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medinews: revisiting 2009
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Eye care snippets by Dr. Narendra Kumar: ANATOMY & PHYSIOLOGY OF THE EYE
Comparable to a camera, the eye has an outer protective layer, sclera; a middle vascular layer, choroid; and an inner photosensitive layer, retina. The fibrous layer consists of sclera and cornea. The sclera resists the intraocular pressure and supports the attachment of extra-ocular muscles. The cornea lies into the central visible portion of sclera, and is transparent and avascular, and receives its nutrition by diffusion from the surrounding area. Due to its converging power, the cornea bends light entering the eye and focuses it on the retina. The vascular layer, choroid, is continuous with the iris and the ciliary body, and the three structures together are known as the uveal tract. The photosensitive layer, retina, contains rods and cones, and nerve cells which extend into the optic nerve. The fovea, a point on the retina, has a high concentration of rods and no cones.
A transparent structure behind the pupil, the crystalline lens, changes its shape when the ciliary muscle acts during the process of accommodation. It is held in place by suspensory ligaments. Anterior chamber is the space between cornea and iris; posterior chamber is the space between iris and lens, and vitreous chamber is the space between lens and retina. While anterior and posterior chambers are filled with a watery fluid (aqueous humour), the vitreous chamber is filled with a gel-like fluid (vitreous humour). The six extra-ocular muscles (four straight and two oblique), attached to sclera, rotate the eyeball which is richly supplied with blood and nerves, and lies in a bony cavity of the skull, called the orbit. The lids (upper and lower) provide protection to the eyeball in front. On the inner side, lids are covered by a mucous membrane, the conjunctiva, which also covers the exposed part of sclera. The lacrimal gland secretes tears. The globe is 25 mm in diameter, the cornea 12 mm, the pupil 3.5 mm (under average light), and the optic disc 1.5 mm.
Nerve supply. Seven pairs of cranial nerves control and carry information to and from the eyes. 2nd optic nerve carries electric impulses from the retina to the brain. 3rd oculomotor nerve controls eye movement, accommodation and pupillary constriction. 4th trochlear nerve controls the muscle that moves the eye to look down and in. 5th trigeminal nerve transmits signals for corneal sensitivity. 6th abducens nerve controls the muscle that moves the eye to look in an outward direction. 7th facial nerve controls the muscles that close the lids and produce facial expressions. 8th acoustic nerve carries signals of loud noises to which eyes or lids respond.
Emmetropia and ametropia. The cornea bends light entering the eye and focuses it on the retina. If a person requires no correction to see clearly in the distance, s/he is called emmetropic. When a person requires lenses to see clearly in the distance, s/he is called ametropic. If the light falls short of the retina, the person is called myopic and if the light falls behind the retina, the person is called hypermetropic. The hyperope, with sufficient accommodation, can bring the image into focus, but the myope cannot (although s/he can improve vision by squeezing lids together to create a small opening and increased depth of focus).
Accommodation and presbyopia. Accommodation is a process of contraction of the ciliary muscle by which lens of the eye is forced to bulge forward, resulting in increased power, to allow focus clearly on objects situated at near distances. A hypermetropic person spends lot of energy by exercising accommodation in order to keep things in focus. As he gets older and begins to lose his ability to accommodate, he may find it difficult to see clearly after a long day's work. Small amount of concentrated near work increases his difficulty in accommodating. Presbyopia is the name given to the age-related loss of accommodation. The ability to focus closely may be completely lost by the age of sixty.
Binocular vision and strabismus. Binocular vision is the fusion, in the brain, of two images one from each eye, resulting in a single stereoscopic perception. If fusion is disrupted by a Maddox rod in front of one eye, the eyes drift (heterophoria). An outward turning is called exophoria; and an inward turning, esophoria. In the absence of fusion, there is no binocular vision. In heterotropia or strabismus, the eyes are not straight. An outward turning is called exotropia; and an inward turning, esotropia. In childhood strabismus, two separate images reaching the brain cause diplopia. Since the condition of double vision is intolerable, the brain ignores image from one eye (suppression), and with passage of time vision in the ignored eye does not develop (amblyopia). If amblyopia is detected early, it is possible to use the affected eye by occluding the other (better) eye. To correct strabismus, however, lenses, orthoptics or surgery (or all together) may be needed.
Glaucoma. Aqueous maintains a constant pressure within the eyeball, which can be measured with the tonometer. The average range of normal intraocular pressure is from 12 mm Hg to 22 mm Hg. Estimating intraocular pressure, examining optic disc and recording visual field are the steps to diagnose glaucoma, which requires life-long treatment with drugs. Surgery is done when the condition cannot be controlled by drugs.