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Transcript of The Eye
Both the lens and the cornea are made up of organized layers. The lens continues to add layers as you age becoming rounder and less flexible.
The structure of the cornea is similar to that of the sclera. The transparency of the cornea may be due to the regular arrangement of its protein fibers. The cornea, but not the lens, includes unmyelinated nerve endings that can sense pressure, temperature, and pain. Function of the Cornea and Lens
Both the cornea and the lens act as lenses, bending light so that a focused, inverted, and reversed image is projected on the retina. The cornea is a better lens than the lens itself. It has a focusing power of 43 diopters compared to 18 diopters for the lens (a lens with a strength of one diopter bends light at a distance of one meter by one cm).
The main reason that the cornea bends light more than the lens is the difference between the refractive index of the atmosphere and the conea. The greater this difference, the stronger the lens. You may have noticed how you lose the ability to see things clearly under water. This is because the refractive index of water is about the same as that of the cornea and the cornea becomes a weaker lens.
The shape of the lens can change when the ciliary muscles contract, and, due to the way things are connected in the eye, the zonules (of Zinn) are loosened and the lens becomes rounder and, thus, a stronger lens (think how a flat sheet of glass doesn't bend light, but a curved magnifying lens does). A stronger lens is needed to focus nearby objects, a weaker one to focus distant objects. As one ages, due either to increase in size or hardness of the lens, this ability to accomodate becomes weaker. The hole in the middle of the Iris, the Pupil The structure of the Iris
The iris is made up of two sets of muscles; one that circles the iris, and another that goes from the pupil to the exterior. When the cirular ones contract, the pupil constricts, when the radial fibers contract, the pupil dilates. The size of the pupil is affected by the pupillary light reflex, which constricts the pupil when the eye is exposed to bright light, and the effects of the sympathetic and parasympathetic nerve systems. The sympathetic system causes the iris to dilate - - this will happen when we become somewhat excited about what we see. A preparation from the belladonna plant was used in the past as a cosmetic. It was placed in the eye causing the iris to dilate making the woman seem younger (younger people have larger irises) and also interested in whoever she was talking to.
The function of the iris
The amount of light entering the eye is obviously controlled by the size of the iris. Although this is a minor cause of our ability to adapt to different illumination levels, it is quicker acting than other causes.
The size of the iris, like size of the aperture of a camera, affects focus. The smaller the iris, the larger the range in distance that everything appears in focus. Aqueous Humour
A thickish clear fluid that carries nourishment to the lens and cornea, and waste away from them. It also is under pressure, maintaining the shape of the eye (and sometimes, when the pressure becomes too great, it is the cause of glaucoma. It is produced in the area around the ciliary muscles in the posterior chamber, and flows out through the border of the sclera and cornea in the anterior chamber. The Aqueous Humour The Vitreous The vitreous is the home of the floaters (muscae volitantes - darting flies) that some of us see when we look up at the sky or other uniform bright area. It is of jellylike consistency with a surface that pushes against the interior wall of the eye. With age, it decreases in size and occasionally causes the retina to separate from the choroid layer. The Retina The retina is brain tissue that migrated early in development to the eye. As well as turning light into nervous impulses in the rods and cones, the retina processes information from light. It emphasizes the borders between light and dark, tranforms color information, and sends in separate pathways information about movement, fine details, and color.
Light travels through the ganglion cells, amacrine cells, bipolar cells, horizontal cells and their connections before reaching the rods and cones where photopigments absorb light and cause chemical reactions changing the amount of neurotransmitter leaving those photoreceptors. The Fovea The portion of the retina that contains tightly packed cones - it is the portion of the retina that provides our fine vision (has the highest acuity). The further one gets from the fovea, the greater the number of rods (cells sensitive to light, but not to color), and the less the number of cones.