Structure of Eye

What is vision

1. The human vision is a very complex process. Vision is a result of the two eyes working almost simultaneously with large portions of the brain to produce the sensation of vision.
2. The first step in the process is the stimulation of the light sensitive receptors in the eye. This information of the image is then converted into electrical signals which are transmitted by the optic nerve to the brain where it undergoes a lot of pre-processing before reaching the visual cortices in the cerebrum. It is at the end of this process that we have a sense of vision.
3. We must remember that the eye is actually an extension of the brain and is only one tool in the entire process of vision.

Optical Parts of the eye

The optical parts of the eye, as can be seen in the accompanying figure are:

1. Cornea: This is the front bulging part of the eye. It is thin, clear as glass and is primarily responsible for protecting the eye from external elements like dust, micro-organisms etc. as well as for blocking out ultraviolet light from entering the eye. It also provides more than half the refracting power of the eye – A substantial part of the focusing – about 65% - is done by the cornea. The sclera is the remaining part of the external eye. In humans this is white and is seen as the white part of the visible portion of the eye. (In other animals this could be a different colour)
2. Anterior - Behind the cornea is the anterior (from the Latin word ‘ante’ meaning front) chamber which is filled with Aqueous Humor (so called because it is in liquid form.)
3. The Iris is behind this. This is what we see as the brown, green blue or grey coloured portion in the eye.
4. At the centre of the Iris is a circular gap called the Pupil (this is circular in human beings but is different in other animals – for example the pupil is shaped like a slit in the case of cats.) The pupil appears to be black in colour because the light that enters it is not reflected back but is absorbed by the interior components of the eye. The pupil acts like the ‘aperture’ of a camera and the iris acts like an ‘aperture stop’. The Iris increases or decreases the pupil size depending on the amount of light.
5. Lens - Behind the pupil is the lens. The focal length of this is variable. The muscles to which the lens is attached stretch it or compress it to cause the focal length to change. As mentioned earlier this is only one part of the focussing system.
6. Posterior - Behind the iris is the posterior (from the Latin ‘post’ meaning after) chamber. This is filled with vitreous (so called because it is a gel like substance) humor.
7. Retina - At the back is the ‘Retina’. The Retina is a light sensitive tissue lining the interior wall of the eye. It is a multi-layered membrane that contains millions of light sensitive cells. As mentioned earlier the cells are sensitive in a variety of ways – some of them distinguish between light and dark (rods) and some are sensitive to different wavelengths of light (cones). The point where the optic nerve connects to the retina is know as the blind spot - light falling on this spot is not seen as the there are no light sensitive cells here. Another portion of the retina called the ‘fovea’ is the place where there is a maximum concentration of all types of cells – rod and cones. When we try to focus and look carefully at something, this is the portion of the retina that gets used. h. The retina is connected to the brain by the Optic Nerve.

How do we see

All these elements work together to form images of the objects that fall in the field of view for each eye. When an object is observed, it is first focused through the convex cornea and lens elements, forming an inverted image on the surface of the retina, In order to reach the retina, light rays focused by the cornea must successively traverse the aqueous humor (in the anterior chamber), the crystalline lens, the gelatinous vitreous body, and the vascular and neuronal layers of the retina before they reach the photosensitive outer segments of the cone and rod cells. These photo sensory cells detect the image and translate it into a series of electrical signals for transmission to the brain.
The images formed of an object by the two eyes are slightly different as the eyes are positioned apart. The two images pass through the independent optic nerves and cross over within the brain. At the cross over points the two images are processed to provide us with stereoscopic vision. As mentioned earlier the number of cones and rods are greatest near the fovea and rapidly decrease elsewhere. This means that the image of the world that is formed on the retina is clearest near the fovea and blurred elsewhere. Despite this we appear to see the world uniformly and clearly. This is because the involuntary movements of the eye allow us to scan objects in the field of view. This scanning helps us to offset the loss of clarity in the periphery and results in a perceived image that is uniformly sharp. In fact, when the image is prevented from moving relative to the retina (via an optical fixation device), the eye no longer senses an image after a few seconds. We need a changed view an object to see it!

Problems with eyes

Failures of the focussing system of cornea and lens

As mentioned earlier the lens changes its shape and therefore its focal length and lets us see things clearly even though they are at different distances from us. Of course, the closer the object is to our eye, the larger the image on the retina and the greater the clarity. The closest distance we can bring on object to our and yet see clearly is known as the ‘least distance of distinct vision’ (LDD). LDD is normally around 25 cms. If, however, the lens is unable to accommodate itself or if the shape of the cornea has changed or distorted then the LDD would be different. If the LDD is closer to the eye then we would find that we are unable to see things that are at a distance clearly. (We would also notice, for example, that we need to hold a book closer to our eye in order to be able to read it.) Such a problem is known as Myopia or Short Sight. This is a problem that can affect people of any age and is generally due to the lens not being able to change its focus properly. If the LDD is farther from the eye – we would have to keep the book farther away form us – the condition is known as Long Sight or Hypermetropia. This generally happens with older people. Occasionally it can happen with young children and is likely to be because of corneal problems. If the cornea bulges in an odd way and so distorts the image that forms on the retina – here the LDD remains the same - then the condition is known as Astigmatism. All these problems can be corrected by using spectacles with appropriate kind of lenses. Nowadays you can also correct this condition by changing the shape of the cornea.

Clouding of the lens

This is a process that happens as people grow old. The lens loses its transparency as people grow older and the amount of light reaching the retina is reduced. This can happen in patches or uniformly. This problem is called Cataract. Nowadays there are simple operations that allow surgeons to replace the lens inside the eye. There are some other conditions that can affect the optic nerve and not allow it send messages to the brain. This can happen gradually and often occurs because there may be a high pressure on the nerve because of insufficient control of the aqueous or vitreous humors. Such a problem is called Glaucoma.

Colour Blindness

d. Lastly there could be problems with the rods and cones in the retina itself. One of the most common such problem is the condition known as colour blindness. Because it is genetic and as this is part of the XY chromosome, this normally affects only men and incidence in women is very rare. There are different kinds of colour blindness: In the first – known as Anomalous Trichromacy - all three kinds of cones work but one of them – the red, blue or green cone does not sense the right wavelength and thus the person sees colours differently. This is the most common kind and roughly about 6% of the male population would suffer from this and most of the failure (about 5% of the population) is in sensing colour green properly. In the second case – known as Anomalous Dichromacy – one of the colour sensitive cones – normally the red or the green – would not be active at all and the world would be seen using only two types of cones. This kind of problem affects about 2 % of the male population. In very rare cases a person may not be able to see colour at all. This can happen either because the cones are non-functional or because the areas of the brain that recognize colour have been damaged.