Download EYE2

VISION
Vision, the act of seeing, is extremely important to human survival. More than half
the sensory receptors in the human body are located in the eyes, and a large part of the
cerebral cortex is devoted to processing visual information.
Electromagnetic Radiation
Electromagnetic radiation, is energy in the form of waves that radiates from the sun.
There are many types of electromagnetic radiation, including gamma rays, x-rays,
UV rays, visible light, infrared radiation, microwaves, and radio waves. This range of
electromagnetic radiation is known as the electromagnetic spectrum.
The distance between two consecutive peaks of an electromagnetic wave is the
wavelength. Wavelengths range from short to long; for example, gamma rays have
wavelengths smaller than a nanometer, and most radio waves have wavelengths
greater than a meter.
The eyes are responsible for the detection of visible light, the part of the
electromagnetic spectrum with wavelengths ranging from about 400 to 700 nm.
Visible light exhibits colors: The color of visible light depends on its wavelength. For
example, light that has a wavelength of 400 nm is violet, and light that has a
wavelength of 700 nm is red. An object can absorb certain wavelengths of visible
light and reflect others; the object will appear the color of the wavelength that is
reflected. For example, a green apple appears green because it reflects mostly green
light and absorbs most other wavelengths of visible light. An object appears white
because it reflects all wavelengths of visible light. An object appears black because it
absorbs all wavelengths of visible light
.
Accessory Structures of the Eye
The accessory structures of the eye include the eyelids, eyelashes, eyebrows, the
lacrimal (tear-producing) apparatus, and extrinsic eye muscles.
Eyelids
The upper and lower eyelids, or palpebrae, shade the eyes during sleep, protect the
eyes from excessive light and foreign objects, and spread lubricating secretions
over the eyeballs. The upper eyelid is more movable than the lower and contains in its
superior region the levator palpebrae superioris muscle.
Sometimes a person may experience an annoying twitch in an eyelid, an involuntary
quivering similar to muscle twitches in the hand, forearm, leg, or foot. Twitches are
almost always harmless and usually last for only a few seconds. They are often
associated with stress and fatigue.
The space between the upper and lower eyelids that exposes the eyeball is the
palpebral fissure. Its angles are known as the lateral commissure, which is narrower
and closer to the temporal bone, and the medial commissure, which is broader and
nearer the nasal bone. In the medial commissure is a small, reddish elevation, the
lacrimal caruncle, which contains sebaceous (oil) glands and sudoriferous (sweat)
glands. The whitish material that sometimes collects in the medial commissure comes
from these glands. From superficial to deep, each eyelid consists of epidermis, dermis,
subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands,
and conjunctiva.
The tarsal plate is a thick fold of connective tissue that gives form and support to the
eyelids. Embedded in each tarsal plate is a row of elongated modified sebaceous
glands, known as tarsal or Meibomian glands, that secrete a fluid that helps
keep the eyelids from adhering to each other. Infection of the starsal glands produces
a tumor or cyst on the eyelid called a chalazion.
The conjunctiva is a thin, protective mucous membrane composed of nonkeratinized
stratified squamous epithelium with numerous goblet cells that is supported by areolar
connective tissue.
The palpebral conjunctiva lines the inner aspect of the eyelids, and the bulbar
conjunctiva passes from the eyelids onto the surface of the eyeball, where it covers
the sclera (the “white” of the eye) but not the cornea, which is a transparent region
that forms the outer anterior surface of the eyeball. Over the sclera, the conjunctiva
is vascular. Dilation and congestion of the blood vessels of the bulbar conjunctiva due
to local irritation or infection are the cause of bloodshot eyes.
Eyelashes and Eyebrows
The eyelashes, which project from the border of each eyelid, and the eyebrows,
which arch transversely above the upper eyelids, help protect the eyeballs from
foreign objects, perspiration, and the direct rays of the sun. Sebaceous glands at the
base of the hair follicles of the eyelashes, called sebaceous ciliary glands, release
a lubricating fluid into the follicles. Infection of these glands, usually by bacteria,
causes a painful, pus-filled swelling called a sty.
