Eyes

What is the eye?
The eye is the organ for sight. The eye is a remarkable organ as it functions as a highly sophisticated biological video camera. The visual capabilities of a human being are amongst the most developed among all animals. The most characteristic feature of the human eye is its capacity for color vision. We can see a wide spectrum of colors, which few other animals can. The eye automatically adjusts to changes in light and distance. The eyes also have built-in protective functions that flush out dirt and dust from the eyes.

It is one of the most important organs in the body, without which a person is severely handicapped. Overcoming the loss of vision is one of the most difficult parts of any kind of rehabilitation.

Where is the eye located?
The eyeball is a firm organ located in two depressions called as orbits on the front of the skull. It is roughly sphere-shaped about 1 inch or 2.5 cm in diameter. A pair of eyes is located appropriately on the front of the head to provide all round vision coupled with the side-to-side movements of the neck.

How can we describe the eye?
The eye consists of basically three layers; sclera, choroid and the retina from outside to within.

• The sclera
  The eyeball has a tough coat on the outside called as the sclera. It is what we call the white of the eye as it is white in color. It maintains the shape of the eye and protects it from foreign bodies. The sclera is absent in the front of the eyeball. The cornea occupies this sclera-deficient portion of the eyeball.
• The conjunctiva
  The outermost layer of the sclera is a thin membrane called as conjunctiva. This layer is continuous on the inner aspect of the eye and also lines the eyelids on the inside.
• The cornea
  The cornea is a transparent structure present in front of the eyeball. It plays a very important part in the focussing of light rays on the retina, which is the screen of the eye. The cornea lies in front of the pupil. The pupil is the circular gap between the muscles of the iris.
• The pupil
  The pupil is the circular opening through which light passes into the eye. It is formed as a gap in the muscles of the colored iris. The pupil is an adjustable aperture. As the incoming light becomes brighter, the pupil decreases in size to regulate the amount of light passing into the eye.
• The iris
  The iris is a colored set of muscles that controls the size of the pupil. In Indians and blacks, the iris is usually black or brown. In whites, the color of the iris is usually blue or green. Behind the iris lies the lens. The muscles of the iris are of two types. There is an outer ring of radial muscles and an inner ring of circular muscles.
• The lens
  The lens is a transparent, elastic structure behind the pupil. The lens can change its shape to focus light on both near and distant objects. As we get older, the elasticity of the cornea and the ciliary body decreases. Thus it is difficult to focus light clearly in the elderly. The microscopic structure of the lens shows accurately placed cells. These cells do not possess a nucleus. This causes the cells to appear transparent, which allows light to pass through them.
• The ciliary body
  The ciliary body is a small, muscular structure in the front of the eye. it lies at he periphery. Its muscles can squeeze or relax to alter the shape of the lens i.e. they can make the lens thicker or thinner. It also secretes the fluid in the front chamber of the eye called as the aqueous humor.
• The suspensory ligaments
  The suspensory ligaments consist of fibres that attach the lens to the ciliary body. They actually stretch the lens when the muscles of the ciliary body squeeze or relax. These movements alter the shape of the lens.
• The aqueous humor
  The aqueous humor is thin, watery fluid produced by the ciliary body. It fills the space between the cornea and the lens. This space is called the anterior chamber of the eye.
• The vitreous humor
  This fluid is a thicker fluid than the aqueous humor. It is a gel-like substance, which fills the portion between the lens in front and the retina of the eye behind. This space in the back portion of the eye is called the posterior chamber.
• The choroid
  The choroid is the second layer of the eye and lies beneath the sclera. The eye is supplied with nourishment by blood vessels that traverse through this layer.
• The retina
  The retina is the innermost layer of the eye. It is the most important layer of the eye. It can be called as the screen of the eye as all images are projected on the retinal screen. The retina contains nerves and light sensitive cells called as the cones and the rods which are responsible for color vision and distinct vision. The part of the retina that is responsible for the most distinct vision contains a large number of cones and rods. This area is called as the macula.
• The fovea
  The fovea lies at the center of the macula and contains a large number of light sensitive cells. This spot is the most sensitive part of the retina. The area surrounding the fovea is responsible for detailed vision. This area is called as the macula.
• The optic disc
  The optic disc is that part of the retina from where the optic nerve leaves the eye. This area is devoid of light sensitive cells and therefore images are not recognized at this spot. The optic disc is also called as the ‘blind spot’.
• The optic nerve
  The optic nerve collects information from the light sensitive cells in the retina. This information is carried in the form of electrical impulses by the optic nerve to the brain.
• The muscles of the eye
  The movements of the eye are due to a set of muscles that surround the eye. These muscles are six in number. Let us study the eyeball in detail. Two muscles are present on the sides of the eyeball. The one on the right of the eyeball squeezes to rotate the eye to the right. The one on the left of the eyeball squeezes to move the eye to the left. They cause the eyeball to rotate sideways. The eyeball is moved up and down by the muscles at the top and bottom of the eyeball respectively. These are the main movers of the eye. The eye is also moved diagonally by two oblique muscles, one that is placed at the top and one placed below. Both these muscles are not attached to the center of the eyeball. This helps them to rotate the eye diagonally. Any disorder in the eye muscles causes squint.

