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Low Vision Facts - Anatomy and Physiology of the Eye

Figure 1: Diagram of the human eye identifying the various segments described below.
The image shows a cross-section of the human eye. It also identifies principle parts of the eye, such as the sclera, iris, cornea, pupil, lens, conjunctiva, vitreous, choroid, optic nerve, macula, and retina.


Illustration by Mark Erickson


The protective outer layer of the eye, sometimes referred to as the “white of the eye” is called the sclera and it maintains the shape of the eye. The front portion of the sclera, called the cornea, is transparent and allows light to enter the eye. The cornea is a powerful refracting surface, providing much of the eye's focusing power (Cassin and Solomon, 1997). Attached to the sclera are six extraocular muscles responsible for movement of the eyes (Bianco, 2002). The choroid is the second layer of the eye and lies between the sclera and the retina. It contains the blood vessels that provide nourishment to the outer layers of the retina (Cassin and Solomon, 1997). The iris is the part of the eye that gives it color. It consists of muscular tissue that responds to surrounding light, making the pupil, or circular opening in the center of the iris, larger or smaller depending on the brightness of the light (Pachler and Rizun, n.d.).


Light entering the pupil falls onto the lens of the eye where it is altered before passing through to the retina. The lens is a transparent, biconvex structure, encased in a thin transparent covering. The function of the lens is to refract and focus incoming light onto the retina for processing (Moorfields Eye Hospital, 2002).


The retina is the innermost layer in the eye. It converts images into electrical impulses that are sent along the optic nerve to the brain where the images are interpreted. The retina can be compared to the film of a camera. It is composed of light sensitive cells known as rods and cones interconnected by a complex mesh of neurons that provide early stage visual processing. Rod cells are primarily in the outer retina, do not discriminate colors, have low spatial resolution, support vision in low light (“night vision”), are sensitive to object movement and provide peripheral vision. Cone cells are densely packed within the central visual field, function best in bright light, process acute images and discriminate colors (Montgomery, 2002).


The macula is located in the back of the eye, in the center of the retina. Within the macula is an area called the fovea centralis. This area contains the highest concentration of cones, produces the sharpest vision, and is used to see details clearly (Moorfields Eye Hospital, 2002).


The inside of the eyeball is divided by the lens into two fluid-filled sections. The larger section at the back of the eye is filled with a colorless gelatinous mass called the vitreous humor. The smaller section in the front contains a clear, water-like material called aqueous humor (Discovery Fund for Eye Research, 1999). A circular canal, called the Canal of Schlemm provides a drainage system for the aqueous humor from the eye into the bloodstream. Blockages in the Canal of Schlemm are believed to be contributing factors in the development of glaucoma (Bianco, 2002).


The conjunctiva is a mucous membrane that begins at the edge of the cornea and lines the inside surface of the eyelids and sclera, which serves to lubricate the eye. Inflammation of this membrane results in conjunctivitis, commonly known as pink eye (Bianco, 2002; Cassin and Solomon, 1997).


©2003, Technology Transfer Rehabilitation Engineering Research Center, University at Buffalo


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