The Lacrimal Apparatus
The lacrimal apparatus is a group of structures that produces and drains lacrimal
fluid or tears. The lacrimal glands, each about the size and shape of an almond,
secrete lacrimal fluid, which drains into 6–12 excretory lacrimal ducts that empty
tears onto the surface of the conjunctiva of the upper lid. From here the tears pass
medially over the anterior surface of the eyeball to enter two small openings called
lacrimal puncta (singular is punctum). Tears then pass into two ducts, the lacrimal
canals, which lead into the lacrimal sac and then into the nasolacrimal duct. This
duct carries the lacrimal fluid into the nasal cavity just inferior to the inferior nasal
concha.
An infection of the lacrimal sacs is called dacryocystitis. It is usually caused by a
bacterial infection and results in blockage of the nasolacrimal ducts. The lacrimal
glands are supplied by parasympathetic fibers of the facial (VII) nerves.
The lacrimal fluid produced by these glands is a watery solution containing salts,
some mucus, and lysozyme, a protective bactericidal enzyme. The fluid protects,
cleans, lubricates, and moistens the eyeball. After being secreted from the lacrimal
gland, lacrimal fluid is spread medially over the surface of the eyeball by the blinking
of the eyelids. Each gland produces about 1 mL of lacrimal fluid per day.
Normally, tears are cleared away as fast as they are produced, either by evaporation or
by passing into the lacrimal canals and then into the nasal cavity. If an irritating
substance makes contact with the conjunctiva, however, the lacrimal glands are
stimulated response to parasympathetic stimulation, the lacrimal glands produce
excessive lacrimal fluid that may spill over the edges of the eyelids and even fill the
nasal cavity with fluid. This is how crying produces a runny nose.
Extrinsic Eye Muscles
The eyes sit in the bony depressions of the skull called the orbits. The orbits help
protect the eyes, stabilize them in three-dimensional space, and anchor them to the
muscles that produce their essential movements. The extrinsic eye muscles extend
from the walls of the bony orbit to the sclera (white) of the eye and are surrounded
in the orbit by a significant quantity of periorbital fat. These muscles are capable of
moving the eye in almost any direction. Six extrinsic eye muscles move each eye: the
superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique,
and inferior oblique. They are supplied by cranial nerves III, IV, or VI.
In general, the motor units in these muscles are small. Some motor neurons serve
only two or three muscle fibers—fewer than in any other part of the body except the
larynx (voice box). Such small motor units permit smooth, precise, and rapid
movement of the eyes.
As indicated in sExhibit 11.B, the extrinsic eye muscles move the eyeball
laterally, medially, superiorly, and inferiorly. For example, looking
to the right requires simultaneous contraction of the right lateral
rectus and left medial rectus muscles of the eyeball and relaxation
of the left lateral rectus and right medial rectus of the
eyeball. The oblique muscles preserve rotational stability of the
eyeball. Neural circuits in the brain stem and cerebellum coordinate
and synchronize the movements of the eyes.
Anatomy of the Eyeball
The adult eyeball measures about 2.5 cm (1 in.) in diameter. Of
its total surface area, only the anterior one-sixth is exposed; the
remainder is recessed and protected by the orbit, into which it
fits. Anatomically, the wall of the eyeball consists of three layers:
(1) fibrous tunic, (2) vascular tunic, and (3) retina.
