The eye works very much like a a camera. In the camera, the light comes in through the shutter, is focused by the lens and falls onto the film.

visual field test eye

In the the eye, light comes in through the cornea, a clear cover that is like the glass of a camera's aperture. The amount of light coming in is controlled by the pupil (the shutter). The pupil widens or constricts to control the amount of light that reaches the retina. In dark conditions, the pupil widens. In bright conditions, the pupil constricts. Then, the light focuses on the retina, a series of light-sensitive cells lining the back of the eye. The retina acts like camera film, reacting to the incoming light and sending the optical stimuli via the optic nerve to the brain (the processor).

Anatomy

visual field test eye anatomy

The human eye is a complex organ.

The clear, small and curved unit in front - the cornea - is linked to the larger (white) unit called the sclera. The cornea and sclera are connected by a ring called the limbus. Together they form the outermost layer of the eye. The middle layer consists of the choroid, ciliary body and iris. The innermost layer is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels. The iris, ciliary body and choroid are also referred to as the uvea.

The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. The iris has an opening in the center called the pupil. The pupil looks black. The iris is responsible for the eye's 'color' which is usually green, blue (low pigment amount), or brown (most common). The pigment involved is melanin, which can be of differing shades from yellow-brown to black. An iris with no pigment is naturally a pale blue, as can be seen in an albino person’s eyes. The eye color is also affected by the presence of blood vessels and the nature of fibrous tissue in the iris. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil.The iris and pupil are the most visible or obvious eye structures that people see immediately when they look at you.

In the iris are two tiny muscles:

  • A sphincter muscle (sphincter pupillae) around the edge of the pupil – it acts to constrict the pupil in bright light, restricting how much light can enter the eye and travel through the lens to the retina. Constricting the pupil makes the iris larger.
  • A dilator muscle (dilator pupillae) structured radially like spokes on a wheel – it widens (dilates) the pupil in dim light, to maximize how much light can enter the eye, and this reduces the size of the iris.

The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea.

The anterior chamber (AC) is is the space inside the eye between the iris and the cornea's innermost surface. The peripheral part of the anterior chamber, between the cornea and the iris region is referred to as the trabecular meshwork (filtration angle).

Drainage after filtration is via the Canal of Schlemm. The Canal of Schlemm is a circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream via the anterior ciliary veins. The canal is essentially an endothelium-lined tube, resembling that of a lymphatic vessel. It is covered by the trabecular meshwork. if the canal is blocked then ocular hypertension is the result.

The Posterior Chamber consists of small space directly posterior to the Iris but anterior to the lens. The iris is the boundary between the posterior and anterior chambers.

The anterior and posterior chambers of the eye are filled with aqueous humor, a fluid with an ionic composition very similar to the blood plasma and with two main functions: to supply nutrients to the avascular structures of the eye, cornea, and lens and to maintain intraocular pressure (IOP) within its physiological range and the eye's shape. The production of the aqueous humor is a constant process and its removal is very important, the balance between production and drainage determines the intraocular pressure. It must be noted that eye pressure varies from person to person, what is 'normal' eye pressure for one person may be high for another. Aqueous humor is secreted to the posterior chamber of the eye by the ciliary processes of the ciliary body in a process known as aqueous humor inflow. Aqueous humor then reaches the anterior chamber by flowing through the pupil. There are two different pathways of aqueous humor outflow, both located in the iridocorneal angle of the eye. The a) uveoscleral or nonconventional pathway, by diffusion through intercellular spaces among ciliary muscle fibers. This route is the target of specific antiglaucoma drugs (latanoprost, a prostaglandin F analog) that increase its functionality. and b) the trabecular or conventional main outflow pathway (through the trabecular meshwork and the Canal of Schlemm).

The posterior chamber should not be confused with the vitreous chamber. The vitreous chamber consists of the space between the lens and the retina, and if filled with the amorphous and somewhat gelatinous material - the vitreous body. This material servers principally to maintain the eye's shape.

High pressure left uncontrolled can lead to damage of the optic nerve and result in vision loss. The optic nerve is a bundle of about one million individual nerve fibers, like an electric cable is made up of many wires and is connected to the retina, which is a layer of light-sensitive tissue lining the back of the eye. The optic nerve transmits visual signals from the retina to the brain, where these signals are interpreted as the images you see. In Glaucoma the optic nerve at the back of the eye is slowly destroyed.

 

visual field test optic nerve

In most cases, damage to the optic nerve  is due to increased pressure in the eye, but there are many people with ocular hypertension who don’t have any eye damage, and many people who have optic nerve damage and visual field test loss despite a normal ocular pressure. When the pressure inside an eye is too high for that particular optic nerve, whatever that pressure measurement may be, glaucoma will develop .

The macula is an oval-shaped highly pigmented yellow spot near the center of the retina. It has a diameter of around 5 mm. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and is responsible for central, high resolution vision.

Cone cells are photoreceptors cells in the retina that are responsible for color vision, they function best in relatively bright light, as opposed to rod cells that work better in dim light. Cone cells are densely packed in the fovea, but quickly reduce in number towards the periphery of the retina. Rod cells are concentrated at the outer edges of the retina and are used in peripheral vision.

 

The largest is the vitreous chamber, filled with the amorphous and somewhat gelatinous material of the vitreous body.
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