The eyes are protected by their location in the bony cavities of the orbits. Only about a third of the eyeball is unprotected by bone. The eyeball itself is roughly spherical and its wall consists of three layers: a tough outter coat, the sclera, which is white in appearance; a pigmented layer called the choroid, which is highly vascular; and the retina, which contains the photoreceprors (rods and cones) togerher with an extensive network of nerve cells. The retinal ganglion cells are the output cells of the retina and they send their axans to the brain via the optic nerves.

At the front of the eye, the sclera gives way to the transparent cornea which consists of a special kind of connective tissue that lacks blood vessels. The health and transparency of the cornea is maintained by the tear fluid secreted by the lachrymal glands and by the aqueous humor that is secreted by the ciliary body within the eye itself. The pigmented iris covers much of the transparent opening of the eye formed by the cornea, leaving a central opening, the pupil, to admit light to the photoreceptors of the retina.

Basic Structure

The pupil diameter is controlled by two muscles, the sphincter pupillae and dilator pupillae of the iris, which are innervated by the autonomic nervous system. The sphincter pupillae receives parasympathetic innervation via the ciliary ganglion while the dilator pupillae receives sympathetic innervation via the superior cervical ganglion.

Behind the iris lies the ciliary body which contains smooth muscle fibers. The lens of the eye is attached to the ciliary body by a circular array of fibers called the zonule of Zinn or the suspensory ligament. The lens is formed as a series of cell layers that arise from the cuboidal epithelial cells that cover its anterior surface. The cells of the lens synthesize proteins known as crystalins that are important for maintaining its transparency. Like the cornea, the lens has no blood vessels and depends on the diffusion of nutrients from the aqueous humor for its nourishment. The lens itself is elastic and can change its shape according to the tension placed on it by the zonal fibers. This is an essential part of the mechanism by which the eye can bring different objects into focus on the retina. This process is controlled by the ciliary muscles and is called accommodation.

The function of our eyes is to enable us to see clearly the objects in our surroundings at variable distances and under various conditions of lights. This function is achieved by a very complex arrangement of structures in the eye. Our eye can be thought of as a very advanced camera. There are many similarities between our eye and a camera. Like in a camera the aim is to provide a well-focused image of the object onto the film at the back of the camera. Similarly in the eye, the retina, which is situated towards the back of the eye, works as the photographic film. The image is formed there and then the signal is sent from there to our brain through the optic nerve, and thus we perceive the objects around us. In fact, the retina is much more advanced than a photographic film because it can automatically change its sensitivity depending upon the amount of illumination present.

The rays of light enters the eye and passes through cornea and lens, which converge them so that it gets focused at the retina and a sharp image is formed. This function is akin to that of the lens of a camera. Once again the eye is superior, because the lens of the eye has the property to automatically adjust its power depending upon the location of the object of interest. Therefore, whenever we see from distance to near object, the lens of eye increases its curvature and thus is able to focus the image clearly onto the retina. The aperture (pupil) in the colored part (iris) of the eye is also adjustable according to the illumination of the surroundings. This is akin to the aperture of the diaphragm in a camera.

The retina is the sensory region of the eye. It consists of eight layers. Starting from the vascular choroid layer, the first, most outward, component of the retina is the pigmented ePithelium. The next three layers contain the photoreceptors-the rods and cones and the terminal regions of the photoreceptors where they make synaptic contact with other retinal cells. Above this are two layers that consist of the cell bodies of the bipolar cells, horizontal cells, and amacrine cells and their processes. The final two layers contain the output cells of the retina, the ganglion cells and their axons. Individual photoreceptors consist of an outer segment 'which contains the photosensitive pigment, an inner segment where the cell nucleus is located, and a rod pedicle which is the site at which the photoreceptors make synaptic contact with the bipolar and amacrine cells of the retina.

A highly schematic diagram of the organization of the retina. Note that light passes through the cell layers to reach the photoreceptors which are located next to the pigmented epithelium. Rods and cones are distributed throughout the retina but, in the central region known as the fovea centralis, the retina is very thin and consists of a densely packed layer of cones. In the surrounding region, the parafoveal region, both rods and cones are present in abundance together with the 1 bipolar, amacrine, and horizontal cells connected to the cones of the fovea. One photoreceptor is connected to one ganglion cell. This is only the case for the central region of the retina; elsewhere the signals from a number of photoreceptors converge on a single ganglion cell. In the extreme periphery, as many as 100 rods are connected to a single ganglion cell. The region where the ganglion cell axons pass out of the eye to form the optic nerve (the papilla or optic disc) is devoid If photoreceptors.

Basic Defination(s):

• Aqueous humor: Clear fluid in the eye that both provides nutrients and determines intraocular pressure.
• Ciliary body: Part of the eye that contains a focusing muscle and connective tissue.
• Choroid: Layer of blood vessels and pigments (usually brown or blue) that lies beneath the sclera (the white of the eye). Often mistakenly called the iris.
• Conjunctiva: Mucous membrane that lines the visible part of the eye and the inner surface of the eyelids.
• Cornea: The transparent front window of the eye. The cornea transmits and focuses light into the eye.
• Iris: The colored part of the eye. The iris helps regulate the amount of light that enters the eye.
• Fovea: Adepression in the retina that contains only cones (not rods), and that provides acute eyesight.
• Lens: The transparent structure behind the iris that focuses light rays onto the retina.
• Macula: A small specialized area in the retina that contains special light sensitive cells. The macula allows us to see fine details clearly.
• Optic Nerve: the nerve that connects the eye to the brain. It carries the impulses formed by the retina to the brain, where it is interpreted as images.
• Pupil: The dark center in the middle of the iris. The pupil changes size in response to various degrees of illumination to control the amount of light that is let into the eye.
• Retina: The nerve layer that lines the back of the eye. The retina senses light and creates impulses that are sent through the optic nerve to the brain.
• Suspensory ligament: Part of the eye that holds the lens in place.
• Vitreous: the clear, jelly-like substance that fills the middle of the eye.

Source(s): All above information & images are based on information collected health.indiamart.com, from chapter on eyes from the book Human Physiology by Gillian Pocock and Christophor D. Richards, an article written by Liz Segrè, allaboutvision 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.

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