The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye.

The innermost layer is the light-sensitive retina. The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it.

Inner Chamber of eye

The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length. The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye. Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended. The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer.

These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area.

At the outer edges are only rod-shaped cells. Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea, a small round area of the retina exists that has no light-sensitive cells. This optic disk forms the blind spot of the eye.

The compound eye or commonly referred as animal eye is composed of hexagonal or rectangular-shaped, closely packed optical units called ommatidia (small eyes); each ommatidium is virtually a single eye. In different species the size, number, and structure of ommatidia vary. An ommatidium is composed of a corneal lens, or facet, which consists of a modified extension of the cuticle (the hard outer covering of arthropods) on the surface of the eye; four cells called Semper's cells or cone cells, which form the crystalline cone; and a sensory region called the retinula (small retina).

In primitive insects (e.g., the springtail Lepisma), in which the transparent cone cells are not specialized, the ommatidia are called acone ommatidia. In the more common eucone ommatidium, which occurs in moths and butterflies, the cone cells have a more complicated structure and contain granules of glycogen, or animal starch; because the granules are packed at various distances from each other, the refractive index varies in different positions in the cell. In certain beetles (e.g., Lampyris) and in the horseshoe crab Limulus, the crystalline cone is an extension of the cornea.

The sensory part of the ommatidium, the retinula, consists of several radially arranged cells (retinular cells); each has a photoreceptor component, or rhabdomere. The rhabdomeres of neighbouring retinular cells may be either in contact (forming a rhabdom) or completely separate. The optical isolation of each ommatidium is enhanced by its being surrounded by light-screening, pigment-containing cells. During adaptation to light and dark conditions, migration of pigment in cells around the crystalline cone, the corneal process, the proximal part of the ommatidium, and within the retinular cells has been reported to occur in most types of compound eyes.

Source(s): All above information & images are based on information collected from chapter on eyes from the book Human Physiology by Gillian Pocock and Christophor D. Richards and from various sources. All rights reserved by respective owners.

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Disclaimer: Any information displayed here is just for educational purposes, and may not be taken as an expert advice and should not be applied in life without consulting your eye doctor/specialist. We here by take no responsiblity of the accuracy of the above content as they have been taken from various sources.

About 200 children are diagnosed with retinoblastoma (eye cancer) each year in the United States. This cancer affects about one out of every 20,000 children, accounting for 3.1% of all childhood cancers. Most children with retinoblastoma are under four years of age. About 75% of children with retinoblastoma have a tumor in one eye. In about 25% of cases, both eyes are affected.

More Information

Basic Structure

Eye as a Camera

Protective Structures

Visual Pathways

Human Eye

Compound Eye