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The Eye is the organ of sight which helps us navigate through the world around us (Hollows, 2015). Although, it is not just the eye involved in vision, the brain also plays a key role in how we see and interep the world. This essay will identify and discuss the key structure and functions of main parts of the eye. It will also look at the transmissive and refractive properties and how they affect sight. Finally, will identify how various diseases and genetic defects can impact visual acuity.
The eye consists of various accessory structures along with the eyeball. The accessories include the eyelids, eyelashes and tear ducts which all are key to maintaining healthy eye. The Eyeball which is an irregularly shaped hollow instrument which is around 24mm in diameter and weighing in at just 8g, However, it is made up of various numbers of layers and components (see appendix 1), all of which have their own role to play in one of the key senses (Hollows, 2015).
The first of three layers of the eye, The Fibrous tunic, the eyes outer layer, formed by a tough layer of fibrous tissue. Situated at the front of the eye is a transparent and non-vascular component known as the cornea, this covers the pupil, iris and the anterior chamber. Another transparent but vascular membrane called conjunctiva covers the cornea and sclera. The sclera gives shape to the eye, protects it and provides the surface for the six extrinsic muscles to attach (Slomiank, 2009).
The layer below the Fibrous Tunic is the Vascular Tunic, within this layer is the Choroid, a vascular structure which supplies a rich blood supplying the retina. The choroid is dark in colour with pigments which absorbs any excess light to avoid reflection. However, in nocturnal mammals this layer contains a component called the tapetum lumen which reflects light to maximise vision at night, by increasing the chances of it hitting the photoreceptors (Swenson, 2008).
The Iris is the coloured part of the eye, which is not just for aesthetics, it actually dilates and constricts controlling the amount of light which enters the eye. The pupil is not actually a physical structure, it is a hole within the iris. The iris and lens seperate part of the eye into a small anterior aqueous chamber and a large vitreous chamber. The aqueous chamber is filled with a watery aqueous humour, and the vitreous chamber is filled with a gelatinous vitreous humour which maintains intraocular pressure and also acts as a refractive medium. The ciliary muscles are also within vascular tunic and are responsible for altering the shape of the lens when trying to focus (Willis, ND).
The final layer is the Neural tunic this houses the Retina, Macula and the Fovea. The retina is comprised of several types of cells. The retina is a layer of various cells such as ganglion, bipolar, pigmented and photoreceptor cells (see below) which change light into electrical impulses which are then transmitted to the brain via the ganglion cells and optic nerve (Peek,2017).
There are two types of photoreceptor cells which both have different roles. Rods, these are found in numbers of approximately 120 million and are highly sensitive to light and provide monochromatic vision. The other photoreceptor cells are called Cones, there are three types which are all sensitive to different colours. Cones are found in much fewer numbers but provide Trichromatic vision, these are found in high concentrations at the area at the back of the retina which is called the Fovea. The Fovea (see below) is the a central part of the retina and is located in the centre of the macula. Consisting only of cones, It is the responsible for producing the sharpest vision and great colour perception (Spring, 2015).
Vision is heavily dependant on the eyes ability to refract light, this refraction process takes place in various parts of the eye. The cornea and the eyes lens, when light first passes through the spherical surface of the cornea it refracts. The majority of the eyes refractive power comes from the cornea, when the eye refracts light correctly this is known as 20/20 vision with no refractive faults (Heiting, 2016).
Visual acuity is defined as the clarity of the image seen by the eye (Day, 2011), and can be affected by various medical, genetic and structural problems (see appendix) . Myopia, also known as short sightedness. Sufferers can see near objects clearly but struggle to see objects at distance. This is due to the lens being too convex, this effects the light being focused correctly on the retina (see below image). This problem is fixed by using eyewear which uses concave lenses to correct the fault (NHS, 2015).
Hyperopia, commomly known as long sightedness, is the opposite to Myopia, this allows sufferers to see objects at distance clearly but struggle with objects that are near (Lowth,2016). This is caused by the eyeball being too short, this effects the correct distance between the lens and the retina, focusing the light to a point behind the retina instead of directly on it (see below image), this is corrects by prescribing the use of a convex lens (NEI, 2016).
Cataract is a condition where the lens of the eye becomes cloudy, this is normally associated with age, but can also be a genetic defect. The lens is made up of mostly water and proteins, as we age the proteins can begin to clump together. The sufferer struggles to see any objects clearly due to the light rays not being able to pass through the cloudy lens (see below image). This problem can be remedied only by a surgical procedure which involved removing the effect lens and replacing it with an clear convex lens (NEI, 2015).
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