Structure of Human Eye

The human eye is an amazing organ. Composed of many intricate parts working together, that results in something that everyone depends on heavily every single day i.e. our vision. Although we value our vision the most above all our senses (accounting for about 80% of all impressions), a lot of people surprisingly know very little about how our eyes work. The human eye can be broken down into parts that are visible externally while looking in the mirror, and parts that aren’t visible since they are situated inside the eye, or further towards the back. Humans possess binocular vision, which means that both the eyes are able to create a single combined image. Optical components help in the formation of an image that further gets perceived and interpreted by the brain through the connecting neurons. The entire machinery works in quite a complex manner.

External Parts of Human Eye

Eyelid: the eyelids cover the eye in order to protect it from dust, grit, and perspiration that can further cause damage. It opens and closes both voluntarily and involuntarily, and allows blinking to help keep the eye hydrated and well-lubricated.

Pupil: it is the part of the eye which we see through, and it keeps on changing size based on light levels. If a person is present in a particularly bright environment, then the pupil contracts to allow only little light to enter, while if in a darker environment, it gets expanded to let more light enter. This arrangement helps us to see well in different environments of light levels, making sure that the correct amount of light reaches the retina which is present at the back of the eye.

Sclera: this represents the white portion of the eye, and provides a protective outer layer. It surrounds the optic nerve and it can also be a good indicator of a person’s eye health. For example, a red sclera may suggest that the eyes are tired or dry, whereas a yellow-tinted sclera is an indication of some liver problems.

Iris: it represents the coloured part of the eye and it actually helps in controlling the size of the pupil. That is it regulates the amount of light that enters into the eye. The iris is actually made from connective tissue and muscles surrounding the pupil, and its structure, pattern and colour are  unique just like our fingerprints.

Internal Structure of Eye

Cornea: it represents the clear surface present in the front of the eyes, thus allowing light to enter the eye. It directly surrounds the iris and pupil, thereby providing a layer of protection. The cornea is mainly the one which we operate on for laser eye surgery procedures, as it is imperfections present in the curves of the cornea which creates an eye prescription, thus requiring us to need glasses. The smoother the surface of the cornea is, the better the vision will be.

Lens: the lens is situated behind the iris and is the part of the eye that provides focus. The lens possesses the ability to change its shape in order to alter the focal distance of the eye, focusing the light rays that pass through it to hit the retina at the right angles. As one gets older, a build-up of protein in the eye means that the lens has become cloudy. This is known as cataract. The lens can be easily removed and can be replaced with an artificial clear lens in order to provide a clear vision again.

Aqueous humour: the aqueous humour represents a watery fluid that the eyes continuously produce to maintain a good eye pressure and to nourish the cornea. It keeps our eyes healthy and, in turn, contributes to a clearer vision. It gets drained from the eye at the same rate in which it is produced (when this rate is no more constant, then it may lead to glaucoma) and its presence is crucial for good vision.

Ciliary muscle: the ciliary muscle is the part of the eye that helps in changing the shape of the lens, therefore, allowing it to focus on different distances. It also helps in holding the lens in the correct position in the eye’s middle layer and regulates the flow of the aqueous humour within the eye.

Medial rectus muscle: there are about six extraocular muscles present in the eyes that helps in movement (such as the medial rectus, lateral rectus, superior oblique, superior rectus, inferior rectus, and the inferior oblique) and the medial rectus is the largest among them. It moves the pupil closer to the midline of the body (towards the nose) and it makes sure that the eye is aligned correctly. If there are problems with the medial rectus, it may lead to strabismus.

Lateral rectus muscle: it is the muscle which is responsible mainly for lateral – or sideways – movement of the eye, usually movements away from the midline. Again, if there are problems with the lateral rectus muscle, then a person may experience esotropia. It is a form of strabismus in which the eyes turn inwards since the muscle becomes either too weak, or isn’t working properly to move it away from the midline.

Retina: the retina represents a layer of tissue located at the back of the eye. The main function of the retina is to receive light through the lens and send signals to the brain to convert it into a visual image. The retina consists of majorly two types of photoreceptor cells: rods and cones. Rods function in picking up on motion, dark and light, whereas cones help in detecting colour vision. Problems with the retina may lead to loss of vision, so preserving the retinal health is very important.

Choroid: it is a major blood vessel that is located between the retina and the sclera at the back of the eye. It helps in nourishing the external layers of the retina and keeps the eye usually at the right temperature. It also helps in providing the correct amount of oxygen and blood flow to the retina, therefore helping the eye to function well.

Macula: the macula is generally the central portion of the retina and is about 5mm in diameter. A healthy macula is an indication of clear vision and the person will be able to see fine details. If the macula becomes diseased, like with macular degeneration, the person’s central vision gets affected. This obviously will have a huge impact on a person’s day to day life, and gets  worsening until all vision is lost completely.

Optic nerve: the optic nerve is the region of the eye that transmits visual signals from the retina to the brain, that is needed to be converted into images. It consists of around a million nerve fibres and is mainly considered to be a part of the central nervous system. One of the very known ways by which the optic nerve gets damaged is by glaucoma. Eye pressure builds up, thus, compressing the optic nerve, meaning visual signals cannot be transmitted effectively anymore.

Vitreous humour: Vitreous humour is a liquid present in the eye; it has a gel-like consistency, and is situated behind the lens but in front of the retina. If any particle enters the vitreous humour, they are known as floaters. It can be either small flecks of blood or clusters of cells and, while they are annoying to see in the line of vision, they are generally harmless. With age, a person’s vitreous layer gets thin and also gets separated from the retina, resulting in “posterior vitreous detachment”. This becomes even more prone to floaters but isn’t sight-threatening.

Conclusion

The eye acts in the same way like the camera in the sense that the image formed on the retina is inverted (i.e. usually upside down). When the brain processes the image, it automatically flips it. If one wears special goggles that make them view everything upside down, after a few days the brain will get adapted to it, thus showing us the “correct” view. Here we come to an end of this topic. We hope that you were able to get a clear concept on the structure of the eye.