EMBRYOLOGY
The central nervous system develops from the neural groove which invaginates to form the neural tube, running longitudinally down the dorsal surface of the embryo tube. A thickening appears on either side of the neural tube in its anterior part, known as the optic plate. The optic plate grows toward the surface to form the optic vesicle. The two eyes develop from these optic vesicles and the ectoderm and mesoderm coming in contact with the optic vesicles.
The optic vesicle invaginates from in front and below to form the optic cup. The line of invagination remains open for sometime as the embryonic fissure. The hyaloid artery enters through the fissure to provide nutrition to the developing structures. Later, it atrophies and disappears.
The inner layer of the optic cup forms the inner nine layers of the main retina and the outer layer develops into the pigment epithelium. The neural ectoderm secretes jelly-like structure, the vitreous, which fills the cavity.
The ciliary body and iris are formed by the anterior portion of the optic cup and mesoderm. The mesoderm around the cup differentiates to form the coats of eye, orbital structures, angle of anterior chamber and main structure of cornea.
Meanwhile, the surface ectoderm thickens to form the lens plate, invaginates to form the lens vesicle and later separates to form the lens. The surface ectoderm remains as the corneal and conjunctival epithelium. The mesoderm in front of the cornea grows in folds, unites and separates to form the lids.
The eye is essentially formed from both ectoderm and mesoderm. The ectoderm is of two types, namely;
- The neural ectoderm derived from the neural tube.
- The surface ectoderm on the side of the head.
PRIMORDIA OF OCULAR STRUCTURES
The eye originates from neural ectoderm, surface ectoderm and mesoderm.
SURFACE ECTODERM | MESODERM | NEURAL ECTODERM |
|---|---|---|
1. Conjunctival epithelium | 1. Corneal stroma | 1. Iris epithelium |
2. Corneal epithelium | 2. Iris | 2. Ciliary body epithelium |
3. Crystalline Lens | 3. Choroid | 3. Smooth muscles of iris i. Sphincter pupillae ii. Dilator pupillae |
4. Lacrimal glands | 4. Sclera | 4. Part of vitreous |
5. Tarsal glands | 5. Part of Vitreous | 5. Melanocytes |
6. Extraocular muscles | 6. Retinal pigment epithelium | |
7. Vascular endothelium of eye and orbit | 7. Sensory retina | |
8. Fibres (Neural part) of optic nerve |
- Zonules (tertiary vitreous)—They develop from surface ectoderm and mesoderm.
- Bruch's membrane—It develops from neural ectoderm and mesoderm.
The Eye at Birth
- Orbit is more divergent (50°) as compared to an adult (45°).
- Eyeball is about 70% of adult length. It is fully developed at the age of 8 years.
- The newborn is hypermetropic by +2.5 D.
- Cornea is approximately 80% of its adult size, being fully grown at the age of 3 years.
- Anterior chamber is shallow and the angle is narrow.
ANATOMY
The eye is the organ of sight situated in the orbital cavity. It is almost spherical in shape and is about 2.5 cm in diameter. The volume of an eyeball is approximately 7 cc. The space between the eye and the orbital cavity is occupied by fatty tissue. The bony wall of the orbit and the fat helps to protect the eye from injury.
Structurally, the two eyes are separate, but they function as a pair. It is possible to see with only one eye, but three-dimensional vision is impaired when only one eye is used, especially in relation to the judgement of distance.
Structure of the Eye
The eyeball has three layers, namely;
- The outer fibrous layer—Sclera and cornea.
- The middle vascular layer—Iris, ciliary body and choroid.
- The inner nervous tissue layer—Retina.
Interior of the Eyeball
The structures inside the eyeball are;
- Aqueous humour
- Lens
- Vitreous.
Accessory Structures of the Eye
- Eyebrows
- Eyelids and eyelashes
- Lacrimal apparatus
- Extraocular muscles of the eye.
STRUCTURE OF THE EYE
1. The Outer Fibrous Layer
- Sclera—The sclera or white of the eye forms the firm, fibrous outermost layer of the eye. It maintains the shape of the eye and gives attachment to the extraocular muscles. It is about 1 mm thick. The sclera becomes thin (seive-like membrane) at the site where the optic nerve pierces it. It is called Lamina cribrosa.
