Human eye

 Human eye is highly developed photo sensory organ. About only 1/6 of the eye ball is out side and remaining part is enclosed in socket of skull. Eye ball is not a perfect sphere but is slightly protruding in the front. Human eye has highly evolved stereoscopic binocular vision which is absent in most other animals.The front portion of eye is protected by upper and lower eye lid. Upper eye lid constantly is engaged in clearing of dust particles like wipers of  car`s wind shield. There are two Lacrimal glands whose secretions called tears keep the eye surface constantly moist. One Lacrimal gland is arranged toward upper right and one towards caruncula. The lacrimal secretions also contain an antibacterial enzyme called Lysozyme which eliminates bacteria keeping eye surface healthy. Opening and closing of eye lids is involuntary and partly voluntary action. We show involuntary reflexes of closing eye lids, when strong light falls on our eye.

Eye ball has three layers namely outer sclerotic layer, middle choroid layer and inner sensory layer called Retina.Sclerotic layer is non vascular and white in color, choroid layer is highly vascularized and inner layer Retina is lined with  several photo receptor cells namely Rods and cones.The choroid layer is located behind the retina and absorbs unused radiation.The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels that lie just under it called the choroid plexus

 Rods are responsible for night vision as they are highly sensitive to brightness of light. Cones are responsible for color vision which play key role in perception of colors. They are ineffective in dim light.The outer sclera as it comes before lens, it becomes transparent and is called as Cornea. Cornea forms a bulge in front of lens, it encloses a chamber called Aqueous chamber which is filled with fluid called Aqueous humour. The cornea contributes to the image-forming process by refracting light entering the eye.The front surface of the adult cornea has a radius of approximately 8mm. About 80% of eye ball`s volume at centre is filled with jelly like substance called Vitreious humour. It maintains the inner pressure of eye ball.

The Iris is a diaphragm of variable size whose function is to adjust the size of the pupil to regulate the amount of light admitted into the eye.It is made of smooth muscle.The iris is the colored part of the eye which occur in many shades of blue, green, brown, hazel, or grey.

The crystalline lens of the eye is a flexible unit that consists of layers of tissue enclosed in a tough capsule. It is suspended from the ciliary muscles by the zonule fibers otherwise called as suspensory ligaments.

The ciliary muscle is a ring-shaped muscle attached to the iris. It is important because contraction and relaxation of the ciliary muscle controls the shape of the lens ultimately  results in adjustment of focal length facilitating perception of distant and near objects. This is called accommodation of eye. Some times defect in lens causes eye sight. This can be corrected by wearing spectacles or contact lenses.

 It is also called Yellow spot.The Papilla is also known as the "blind spot" and is located at the position from which the optic nerve leaves the retina. It is so called because there are no receptors in this region and no visual perception takes place in this region.

Rods have visual pigment called Rhodopsin which is synthesized from vitamin A.This pigment rapidly disintegrates on exposure to light and quickly formed. Cones are typically cone-shaped, light-sensitive receptor cells that are responsible for sharp focus and color perception, particularly in bright light. The rods and cones are distributed unevenly throughout the retina.Cones are concentrated on the part of the retina called the fovea . The fovea is a particularly sensitive region of the retina.Cones are primarily responsible for the sharply focused perception of color, particularly in brightly lit situations.Cones have visual pigment called Idopsin.Rods and cones also are involved in dark adaptation, the phenomenon of adjusting to dim light after being in brighter light. You might have experienced while entering  movie theater, you see almost nothing but after few minutes your vision quickly improves.

How eye works?

Eye works like a camera. When you look at an object, the beam of light travels through the  transparent cornea, pupil, lens and finally gets focused on rare of eye on retina.The lens focuses light by changing its own thickness by a process called accommodation.

This is controlled by suspensory ligaments. The iris changes the size of the pupil depending on the intensity of light.The wider the pupil is the less is focus on distant objects.Hence while observing distant objects, Pupil takes narrow position. The lens focuses the light onto the back of the eye on retina. The retina has  several millions  of light-sensitive photoreceptor called Rods and Cones, which change light signals into electrical signals.These electrical signals are very week in order of 0.05 volts.Stimulation of the nerve cells in the eye triggers a neural response that is transmitted to other nerve cells in the retina called bipolar cells and ganglion cells. Bipolar cells receive information directly from the rods and cones and communicate that information to the ganglion cells.The ganglion cells collect and analyze visual information, which is then carried out to the back of the eyeball and sent to the brain through a bundle of ganglion axons called the optic nerve.




 These electrical signals are transmitted to optic center of brain.Brain rearranges the inverted image data from two optic nerves of two eyes and makes us to perceive the true world in colors and depth.Different parts of the brain process nerve impulses in several individual systems simultaneously. For instance, one system relates to shapes, one to colors, and others to movement, location, and depth.Scientists discovered some specialized receptors are sensitive in identifying the boundaries of the object and their motion, they can detect any moving subject in a still back ground such as a lion hiding behind bushes.

Our brain some times fail to interpret the electrical signals sent by eye making us to see optical illusions.Optical illusions are those strange visuals which we see which actually does not exist. Magicians exploit these illusions to vanish objects before open eyes.

Observe the  white dots among black boxes, suddenly turning grey and vanishing. This is no animation, it is purely an illusion.




How eye sight occurs?

People who are nearsighted or farsighted have abnormally-shaped eyeballs, meaning their eyes don’t refract, or bend, light properly. Incoming light rays do not converge on the retina at exactly the right depth inside the eye, causing vision to blur. Fortunately, glasses, contact lenses, and surgery can fix both of these common vision problems. 

Myopia (Nearsightedness)

 Myopia occurs because the eye is larger than average. Since myopia is caused by the eye being longer than average, the light that is focused in the eye lands in front of the retina rather than on the retina causing perception of a blurred image.This condition can be corrected by glasses, contact lenses, and laser refractive surgery.

Hyperopia (Farsightedness)

 Hyperopia occurs because the eye is smaller than average. Since hyperopia is caused by the eye being smaller than average, the light that is focused in the eye lands behind the retina rather than on the retina. Farsighted glasses, contact lenses, and refractive surgery are all used to move the image from behind the retina on to the retina

Astigmatism

 Astigmatism is a condition with   distortion of vision in addition to blurring of vision. The reason behind this problem is over curved cornea. A normally curved cornea refracts the light at normal angle but a cornea with irregular curvature produces a distorted image. Recollect your experience while you see reflected objects in spoon.