The central nervous system develops from the neural tube. A thickening appears on either side of the neural tube in its anterior part, known as the optic plate. The optic plate grows towards 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 invaginates 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.
PRIMORDIA OF OCULAR STRUCTURES
The eye originates from neural ectoderm, surface ectoderm and mesoderm.
1. Conjunctival epithelium
2. Corneal epithelium
3. Crystalline lens
5. Epithelium of
1. Corneal stroma
2. Corneal endothelium and Descemet's membrane
3. Iris stroma
7. Extraocular muscles
8. Ciliary muscles
9. Bony orbit
1. Sensory retina
2. Retinal pigment epithelium
3. Pigment epithelium of iris
4. Ciliary body epithelium
5. Sphincter pupillae
6. Dilator pupillae
8. Neural part of optic nerve
- Eyelids—They develop from both surface ectoderm and mesoderm
- 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.
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 specially 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
Accessory Structures of the Eye
- 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 1/6 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 endothelium, stroma, pigment cells and two groups of plain muscle fibres, one circular (sphincter pupillae) and the other radiating (dilator pupillae).
- Ciliary body—Ciliary body is triangular in shape with base forwards. The iris is attached to the middle of the base. It consists of non-striated muscle fibres (ciliary muscles), stroma and secretory epithelial cells. It consists of two main parts, namely pars plicata and pars plana.
- 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 ten layers of nerve cells and nerve fibres lying on a pigmented epithelial layer. It lines about 3/4 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 mmIntraorbital—25 mmIntraosseous—4-10 mmIntracranial—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 in the angle of anterior chamber.
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.
Vitreous is a transparent, colourless, inert gel which fills the posterior 4/5 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.
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, the 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 superior rectus rotates the eyeball upwards.
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
The eye is supplied by the short (about 20 in number) and long ciliary (2 in number) arteries and the central retinal artery. These are branches of the ophthalmic artery, which is one of the branch of the internal carotid artery.
Venous drainage is done by the short ciliary veins, anterior ciliary veins, 4 vortex veins and the central retinal vein. These eventually empty into the cavernous sinus.
NERVE SUPPLY TO THE EYE
The eye is supplied by three types of nerves, namely motor, sensory and autonomic.
1. The Motor Nerves
- The third 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
- Muller'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.