Anatomy Of The Eye
The human eye works exactly like a photographic instrument (a camera), working on memory plates to register images which are then transferred to the brain for processing. Its delicate nature and extreme sensitivity to light makes it extremely vulnerable to certain kinds of abuse, and so it’s extremely important to have a proper idea about how the human eye functions. The human eye, much like any other composite organ of the body, is a cluster of different sub-organs that are involved in the different steps of the actual process of vision.
Seeing actually refers to a combination of the process of viewing objects, and registering that image at the brain. The ocular apparatus (the human eye) performs the first function – accepting incoming light through the pupil. Much like a camera, the pupil is the opening from which light enters, while the iris and the cornea regulate the amount of light that reflects on exposed part of the eye-lens, as well as the duration of exposure. The eye-lens encloses a structure known as the retina, or the nerve-centre of the visual apparatus. The lens curves the direction of the incoming ray of light onto the retina, which constitutes of a number of photosensitive cells called ‘rods’ and ‘cones’. The incoming photons (light particles) trigger a photochemical reaction within these cells, which appropriates the image into electrical pulses right on the retinal disk (optic disk). These pulses are then transmitted to the brain via optic nerve fibres for processing.
What constitutes the ‘Eye’?
The major sub-parts of the human eye are –
Iris – The transparent, tinted portion of the eye that regulates the amount and duration of light entering the pupil at any given point (through dilation and contraction) is the Iris. It’s the Iris that lends the eye its natural colours of blue, or brown or green.
Pupil – It’s the circular opening within the Iris which allows light to be reflected on the eye-lens. It could be opened and closed by the Iris.
Cornea – Cornea is the transparent refractory plate which curves the rays of light towards the eye-lens, and focuses the light onto the retina. This part of the eye is thought to be extremely sensitive to pain, and intense physical triggers like excess light.
Lens – The Lens is a transparent, membranous refractive medium which the rays of light could access after passing the pupil. The light rays undergo refraction in this lens and are focused onto the retina. With age, a person may develop a milky covering on the lens called a cataract. A cataract operation usually involves a prosthetic, plastic lens to be placed in place of this actual lens.
Choroid – The Choroid fills up the interval between the sclera and the retina, and the choroid-pigments absorb excess light to maintain sharpness of vision.
Sclera – The Sclera is the visible white portion of the eye that hosts the Cornea in place. Its function is mainly protective. The Sclera often shows an active network of blood vessels.
Ciliary body – The Ciliary body forms an interface between the Iris and the Choroid.
Retina – The Retina forms the interior portion of the eye, and exists in the form of a photosensitive layer that receives the light and transmits neural information to the brain. The Retinal layer is made up of Rods and Cones, as explained earlier, with the number of rods being about 125 million. Rods are essential for seeing in dim light, or in relative darkness, while the Cones, which are easily about 6-7 million in number, helps the eye perceive objects in bright light. The cones are also responsible for colour perception, and help the eye place emphasis on details, thereby generating a sharper and more accurate image.
Macula – The Macula is a yellow area of the retina, located at the anterior section of the eyes surrounding the Fovea. The Macula essentially protects and hosts the Fovea, which is basically the most concentrated cluster of cone cells in the eye. This Part receives the most concentrated beams of light that is reflected from an object, and it is the fovea that is responsible for sending the most precise description of the object and relevant details to the brain.
Optic Disk – The Optic Disk is the externally visible potion of the optic nerve that is present in the Retinal area. The Optic disk is the point of inception for the optic nerve, and signifies the area from where the impulse is actually being transmitted via the optic nerve to the brain. Usually known as the ‘blind spot’, this area provides an interface for the conversion of the light image into impulses.
Optic Nerve – The Optic Nerve is the connecting nerve between the brain and the optic disk of the eye, and carries electric information from the eye to the brain for processing of the image.
Lachrymal Glands (or Tear glands) – The Lachrymal glands, or tear glands as they are commonly known, are a cluster of several glands around the eyelid-region. Each gland is, again, a cluster of about twelve tear ducts, which release tears to keep the cornea moist and well-protected.