:

Trigeminal nerve --> ophthalmic division --> nasociliary nerve --> 2 long ciliary nerves (enter the eye directly) and sensory root of ciliary ganglion --> pass in the ganglion without relay --> short ciliary nerves.

Both long and short ciliary nerves pierce the sclera around the optic nerve, pass in the suprachoroidal space to the ciliary body. They supply Cornea, Iris and ciliary body. So that their diseases are painful.

N.B. The lens, vitreous, choroid and retina have no pain supply, so that their diseases are painless.

NB. Any ocular irritation is associated with referred headache and reflex lacrimation, photophobia and blepharospasm.

3. Parasympathetic innervation:

It arises from the Edinger Westphal nucleus (EWN) --> oculomotor nerve --> its inferior division --> parasympathetic root of ciliary ganglion --> ciliary ganglion (relay) --> short ciliary nerves pierce the sclera around the OPTIC NERVE, pass in the suprachoroidal and supraciliary space to form a plexus in the ciliary body.

It supplies:-

- Sphincter pupillae muscle.

- Ciliary muscle for near vision accommodation.

4. Sympathetic innervation:

It consists of 3 order neurons:

1. The first order neurons: They arise from hypothalamus and descends uncrossed to the spinal canter.

2. The second order neurons: They arise from C8 and T1-2 forming the preganglionic fibers. They ascend through the sympathetic chain to rely in the superior cervical sympathetic ganglion.

3. The third order neuron: They are the postganglionic fibers. They accompany the carotid arteries and their branches to the orbit. Sympathetic supply reaches the globe via the long and short ciliary nerves.

* The sympathetic system supplies:

- The dilator pupillae muscle.

- Muller's muscles of both upper and lower eyelids.

- Blood vessels vasomotor tone.

N.B. The Ciliary Ganglion:

- It is a small quadrilateral body about the size of pin's head situated between the optic nerve & lateral rectus near the apex of the orbit. It has 3 roots:

1) Sensory root: Comes from naso-ciliary N (no relay).


It drains the inner 5 layers of the retina into superior ophthalmic vein into the cavernous sinus.

 

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2) Parasympathetic root: Comes from 3rd N. The fibers relay in the ganglion.

3) Sympathetic root: Comes from the sympathetic plexuses around the internal carotid artery. They are post-ganglionic fibers, so that they pass in the ganglion without further relay.

- The ciliary ganglion gives 6 - 10 short ciliary nerves which contain the 3 types of innervation.

Applied anatomy:-

We can completely anaesthetise the eyeball by one injection of a local anaesthetic around the ciliary ganglion.

 

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