The Eye
The eye is the body’s organ of sight. It gathers light from the environment and forms an image on nerve cells of the retina. The
image is then transformed into nerve impulses, which are interpreted by the brain, mainly in the occipital lobe of the cerebrum.
The eye is a fluid-filled movable sphere serviced by the anterior end of the optic nerve, a cranial nerve extending from and to the
brain. Visual images initiated by receptors in the retina of the eye travel via optic nerve, with medial fibers crossing at the optic
chiasma. Then they pass through the optic track, finally reaching the visual cortex in the occipital lobe of the cerebrum, where they
are interpreted.
Anatomically, the “eye” is synonymous with the eyeball. The eyeball is a bit longer than wider, with the anterior portion extending
out from the sphere. The wall of the eye is composed of three layers: an outer, tough fibrous coat consisting of the cornea and
sclera; a middle, highly vascular coat containing the choroid layer, iris, ciliary bodies, lens and pupil; and an inner coat, the retina,
which contains the photoreceptors (cones and rodes), bipolar cells, ganglion cells, optic nerve, optic disc or blind spot.
The eye contains two fluid-filled cavities: the anterior and the posterior cavities. One region of the anterior cavity, the anterior
chamber, lies between the iris and the cornea. Another region, the posterior chamber, is between the iris and the lens. Both
chambers are filled with a watery fluid called aqueous humor. The posterior cavity, which lies between the lens and the retina,
contains a jellylike substance called vitreous humor.
The pupil of the eye is an opening in the iris of the eye. Two layers of smooth muscle compose the iris: sphincter muscles, which
constrict the pupil and make it smaller, and a dilator muscle layer, which makes the pupil larger. The iris contains pigments that give
color to the eye. The “white” of the eye is the visible portion of the sclera of the eye.
Behind the iris is the lens, a transparent, biconvex disk of fibrous protein material in concentric layers. The lens is attached to the
ciliary body by the suspensory ligament. Most of the ciliary body, which changes the shape of the lens for focusing, consists of the
ciliary muscle.
The innermost layer of the eyeball is the retina. The retina extends anteriorally as far as the posterior aspect of the ciliary body. Two
layers comprise the retina: an outer pigmented layer with melanin, and an inner layer of nerve tissue, the retina proper. The inner,
nerve layer of the retina consist of three layers of neurons. Closest to the choroid is a layer of receptor neurons (rods and cone
cells). Second is a layer of bipolar neurons, the nerve cells that receive impulses initiate by the rod and cone cells. Third is an inner
layer of ganglionic neurons attached directly to the optic nerve.
The accessory structures of the eye include the eyebrows, eyelids, eyelashes, conjunctiva, and lacrimal apparatus (lacrimal glands
that produce tears to keep the eyeball moist).
Physiology of Vision:
The sense of sight relies most upon the rods and the cones cells. The rods permit vision when there is dim light. They form outlines
of objects and are primarily concerned with twilight vision. The cones are most accurate where there is sufficient light to permit
close, detailed vision. These receptors are most concerned with daylight vision. The cones enable one to see detail and are
responsible for color vision. They are found most concentrated in the fovea centralis, while rods are most concentrated at the outer
edge of the retina.
Both rods and cones detect movement in the environment. They use a visual pigment called rhodospin. The protein opsin, portion
of rhodospin, varies between the rodes and the different types of cones, allowing differential sensitivity to different colors and
intensities of light.
The nerve impulses are transported by the optic nerve and optic tract to the visual cortex of the brain, where they are interpreted.
Because the optic nerve exits at the optic disk, there are no visual receptors, and therefore, this portion of the retina is called the
blind spot. The image reaching the retina is inverted due to the optical properties of the lens of the eye, but the image is perceived
as upright by the cortex of the occipital lobe of the cerebrum.
The pathway of light to the eye begins at the clear cornea, then passes through the pupil, which changes in size depending on the
intensity of the light and the distance of the object being viewed. Light rays then pass through the aqueous humor to the lens, the
principal structure of light focusing. The lens is elastic and focuses light rays on the retina. This process of light focusing is called
accommodation. When the object is distant, the lens becomes flattened; when the object is near, the lens becomes more rounded
or convex.
The lens, cornea, aqueous humor, and vitreous humor are all refracting media (media that focus light rays and cause them to
converge on the fovea centralis to the retina where the image is formed). The area served by an eye is called the external visual
field. The external visual field of an eye usually overlaps that of the other eye, this overlap is responsible for the perception of a
three dimensional image.