Fibrous Tunic
The fibrous tunic (TOO-nik) is the superficial layer of the eyeball
and consists of the anterior cornea and posterior sclera (Figure
17.7). The cornea (KOR-ne¯-a) is a transparent coat that covers
the colored iris. Because it is curved, the cornea helps focus
light onto the retina. Its outer surface consists of nonkeratinized
stratified squamous epithelium. The middle coat of the cornea
consists of collagen fibers and fibroblasts, and the inner surface is
simple squamous epithelium. Since the central part of the cornea
receives oxygen from the outside air, contact lenses that are worn
for long periods of time must be permeable to permit oxygen to
pass through them. The sclera (SKLE-ra; scler- _ hard), the
“white” of the eye, is a layer of dense connective tissue made up
mostly of collagen fibers and fibroblasts. The sclera covers the entire
eyeball except the cornea; it gives shape to the eyeball, makes
it more rigid, protects its inner parts, and serves as a site of attachment
for the extrinsic eye muscles. At the junction of the sclera
and cornea is an opening known as the scleral venous sinus
(canal of Schlemm). A fluid called aqueous humor, which will be
described later, drains into this sinus (Figure 17.7).
Vascular Tunic
The vascular tunic or uvea (U¯ -ve-a) is the middle layer of the
eyeball. It is composed of three parts: choroid, ciliary body, and
iris (Figure 17.7). The highly vascularized choroid (KO¯ -royd),
which is the posterior portion of the vascular tunic, lines most of
the internal surface of the sclera. Its numerous blood vessels provide
nutrients to the posterior surface of the retina. The choroid
also contains melanocytes that produce the pigment melanin,
which causes this layer to appear dark brown in color. Melanin in
the choroid absorbs stray light rays, which prevents reflection and
scattering of light within the eyeball. As a result, the image cast
on the retina by the cornea and lens remains sharp and clear. Albinos
lack melanin in all parts of the body, including the eye.
They often need to wear sunglasses, even indoors, because even
moderately bright light is perceived as bright glare due to light
scattering.
In the anterior portion of the vascular tunic, the choroid becomes
the ciliary body (SIL-e¯-ar_-e¯). It extends from the ora serrata
(O¯ -ra ser-RA¯ -ta), the jagged anterior margin of the retina, to
a point just posterior to the junction of the sclera and cornea. Like
the choroid, the ciliary body appears dark brown in color because
it contains melanin-producing melanocytes. In addition, the ciliary body consists of ciliary processes and ciliary muscle. The ciliary
processes are protrusions or folds on the internal surface of
the ciliary body. They contain blood capillaries that secrete aqueous
humor. Extending from the ciliary process are zonular fibers
(suspensory ligaments) that attach to the lens. The fibers consist
of thin, hollow fibrils that resemble elastic connective tissue
fibers. The ciliary muscle is a circular band of smooth muscle.
Contraction or relaxation of the ciliary muscle changes the tightness
of the zonular fibers, which alters the shape of the lens,
adapting it for near or far vision.
The iris (_ rainbow), the colored portion of the eyeball, is
shaped like a flattened donut. It is suspended between the
cornea and the lens and is attached at its outer margin to the ciliary
processes. It consists of melanocytes and circular and radial
smooth muscle fibers. The amount of melanin in the iris
determines the eye color. The eyes appear brown to black when
the iris contains a large amount of melanin, blue when its melanin
concentration is very low, and green when its melanin concentration
is moderate.
A principal function of the iris is to regulate the amount of
light entering the eyeball through the pupil (pupil _ little person;
because this is where you see a reflection of yourself when looking
into someone’s eyes), the hole in the center of the iris. The
pupil appears black because, as you look through the lens, you see
the heavily pigmented back of the eye (choroid and retina). However,
if bright light is directed into the pupil, the reflected light is
red because of the blood vessels on the surface of the retina. It is
for this reason that a person’s eyes appear red in a photograph
(“red eye”) when the flash is directed into the pupil. Autonomic
reflexes regulate pupil diameter in response to light levels (Figure
17.8). When bright light stimulates the eye, parasympathetic
fibers of the oculomotor (III) nerve stimulate the circular muscles
or sphincter pupillae (pu-PIL-e¯) of the iris to contract, causing
a decrease in the size of the pupil (constriction). In dim light,
sympathetic neurons stimulate the radial muscles or dilator
pupillae of the iris to contract, causing an increase in the pupil’s
size (dilation).
