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2.2.1 Structure of the eyeFigure 2.1 shows the main structures of the eye. Each structure (in bolded italics) is explained below.
To maintain its shape, the eye is kept firm by a clear fluid flowing continuously in and out of the anterior chamber, which nourishes nearby tissues with oxygen, sugars and other nutrients. Circulating around the structures inside the eye, the fluid leaves the chamber at the open angle where the cornea and iris meet. When the fluid reaches the angle, it flows through a spongy meshwork, like a drain, and leaves the eye. Drainage is against resistance, so the eye’s pressure is kept higher than air pressure but lower than blood pressure.
The lens is a clear structure at the front of the eye that helps to focus light, or an image, on the retina. The retina is a thin layer of neural tissue that lines the back of the eye. In a normal eye, light passes through the transparent lens to the retina. The lens must be clear for the retina to receive a sharp image.
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Figure 2.1 Anatomy of the eye and retinaThe iris is the coloured part of the eye that helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light; when there is low light, the iris opens up the pupil to let in more light.
The retinal pigment epithelium (RPE) is the layer between the retina and the blood vessels underneath, which are known as the choroid. The RPE passes oxygen, sugar and other nutrients up to the retina and moves waste products (such as cellular debris from the tips of the photoreceptors as they renew themselves) down to the choroid.
2,2,2 Interactions between the eye and the brainOnce light reaches the retina, the light-sensing cells (photoreceptors) convert light into electrical impulses. Photoreceptor cells are either rods or cones. Rods are concentrated along the outer perimeter of the retina; they help us to see images in our peripheral vision and also to see in dark and dimly lit environments. Cones are concentrated in the macula, the centre of the retina, allowing us to see fine visual detail in the centre of our vision; they also allow us to perceive colour. Together, the photoreceptors convert light into electrical impulses that are passed through the rest of the retina, through the optic nerve to the brain. The visual centres in the brain interpret the information received from the retina, enabling us to see.
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2.2.3 How we focus on an objectThe eye can change its optical power to maintain a clear image (focus) on an object as it moves towards and away from the retina. The main focusing ability of the eye is due to the difference in refractive index between air and the curved cornea. The cornea cannot change its curvature; however, the curvature of the lens can change, and this process is called accommodation.
Normally, parallel rays from distant objects will converge onto the retina. If an object is moved closer to the eye, and if the lens does not change its curvature, then the sharp image will remain behind the retina and our brain will only be able to detect a blurry image. If the lens changes it curvature (becoming thicker) then the light rays will converge on the retina, and the image will be clear (see Figure 2.2).