The human eye is an organ which reacts to light
for several purposes.
The eye is not properly a sphere, rather it is a
fused two-piece unit. The smaller, less curved
unit called the cornea, is linked to the larger
unit called the sclera.
The cornea and sclera are connected by a ring
called the limbus.
The iris and its black center, the pupil, are
seen instead of the cornea due to the cornea's
transparency.
Dimensions differ among adults by only one or
two millimeters. The vertical measure, generally
less than the horizontal distance, is about 24
mm among adults, at birth about 16-17 mm. The
eyeball grows rapidly, increasing to 22.5-23 mm
(approx. 0.89 in) by the age of three years.
From then to age 13, the eye attains its full
size. The volume is 6.5 ml (0.4 cu. in.) and the
weight is 7.5 g. (0.25 oz.)
Components
The eye is made up of three coats, enclosing
three transparent structures. The outermost
layer is composed of the cornea and sclera. The
middle layer consists of the choroid, ciliary
body, and iris. The innermost is the retina,
which gets its circulation from the vessels of
the choroid as well as the retinal vessels,
which can be seen in an opthalmoscope.
Within these coats are the aqueous humor, the
vitreous body, and the flexible lens.
The aqueous humor is a clear fluid that is
contained in two areas: the anterior chamber
between the cornea and the iris and exposed area
of the lens; and the posterior chamber, behind
the iris and the rest. The lens is suspended to
the ciliary body by the suspensory ligament (Zonule
of Zinn), made up of fine transparent fibers.
The vitreous body is a clear jelly that is much
larger than the aqueous humor, and is bordered
by the sclera, zonule, and lens.
There are six Orbital
Muscles
Four of the muscles control the movement of the eye
going up and down and side to side.
Two of the muscles control the eye to move while the
head moves. For instance look in the mirror sometime,
you can see your eyes looking right at it.
Then stare at the mirror while moving your head side to
side. You can see your eyes move but you still are
looking at the mirror. That is what the other two
muscles do.
Muscle
Innervation
Primary function
Secondary function
Tertiary function
Levator
palpebrae superioris
Oculomotor
nerve
Elevation of
the superior (upper) eyelid
.
.
Superior rectus
Oculomotor
nerve
Elevation
Intorsion
Adduction
Inferior rectus
Oculomotor
nerve
Depression
Extorsion
Adduction
Lateral rectus
Abducens nerve
Abduction
.
.
Medial rectus
Oculomotor
nerve
Adduction
.
.
Superior
oblique
Trochlear nerve
Intorsion
Depression
Abduction
Inferior
oblique
Oculomotor
nerve
Extorsion
Elevation
Abduction
This faithful reproduction
of a lithograph plate fromGray's Anatomy,
a two-dimensional work of art, is not copyrightable in the U.S.
as per Bridgeman Art Library v. Corel
Corp.; the same is
also true in many other countries, including Germany. Unless
stated otherwise, it is from the 20th U.S. edition of
Gray's Anatomy of the Human Body, originally published in
1918 and therefore lapsed into the public domain. Other copies
of Gray's Anatomy can be found on
Bartleby and also on
Yahoo