Retina
The third and inner layer of the eyeball, the retina, lines the
posterior three-quarters of the eyeball and is the beginning of
the visual pathway (see Figure 17.7). This layer’s anatomy
can be viewed with an ophthalmoscope (of-THAL-mo¯-sko¯p;
ophthalmos- _ eye; -skopeo _ to examine), an instrument that
shines light into the eye and allows an observer to peer through
the pupil, providing a magnified image of the retina and its blood
vessels as well as the optic (II) nerve (Figure 17.9). The surface of
the retina is the only place in the body where blood vessels can be
viewed directly and examined for pathological changes, such as
those that occur with hypertension, diabetes mellitus, cataracts,
and age-related macular disease. Several landmarks are visible
through an ophthalmoscope. The optic disc is the site where the
optic (II) nerve exits the eyeball. Bundled together with the optic
nerve are the central retinal artery, a branch of the ophthalmic
artery, and the central retinal vein (see Figure 17.7). Branches of
the central retinal artery fan out to nourish the anterior surface of
the retina; the central retinal vein drains blood from the retina
through the optic disc. Also visible are the macula lutea and fovea
centralis, which are described shortly.
The retina consists of a pigmented layer and a neural layer.
The pigmented layer is a sheet of melanin-containing epithelial
cells located between the choroid and the neural part of the
retina. The melanin in the pigmented layer of the retina, as in
the choroid, also helps to absorb stray light rays. The neural
(sensory) layer of the retina is a multilayered outgrowth of the
brain that processes visual data extensively before sending
nerve impulses into axons that form the optic nerve. Three distinct
layers of retinal neurons—the photoreceptor layer, the
bipolar cell layer, and the ganglion cell layer—are separated
by two zones, the outer and inner synaptic layers, where synaptic
contacts are made (Figure 17.10). Note that light passes
through the ganglion and bipolar cell layers and both synaptic
layers before it reaches the photoreceptor layer. Two other
types of cells present in the bipolar cell layer of the retina are
called horizontal cells and amacrine cells (AM-a-krin). These
cells form laterally directed neural circuits that modify the signals
being transmitted along the pathway from photoreceptors
to bipolar cells to ganglion cells.
Photoreceptors are specialized cells that begin the process by
which light rays are ultimately converted to nerve impulses. There
are two types of photoreceptors: rods and cones. Each retina has
about 6 million cones and 120 million rods. Rods allow us to see
in dim light, such as moonlight. Because rods do not provide color
vision, in dim light we can see only black, white, and all shades of
gray in between. Brighter lights stimulate cones, which produce
color vision. Three types of cones are present in the retina: (1) blue
cones, which are sensitive to blue light, (2) green cones, which are
sensitive to green light, and (3) red cones, which are sensitive to
red light. Color vision results from the stimulation of various combinations
of these three types of cones. Most of our experiences
are mediated by the cone system, the loss of which produces legal
blindness. A person who loses rod vision mainly has difficulty seeing
in dim light and thus should not drive at night.
From photoreceptors, information flows through the outer
synaptic layer to bipolar cells and then from bipolar cells through
the inner synaptic layer to ganglion cells. The axons of ganglion
cells extend posteriorly to the optic disc and exit the eyeball as the
optic (II) nerve. The optic disc is also called the blind spot. Because
it contains no rods or cones, we cannot see images that
strike the blind spot. Normally, you are not aware of having a
blind spot, but you can easily demonstrate its presence. Hold this
page about 20 in. from your face with the cross shown at the end
of this paragraph directly in front of your right eye. You should be
able to see the cross and the square when you close your left eye.
Now, keeping the left eye closed, slowly bring the page closer to
your face while keeping the right eye on the cross. At a certain
distance the square will disappear from your field of vision because
its image falls on the blind spot.