- Cornea—Cornea forms the anterior one-sixth of the eye. The transparent, ellipsoid, anterior part of the eyeball is known as the cornea. It is the main refracting surface of the eye. The dioptric power is + 43 to + 45 D.
- Limbus—The junction of cornea and sclera is known as the limbus. There is a minute arcade of blood vessels about 1 mm broad present at the limbus.
2. The Middle Vascular Layer
- Iris—Iris is a coloured, free, circular diaphragm with an aperture in the centre—the pupil. It divides the anterior segment of the eye into anterior and posterior chambers which contain aqueous humour secreted by the ciliary body. It consists of anterior limiting (border) membrane, stroma, anterior and posterior epithelial layers, pigment cells and two groups of smooth muscle fibres, one circular (sphincter pupillae) and the other radiating (dilator pupillae).
- Choroid—Choroid is a dark brown, highly vascular layer situated between the sclera and retina. It extends from the ora serrata up to the aperture of the optic nerve in the sclera.
3. The Inner Nervous Tissue Layer
- Retina—Retina is composed of 10 layers of nerve cells and nerve fibres lying on a pigmented epithelial layer. It lines about three-fourth of the eyeball. Macula lutea is a yellow area of the retina situated in posterior part with a central depression called fovea centralis. It is the most sensitive part of retina.
- Optic disc—Optic disc is a circular, pink coloured disc of 1.5 mm diameter. It has only nerve fibre layer so it does not excite any visual response. It is known as the blind spot.
- The optic nerve—The optic nerve extends from the lamina cribrosa up to the optic chiasma. The total length of the optic nerve is 5 cm. It has four parts, namely;
- Intraocular — 1 mm
- Intraorbital — 25 mm
- Intraosseous — 4–10 mm
- Intracranial — 10 mm (Duke-Elder).
INTERIOR OF THE EYEBALL
1. Aqueous Humour
Both anterior and posterior chambers contain a clear aqueous humour fluid secreted into the posterior chamber by the ciliary epithelium. It passes in front of the lens, through the pupil into the anterior chamber and returns to the venous circulation through the canal of Schlemm situated at the angle of anterior chamber.
2. Lens
Lens is a transparent, circular, biconvex structure lying immediately behind the pupil. It is suspended from the ciliary body by the suspensory ligament or zonule of Zinn. It is enclosed within a transparent capsule.
3. Vitreous
Vitreous is a transparent, colourless, inert gel which fills the posterior four-fifth of the eyeball. It contains few hyalocytes and wandering leucocytes. It consists of 99% water, some salts and mucoproteins.
ACCESSORY STRUCTURES OF THE EYE
The eye is a delicate organ which is protected by several structures such as eyebrows, eyelids, eyelashes and extraocular muscles.
1. Eyebrows
Eyebrows are two arched ridges of the supraorbital margins of the frontal bone. Numerous hair (eyebrows) project obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.
2. Eyelids and Eyelashes
The eyelids are two movable folds of tissue situated above and below the front of each eye. There are short curved hair, i.e. eyelashes, situated on their free edges.
The eyelid consists of;
- A thin covering of skin
- Three muscles—the orbicularis oculi, levator palpebrae superioris and Müller's muscles
- A sheet of dense connective tissue, the tarsal plate
- A lining of the conjunctiva.
3. Lacrimal Apparatus
Lacrimal apparatus consists of;
- Lacrimal gland and its ducts
- Accessory lacrimal glands
- Lacrimal canaliculi
- Lacrimal sac
- Nasolacrimal duct.
The tears are secreted by the lacrimal gland and accessory lacrimal glands. They drain into the conjunctival sac by small ducts. The tears then pass into the lacrimal sac (via the two canaliculi), nasolacrimal duct and finally into the nasal cavity (inferior meatus).
4. Extraocular Muscles of the Eye
The eyeballs are moved by six extrinsic muscles, attached at one end to the eyeball and at the other to the walls of the orbital cavity. There are four straight and two oblique muscles.
They consist of striated muscle fibres. Movement of the eyes to look in a particular direction is under voluntary control but co-ordination of movement needed for convergence and accommodation to near or distant vision, is under autonomic control.
The medial rectus rotates the eyeball inwards.
