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The Visual Pathway

As the retinal fibres leave the optic nerves, half of them cross to the opposite side

The fibres that cross are derived from the retinal receptors nasal to the macula. The structure so formed by the mutual crossing of nasal fibres by both optic nerves is the optic chiasm. From the optic chiasm the nasal fibres emanating from the nasal half of the retina of one eye intermingle with the fibres derived from the temporal sector of the retina of the opposite eye, forming a band called the optic tract.

(Fig. 8)

Fibres in the optic tract continue toward a cell station in the brain called the lateral geniculate body, so named because this body in the brain is shaped like a knee (Latin genu). The geniculate body is a relay station. From here, fibres spread out in a fan shaped manner and extend to the parietal and temporal lobe of the brain. They continue to their final destination, the posterior portion of the brain called the occipital lobe in an area denoted as the visual cortex. It is in this area of the brain that it is thought that the first step in conscious recognition of visual impulses takes place.

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The Optic Nerve

The optic nerve is located at the posterior portion of the globe and transmits visual impulses from the retina to the brain itself. Only the head of the optic nerve, called the optic disc, can be seen by ophthalmoscopic examination. The optic nerve contains no sensory receptors itself, and therefore its position corresponds to the normal blind spot of the eye. Branching out from the surface of the optic disc are the retinal arterioles and veins, which divide soon after leaving the optic disc and extend out on the surface of the retina to supply the inner one third with nutrients. As the optic disc enters the globe, it goes through a fibrous, sieve like structure, visible on ophthalmoscopic examination, called the lamina cribrosa. When the lamina cribrosa is prominent, it forms the base of a depression in the disc called the physiologic cup. The optic nerve consists of 1 million axons arising from the ganglion cells of the retina. The nerve emerges from the back of the eye through a small circular opening, it extends for 25 to 30mm and travels within the external eye muscle cone to enter the optic foramen, the aperture at the back of the eye’s boney socket, where it travels for 4 to 9mm to pass into the intra-cranial cavity and joins its fellow optic nerve to form the optic chiasm.

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The Retina

The retina, which contains all the sensory receptors for the transmission of light, is really part of the brain. The retinal receptors are divided into two main populations - the rods and the cones. The rods function best in dim light; the cones function best under daylight conditions. The cones are far fewer in number than the rods, numbering 6 million, whereas the rods number 125 million.

Cones enable us to see small visual details with great acuity. Vision with the rods is relatively poor but they detect movement and they also become extremely sensitive in low illumination and at night. Colour vision is totally dependent on the integrity of the cones.

The cones form a concentrated area in the retina known as the fovea, which lies in the centre of the macula lutea. Damage to this area can severely reduce the ability to see directly ahead. The rods are distributed in the periphery of the retina.

The junction of the periphery of the retina and the ciliary body is called the ora serrata. In the extreme periphery of the retina there are no cones and only a few rods. The retina is firmly attached to the choroid at the ora serrata. The other site of firm attachment of the retina is at the circumference of the optic nerve. The outer layer of the retina is composed of pigment epithelium cells and lies adjacent to the choroid, insulating the light receptor signals and making vision possible. Retinal detachment occurs as a result of cleavage between its anterior layers and the posterior pigment layer

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     A wide-field retinal image of a normal eye captured with the Optomap^®

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(Fig. 7)  The Lens      

The lens of the eye is a transparent biconvex structure situated between iris and the vitreous

Only that portion of the lens not covered by iris tissue (that is, only that portion directly behind the pupillary space) is visible. The centre of the anterior surface of the lens, known as its anterior pole, is only about 3mm from the back surface of the cornea. The diameter of the lens is about 9 to 10mm. Its peripheral margin, called the equator, lies about 0.5mm from the ciliary processes. It is attached to the ciliary processes and to the posterior portion of the ciliary body by means of the fine suspensory ligaments referred to as the zonular fibres.

The lens is surrounded by a capsule, which is a transparent, highly elastic envelope. The lens material within this elastic bag is rather soft in infants. With age it tends to grow harder, especially toward the centre of the lens. The harder central portion of the lens found in adults 30 years of age or over is referred to as the nucleus of the lens, and the outer lens fibres form the lens cortex. The harder nucleus is a product of the normal development growth of the lens. As new lens fibres are produced, the older fibres are pushed more toward the centre and are compressed in a concentric fashion.

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The Cilliary Body

The ciliary body is in direct continuity with the iris and is adherent to the sclera. Directly posterior to the iris, the ciliary body is plump and thrown into numerous folds referred to as the ciliary processes. This portion of the ciliary body is only about 2.5mm in length and is responsible for the major production of aqueous fluid. The equator of the lens is only 0.5mm from the ciliary processes and is suspended by fine fibres known as the zonular fibres of the lens. The anterior side of the ciliary body forms the rear of the anterior chamber. From its middle portion the iris takes root

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4. WHO GETS GLAUCOMA?

It is not an understatement to say that EVERYONE, children to older adults, should be concerned about glaucoma, because early detection and treatment are the only ways to prevent irreversible blindness. See below for risk factors that place certain people at greater risk.

5. ARE THERE RISK FACTORS FOR THE DEVELOPMENT OF GLAUCOMA?

There are several conditions that put some people at greater risk:

• Age.

  Although glaucoma can occur in younger persons, the risk for glaucoma increases with age, generally beyond 40.

• Family History

  Glaucoma can be inherited, and therefore family history is a risk factor. The Baltimore Eye Survey found that open-angle glaucoma is increased approximately 3.7 fold for individuals having a sibling with this most common form of glaucoma. However just because there is glaucoma in the family does not mean that a given individual will develop the disease.

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1. WHAT IS GLAUCOMA, AND HOW PREVALENT IS THE DISEASE? Approximately 1.2 million nerve fibers from the retina come together and form a "cable" called the optic nerve. This nerve connects the back of the eye to the brain. When light bounces off an image and strikes the retina, it organizes and "packages" the image, and sends the electrical impulses to the brain via the optic nerve. The brain then "decodes" these electrical stimuli into what we know as vision. Glaucoma is a disease that causes damage to the optic nerve which threatens blindness. The optic nerve is the "cable" that connects the eye with the brain and which carries visual information from the retina (back of the eye) to the seeing centers of the brain. (See Eye Anatomy for details.) The optic nerve is composed of over one million nerve fibers and is approximately 1/16th of an inch in diameter. There are many forms and causes of glaucoma (see below) which can be extremely difficult disease to diagnose at times. Glaucoma is considered a major health problem, and it is estimated that over 2 million Americans have the disease, and as much as half of them may be unaware of the problem. Every year, as much as 100,000 Americans develop glaucoma. According to the American Academy of Ophthalmology, in the United States alone, glaucoma of all types is the second most common cause of legal blindness, and is the leading cause of legal blindness among African Americans. Approximately 80,000 Americans are legally blind from glaucoma, and many more have visual impairment. Glaucoma is one of the most common causes of preventable blindness. There is no cure for glaucoma. However, with proper treatment, blindness can be prevented in 90% of the cases.

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The opaque sclera forms the posterior five sixths of the protective coat of the eye. Its anterior portion is visible and constitutes the white of the eye. In children the sclera is thin, and therefore it appears bluish because the underlying pigmented structures are visible through it. In old age it may become yellowish because of the deposition of fat. Attached to the sclera are all the extra ocular muscles. Through the sclera pass the nerves and the blood vessels that penetrate into the interior of the eye. At its most posterior portion, the site of attachment of the optic nerve, the sclera becomes a thin, sieve like structure called the lamina cribrosa. It is through this sieve that the retinal fibres leave the eye to form the optic nerve

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