Vision Mechanism of Eye

The eye works on the same principle as that of the camera.

The light rays from the object pass through the conjuctiva, cornea, aqueous humour, lens and vitreous humour in that order. All these structures refract the light such that it falls on the retina. This is called focussing. Maximum focussing is done by the cornea and the lens. The light then falls on the retina.

This light is received by the photoreceptors - rods and cones, on the retina. The absorbed light activates the pigments present in the rods and cones. The pigments are present on the membranes of the vesicles. Thus, the light is then converted into action potentials in the membranes of the vesicles. These travel as nervous impulses through the rod or the cone cell and reach the synaptic knobs. From here the impulses are transmitted to the bipolar nerve cells, then to the ganglions and then to the optic nerves. Thus the nervous impulses generated in the retina are carried to the brain by about a million neurons of the optic nerve.

The vision is controlled by the occipital lobe at the back of the brain. The information received is processed and we are able to see the image. The image formed on the retina is inverted. However, the brain makes us see the image erect. So, though the eyes are essential for vision, any damage to the optic nerves also results in impairment of vision.

Accommodation:

Accommodation is a reflex action of the eye to focus the light from an object on the retina.

The adjustment to the distance of the object is done by the ciliary muscles. The ciliary muscles contract and expand to make the lens thin and thick, respectively. This changes the focal length of the lens. If the object is far, then the focal length is increased and if the object is near, then the focal length is decreased. The optimal focal length is 6 metres or 20 ft.

Muscular control of the eyes:

There are three types of movements associated with the eye that are due to the action of muscles. They are the movement of the eye, the change in size of the pupil and the change in thickness of the lens. The movement of the eye is controlled by six eye muscles that attach the sclera to the bones lining the optic cavity. These six muscles are superior and inferior rectus, internal and external rectus and superior and inferior oblique muscles.

Binocular and stereoscopic vision:

In man, the eyes are frontal and thus the vision is binocular.

This means that the image finally perceived is the result of the information received from both the eyes. Binocular vision results in stereoscopic vision that gives a 3-dimensional image. Thus, this capability of stereoscopic vision gives more information about the object. This vision is, therefore, present in predatory animals like the hawks, eagles and the members of the cat family.

The other animals like the herbivores (horse, cow, etc.) have laterally place eyes. These animals have limited stereoscopic vision. The region of the environment from which each eye collects light is called the visual field.

Eye Physiology and Anatomy

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 7cc. 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 judgment 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 & choroids
• The inner nervous tissue layer – Retina

Interior of the Eyeball:

The structures inside the eyeball are:

• Aqueous humor
• Lens
• Vitreous

Accessory Structures of the Eye:

• Eyebrows
• Eyelids & eyelashes
• Lacrimal apparatus
• Extraocular muscles of the eye

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:

Eyebrows are two arched ridges of the supraorbital margins of the frontal bone. Numerous hair projects obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.

Eyelids and Eyelashes:

The eyelids are two movable folds of tissue situated above and below the front of each eye. There is 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
• Muller’s muscles
• A sheet of dense connective tissue, the tatsal plate
• A lining of the conjunctiva

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).

The lacrymal glands are present one on the outer upper border of each eye. The lacrymal secretion is watery, alkaline and carries out the following functions

• cleans the eyes
• keeps the eyes moist
• keeps the eyes free of bateria as it contains bacteriolytic lysozyme
• provides nutrition to the cornea

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 medical rectus rotates the eyeball inwards.
• The lateral rectus rotates the eyeball outwards.
• The superior rectus rotates the eyeball upwards.
• The infertor rectus rotates the eyeball downwards.
• The superior oblique rotates the eyball so that the cornea turns in a downwards and outward directions.
• The inferior oblique rotates the eyeball so that cornea turns ouwards and outwards.

Arterial Supply:

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, one of the branches of the internal carotid artery.

Venous Drainage:

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

• The motor nerves
• The sensory nerves
• The autonomic nerves

Interior of the Eyeball

Aqueous Humor:

Both anterior and posterior chambers contain a clear aqueous humor fluid secreted into the posterior chamber by the ciliary epithelium. It passes infront 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:

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:

Vitreous is a transparent, colorless, inert gel which fills the posterior 4/5 of the eyeball. It contains few hyalocytes & wandering leucocytes. It contains of 99% water, some salts and mucoproteins.

Structure of the Eye

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 & gives attachment to the extraocular muscles. It is about 1mm thick. The sclera becomes thin (sieve- like membrane) at the site where the optic nerve pierces it. It is called Lmina 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 & sclera is known as the limbus. There is a minute arcade of blood vessels about 1mm broad present at the limbus.

The Middle Vascular Layer:

Iris:

Iris is a colored free circular diaphragm with an aperture in the centre the pupil. It divides the anterior segment of the eye into anterior & posterior chambers which contain aqueous humor 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 & pars plana.

Choroid:

Choroid is a dark brown highly vascular layer situated between the sclera and retina. It extends from the ora serrata upto the aperture of the optic nerve in the sclera.

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 ¾ of the eyeball. Macula lutea is a yellow area of the retina situated in the posterior part with a central depression called fovea centralis. It is the most sensitive part of the retina.

Optic disc:

Optic disc is a circular, pink colored disc of 1.5 mm diameter. It has only nerve fibre layer so it does not excite any visual response. It is know 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 5cm. It has four parts namely.

• Intraocular - 1 mm
• Intraorbital - 25 mm
• Intraosseous - 4-10 mm
• Intracranial - 10 mm (Duke–Elder)