The lateral rectus rotates the eyeball outwards.
The inferior rectus rotates the eyeball downwards.
The superior oblique rotates the eyeball so that the cornea turns in a downward and outward directions.
The inferior oblique rotates the eyeball so that the cornea turns upwards and outwards.
BLOOD SUPPLY TO THE EYE
Arterial Blood Supply
The eye is supplied by the short posterior (about 20 in number), long posterior ciliary (two in number) arteries, anterior ciliary arteries and the central retinal artery. These are the branches of the ophthalmic artery, which is one of the branch of the internal carotid artery.
- The short posterior ciliary arteries supply the whole of choroid being reinforced by recurrrent branches from the ciliary body.
- The ciliary body and iris are supplied by the long posterior and anterior ciliary arteries.
- The two long posterior ciliary arteries pass forward between the choroid and the sclera, without dividing up to the posterior part of ciliary body. Here each divides into two branches and bend around in a circular direction anastomosing with each other and forming;
- Circulus arteriosus iridis major—it is situated in the ciliary body at the base of the iris.
- Circulus arteriosus iridis minor—it is situated just outside the pupillary margin of iris.
- The retina is supplied by the central retinal artery which enters the optic nerve on its lower surface, 15–20 mm behind the globe. The central retinal artery divides on or slightly posterior to the surface of the optic disc into the main retinal trunks. The retinal arteries are the end-arteries and have no anastomoses at the ora serrata. The only place where the retinal system anastomoses with any other is near lamina cribrosa.
- The blood supply of the optic nerve head, near lamina cribrosa, is served by the fine branches from the arterial circle of Zinn, but mainly from the branches of posterior ciliary arteries.The retinal vein do not follow the course of arteries accurately, but they also unite at or slightly behind the surface of the disc to form central retinal vein.The central retinal artery does not supply the prelaminar region and lamina cribrosa.The prelaminar region is supplied by centripetal branches from the peripapillary choroidal vessels along with the vessels in the lamina cribrosa region.The surface layer of the optic disc contains the main retinal vessels and a large number of capillaries, which are derived from branches of the retinal arterioles. In this part of the disc, vessels of choroidal origin may be seen in the temporal sector and one of them may enlarge to form a cilioretinal artery.The retrolaminar part of the optic nerve is supplied by blood vessels, namely;
- Intraneural branches of the central artery of the retina
- Pial branches of the choroidal arteries
- Circle of Zinn
- Central artery of the retina
- Ophthalmic artery.
Venous Drainage
Venous drainage of the eye is carried out by the short ciliary veins, anterior ciliary veins, four vortex veins and the central retinal vein. These eventually empty into the cavernous sinus.
The ciliary veins also form three groups;
- The short posterior ciliary veins.
- The anterior ciliary veins.
- The four vortex veins.
The short posterior ciliary veins receive blood only from the sclera.
The anterior ciliary veins are smaller than arteries and receive blood from only the outer part of ciliary muscles.
The four vortex veins or venae vorticosa are the most important. The tributaries of the vortex veins receive blood from the whole of choroid and iris. They are arranged radially so as to give a whorled appearance. They enter the sclera slightly behind the equator of the globe, two above and two below and pass obliquely through the sclera. They open into the ophthalmic vein.
NERVE SUPPLY TO THE EYE
The eye is supplied by three types of nerves, namely motor, sensory and autonomic.
1. The Motor Nerves
- The 3rd cranial nerve (Oculomotor).
- The 4th cranial nerve (Trochlear)—it supplies the superior oblique muscle.
- The 6th cranial nerve (Abducens)—it supplies the lateral rectus muscle.
- The 7th cranial nerve (Facial)—it supplies the orbicularis oculi muscle.
2. The Sensory Nerve
The 5th cranial nerve (Trigeminal)—the ophthalmic division supplies the whole eye.
3. The Autonomic Nerves
- The sympathetic nerve supply is through the cervical sympathetic fibres to;
- Iris—dilator pupillae muscle.
- Ciliary body.
- Müller's muscle in the lids.
- Lacrimal gland.
- The parasympathetic nerve supply originates from the nuclei in the midbrain. It gives branches to;
- Iris—sphincter pupillae muscle.
- Ciliary body.
- Lacrimal gland.