Endocrine System
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2.29 Eye – The outer surface of the eye is covered by the cornea, a transparent membrane that protects the eye but allows light to enter. Behind the cornea is the iris, a thin, coloured, circular membrane. In the centre of the iris is an opening called the pupil. Muscles in the iris control the amount of light that is let in by altering the diameter of the pupil. The pupil dilates, or enlarges, to let more light in and contracts to let in less light. Behind the pupil is the lens, a thick, curved structure that focuses the light rays so that they will fall on the retina, the thin lining on the back of the eye. The image on the retina is inverted.
30. Receptor Cells – There are approximately 1 billion receptor cells in the retina. Rods are cells that are extremely sensitive to light; however, they cannot detect colour, and they produce poorly defined images. Cones are cells that detect colour and produce sharp images. At the centre of the retina is a concentration of about 3 million cone cells. People usually move their eyes so that the light rays from an object they want to see fall on this area. There are no cones or rods at the point where the optic nerve enters the retina. Light that falls on this area does not stimulate any sight receptors, resulting in a “blind spot” in the field of vision.
Eye Disorders
2.31 Image Forming – When the eyes view an image, each eye sees the image from a slightly different angle. About one-third of the visual fields overlap, but the other two-thirds differ. This overlap produces binocular vision, which enables people to perceive depth. Also, the optic nerves do not connect to only one hemisphere. About one-half of each optic nerve crosses over into the opposite side of the brain; because of this, each hemisphere receives visual information from both eyes. The brain combines all the information it receives to produce one image.
2.32 Where do tears come from and what is their purpose? – Tears are produced by a gland found above and to the outside of each eyeball. Each gland produces a watery secretion that mixes with a slightly oily secretion from accessory glands. The tear glands have between 10 and 12 ducts, and every time the eyes blink, tears are spread across the surface of the eye. Tears protect the eyes by lubricating them and washing away debris. Also, they contain an enzyme that helps prevent eye infection. Tears drain into two ducts in the inner corners of the eye. These ducts drain into the back of the nose.
