> Anatomy > Basic Structure
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.
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.
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
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
humor: Clear fluid in the eye that both provides nutrients
and determines intraocular pressure.
body: Part of the eye that contains a focusing muscle and
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.
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.
The colored part of the eye. The iris helps regulate the amount
of light that enters the eye.
Adepression in the retina that contains only cones (not rods),
and that provides acute eyesight.
The transparent structure behind the iris that focuses light rays
onto the retina.
A small specialized area in the retina that contains special light
sensitive cells. The macula allows us to see fine details clearly.
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
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.
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.
ligament: Part of the eye that holds
the lens in place.
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
Segrč, allaboutvision and from various sources. All rights reserved
by respective owners.
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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.
Did you know ?
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.