How does the eye work?
The eye is an amazing organ. It functions as a sophisticated biological video camera. The basic principle of vision is the focussing of light rays that enter the eye through the pupil. We see objects because they reflect light in the atmosphere, which falls on them.

This reflected light first enters the cornea, where it is slightly refracted. This means that light rays are slightly bent by the cornea. These rays then pass through the pupil. The iris muscles control the diameter of the pupil. In dim light the pupil enlarges to accommodate more light. In bright light the pupil becomes smaller to regulate the amount of light falling in the eye necessary for clear vision.

The accommodation reflex
The light then passes through the lens. The muscles of the ciliary body alter the shape or thickness of the lens. It changes the thickness of the lens to focus the rays of light accurately on the retina. Rays from nearer objects need to be refracted more i.e. bent more to create a sharp image on the retina. The muscles of the ciliary body contract or squeeze to give the lens a more spherical shape. This bends the light more and results in a sharp image on the retina.

Similarly for a far-off object, the ciliary body relaxes to make the lens flatter. This does not bend the rays of light significantly. Hence a clear image is formed on the retina.

The changing of the shape of the lens to focus light accurately on the retina is called as the accommodation reflex.

The light rays then travel through the fluid system of the eye i.e. the aqueous and vitreous humor and focus on the retina. An inverted or upside down image of the object is formed on the retina. As soon as light falls on the retina, the light sensitive cells in the retina produce electrical signals. These signals are carried from the retina to the brain via the optic nerve. The optic nerves from each side meet at a point called as the optic chiasm. From this point some of the fibres of the optic nerve continue on the same path to the brain while the rest cross over to the opposite side to the opposite side of the brain.

The brain has an area called as the visual cortex located at the back of the brain. The images from both eyes are sent to the brain at the same time. These images are seen by each eye at a slightly different angle from the other. The visual cortex processes the information it receives to produce a single picture from both images. This effect is called as stereoscopic vision.

How does the pupil change its size?
The pupil has two sets of muscles. The outer layer has radially placed muscles (like spokes of a wheel). When these contract or squeeze, the inner circularly arranged muscles relax and thus the pupil is dilated or widened. This occurs in dim light.

In bright light, the outer radially arranged muscles relax and the inner circular ring of muscle contracts. This constricts or narrows the pupil. In this manner the size of the pupil changes to regulate the amount of light entering the eye.

How do the light sensitive cells work?
The light sensitive cells are present in the retina. The fovea is a spot on the retina with the maximum density of these cells. These cells are of two types; the rods and the cones. As the names suggest the rods are rod shaped cells while the cones are cone shaped ones.

As soon as light falls on the retina, the rods and cones produce electrical signals. There are pigment cells present behind the rods and cones that prevent reflection of light within the eye itself.

The rods (about 120 million in number) contain only one type of pigment and therefore cannot distinguish colors. They are mainly responsible for night vision.

The cones (about 6.5 million in number) respond to red, green and blue light only in bright light. They are thus responsible for color vision.

The electrical impulses produced by these cells are then carried by fibers of the optic nerve to the brain.