> Anatomy > Types of Eyes
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
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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.