The eye

The eyeball can be viewed as the fusing together of a small
portion of a small, strongly curved sphere with a large portion
of a large, not so strongly curved sphere. The small
piece, occupying about one-sixth of the whole, has a radius
of 8 mm (0.3 inch); it is transparent and is called the cornea;
the remainder, called the scleral segment, is opaque
and has a radius of 12 mm (0.5 inch). The ring where the
two areas join is called the limbus. Thus, on looking
directly into the eye from in front one sees the white sclera
surrounding the cornea; because the latter is transparent
one sees, instead of the cornea, a ring of tissue lying within the eye, the iris.
The iris is the structure that determines
the colour of the eye.
The centre of this ring is
called the pupil. It appears dark because the light passing
into the eye is not reflected back to any great extent. By
use of an ophthalmoscope, an instrument that permits
the observer to illuminate the interior of the eyeball while
observing through the pupil, the appearance of the interior
lining of the globe can be made out. The lining, called
the fundus oculi, is characterized by the large blood vessels
that supply blood to the retina; these are especially
distinct as they cross over the pale optic disk, or papilla,
the region where the optic nerve fibres leave the globe.
The dimensions of the eye are reasonably constant,
varying among healthy individuals by only a millimetre or
two; the sagittal (vertical) diameter is about 24 mm (about
1 inch) and is usually less than the transverse (horizontal)
diameter. At birth the sagittal diameter is about 16 to 17
mm (about 0.65 inch); it increases rapidly to about 22.5 to
23 mm (about 0.89 inch) by age 3; between ages 3 and 13 the
globe attains its full size. The weight is about 7.5 grams
(0.25 ounce), and its volume 6.5 mm (0.4 cubic inch).
The eye is made up of three coats, which enclose the
optically clear aqueous humour, lens, and vitreous body.
The outermost coat consists of the cornea and the sclera;
the middle coat, or uvea, contains the main blood supply
to the eye and consists, from the back forward, of the choroid,
the ciliary body, and the iris. The innermost layer is
the retina, lying on the choroid and receiving most of its
nourishment from the vessels within the choroid, the
remainder of its nourishment being derived from the retinal
vessels that lie on its surface and are visible in the
ophthalmoscope. The ciliary body and iris have a very thin
covering, the ciliary epithelium and posterior epithelium
of the iris, which is continuous with the retina.
Within the cavities formed by this triple-layered coat
there are the crystalline lens, suspended by fine transparent
fibres—the suspensory ligament or zonule of
Zinn—from the ciliary body; the aqueous humour, a clear
fluid filling the spaces between the cornea and the lens
and iris; and the vitreous body, a clear jelly filling the much
larger cavity enclosed by the sclera, the ciliary body, and
the lens. The anterior chamber of the eye is defined as the
space between the cornea and the forward surfaces of
the iris and lens, while the posterior chamber is the much
smaller space between the rear surface of the iris and the
ciliary body, zonule, and lens; the two chambers both contain
aqueous humour and are in connection through the
pupil. This connection allows the aqueous humour to flow
through the pupil from the posterior chamber to the anterior
chamber. From there, the fluid flows out of the eye
through the trabecular meshwork and Schlemm’s canal,
which encircles the peripheral iris. Some aqueous humour
also exits the eye directly through the ciliary body. The
ciliary muscle attachments and the lens separate the aqueous
humour in front from the vitreous humour behind.
The eye also contains special light receptors, called
photoreceptors, and its construction is much like that of a
simple camera. The retina, an extremely metabolically
active layer of nerve tissue, is made up of millions of photoreceptors.
Various structures of the eye function to
focus light onto the retina, with the cornea providing the
greatest focusing power of the eye. The cornea is curved
to a much greater extent than the rest of the eyeball.
However, the shape and focusing power of the cornea are
not adjustable. This is in contrast to the lens, the shape of
which is adjustable and is controlled by the action of the
ciliary body, altering the focusing power of the lens as
needed. The cornea and lens focus an image onto the retina
at the back of the eye. If the image is projected too far
in front of the retina, it causes the visual defect called
myopia, or nearsightedness. If the image is theoretically
focused “behind” the retina, the result is hyperopia, or farsightedness.
If no deformation of the lens is present,
the image is projected onto the fovea, a structure near the
centre of the retina that contains a large number of cone
photoreceptors and that provides the sharpest vision.
When stimulated by light, retinal photoreceptor cells
send signals to neighbouring cells in the retina that then
relay the signals through the optic nerve to the visual centres
of the brain.