__
The macula lutea (MAK-u¯-la LOO-te¯-a; macula _ a small,
flat spot; lute- _ yellowish) is in the exact center of the posterior
portion of the retina, at the visual axis of the eye (see Figure 17.9).
The fovea centralis (FO¯ -ve¯-a) (see Figures 17.7 and 17.9), a
small depression in the center of the macula lutea, contains only
cones. In addition, the layers of bipolar and ganglion cells, which
scatter light to some extent, do not cover the cones here; these layers
are displaced to the periphery of the fovea centralis. As a result,
the fovea centralis is the area of highest visual acuity (a-KU-i-te¯)
or resolution (sharpness of vision). A main reason that you move
your head and eyes while looking at something is to place images
of interest on your fovea centralis—as you do to read the words in
this sentence! Rods are absent from the fovea centralis and are
more plentiful toward the periphery of the retina. Because rod vision
is more sensitive than cone vision, you can see a faint object
(such as a dim star) better if you gaze slightly to one side rather
than looking directly at it.
Lens
Behind the pupil and iris, within the cavity of the eyeball, is the
lens (see Figure 17.7). Within the cells of the lens, proteins called
crystallins (KRIS-ta-lins), arranged like the layers of an onion,
make up the refractive media of the lens, which normally is perfectly
transparent and lacks blood vessels. It is enclosed by a clear
connective tissue capsule and held in position by encircling zonular
fibers, which attach to the ciliary processes. The lens helps focus
images on the retina to facilitate clear vision.
Interior of the Eyeball
The lens divides the interior of the eyeball into two cavities: the
anterior cavity and vitreous chamber. The anterior cavity—the
space anterior to the lens—consists of two chambers. The anterior
chamber lies between the cornea and the iris. The posterior
chamber lies behind the iris and in front of the zonular fibers and
lens (Figure 17.11). Both chambers of the anterior cavity are
filled with aqueous humor (AK-we¯-us HU¯ -mer; aqua_water), a
transparent watery fluid that nourishes the lens and cornea. Aqueous
humor continually filters out of blood capillaries in the ciliary
processes of the ciliary body and enters the posterior chamber. It
then flows forward between the iris and the lens, through the
pupil, and into the anterior chamber. From the anterior chamber,
aqueous humor drains into the scleral venous sinus (canal of
Schlemm) and then into the blood. Normally, aqueous humor is
completely replaced about every 90 minutes.
The larger posterior cavity of the eyeball is the vitreous chamber
(VIT-re¯-us), which lies between the lens and the retina.
Within the vitreous chamber is the vitreous body, a transparent
jellylike substance that holds the retina flush against the choroid,
giving the retina an even surface for the reception of clear images.
It occupies about four-fifths of the eyeball. Unlike the aqueous
humor, the vitreous body does not undergo constant replacement.
It is formed during embryonic life and consists of mostly water
plus collagen fibers and hyaluronic acid. The vitreous body also
contains phagocytic cells that remove debris, keeping this part of
the eye clear for unobstructed vision. Occasionally, collections of
debris may cast a shadow on the retina and create the appearance
of specks that dart in and out of the field of vision. These vitreal
floaters, which are more common in older individuals, are usually
harmless and do not require treatment. The hyaloid canal (HI¯-aloyd)
is a narrow channel that is inconspicuous in adults and runs
through the vitreous body from the optic disc to the posterior aspect
of the lens. In the fetus, it is occupied by the hyaloid artery
(see Figure 17.27d).
The pressure in the eye, called intraocular pressure, is produced
mainly by the aqueous humor and partly by the vitreous
body; normally it is about 16 mmHg (millimeters of mercury).
The intraocular pressure maintains the shape of the eyeball and
prevents it from collapsing. Puncture wounds to the eyeball may
cause the loss of aqueous humor and the vitreous body. This in
turn causes a decrease in intraocular pressure, a detached retina,
and in some cases blindness.
Table 17.1 summarizes the structures associated with the
eyeball.