Chapter 17 Module 3 The Structures and Functions of the Eye

This is Chapter 17,
Module 3– Structures and Functions of the Eyes. The learning objectives
of this module are, one, describe
the structures of the eye and their
functions; and two, explain the mechanism of vision. There are several accessory
structures of the eye. The palpebra are the eyelids. They act like windshield wipers
for the surface of the eye, removing dust and debris. The two eyelids are connected
medially and laterally by the medial and
lateral canthus. The eyelashes also
protect the surface of the eye from foreign matter. Tarsal glands are along the
inner margin of the lid. These are sebaceous
glands that secrete a lipid-rich product that
keeps the eyelids from sticking together. A cyst can result from an
infection of the tarsal glands. The lacrimal caruncle
is a massive soft tissue at the medial canthus. This contains
glands that produce thick secretions that
appear after sleeping, like sleepy heads. The conjunctiva is a
transparent epithelium that covers the inner
surface of the eyelids and the outer
surface of the eye. The palpebral conjunctiva
covers the inner surface of the eyelid. The ocular conjunctiva
covers the anterior surface of the eye up to the
edges of the cornea. The palpebral conjunctiva
and the ocular conjunctiva are continuous. Conjunctivitis is
the inflammation of the conjunctiva. The eyes get red due to
the dilation of blood vessels deep to the conjunctiva. This can be caused by
pathogens or caused by chemical irritation. The lacrimal apparatus produces,
distributes, and removes tears. The lacrimal gland is the tear
gland that secretes tears. It is located just
outside the orbit and is superior and
lateral to the eyeball. The tears contain
an enzyme called lysozyme that is antibacterial. Blinking sweeps the tears
to the medial canthus, and the tears drain into
two small pores that empty into the lacrimal canaliculi. The lacrimal canaliculi
drain into the lacrimal sac that then drains into
the nasolacrimal duct and into the nasal cavity. If the canaliculi can’t
provide enough draining, tears rush into the nasal
cavity and cause a runny nose. The wall of the eye
contains three layers– the fibrous layer, the vascular
layer, and the neural layer. The fibrous layer is the
outermost layer of the eye. It consists of the
sclera and the cornea. The functions of
the fibrous layer are to, one, support and
protect the eye; two, act as the attachment of
the extrinsic eye muscles; and three, to help
focus the lens. The sclera is the
white of the eye. The sclera is the
site of attachment for the extrinsic eye muscles,
the superior and inferior rectus muscles, the medial
and lateral rectus muscles, and the superior and
inferior oblique muscles. These extrinsic eye muscles
control the movements of the eyeball. They are innervated
by cranial nerves III, which is the ocular
motor; IV, trochlear; and VI, abducens cranial nerves. The sclera is continuous
with the cornea. The cornea is a
transparent epithelium. The cornea has no blood
vessels and must receive oxygen and nutrients from the tears. Also, the cornea has
numerous free nerve endings, so it is the most sensitive
portion of the eye. The vascular layer is a
pigmented middle layer of the eye. It consists of the iris,
ciliary body, and choroid. It contains numerous blood
vessels, lymphatic vessels, as well as the intrinsic
smooth muscles of the eye. The function of
this layer is, one, to provide a route
for blood vessels to supply the tissues
of the eye; two, to regulate the
amount of light that enters the eye; three,
to secrete and absorb aqueous humor that circulates
within the chambers of the eye; and four, to control the shape
of the lens in order to focus. The iris is the pigmented part
of the eye, or the colored part. It contains blood vessels,
pigment cells, and two layers of smooth muscles. When the smooth
muscle contracts, it controls the
diameter of the pupil. The dilator’s
smooth muscles have muscle fibers that radiate away
from the edge of the pupil. When they contract,
the pupil dilates. The constrictor muscle forms
a series of concentric circles around the pupil. When they contract,
the pupil constricts. Both muscle groups
are controlled by the autonomic nervous system. In bright light, the
parasympathetic nervous system is activated and the
pupils constrict. In dim light, the sympathetic
nervous system is activated and the pupils dilate. The ciliary body is
a thickened region that begins deep to the
junction between the cornea and the sclera. The bulk of the ciliary body
consists of the ciliary muscle. The ciliary muscle
is a smooth muscle, the suspensory ligaments
of the lens attached to the ciliary muscle. These ligaments keep the
lens centered on the pupil so that any light
passing through the pupil must also pass through the lens. The inner layer of the
eye is the neural layer, and it contains the
retina and optic nerve. The retina has an
outer layer that contains the photoreceptors. This layer of the eye
consists of two layers– the pigmented part
and the neural part. The pigmented part
absorbs light that passes through the neural part. The neural part of
the retina forms a cup that establishes the posterior
and lateral boundaries of the posterior cavity. The inner layer of the retina
contains the photoreceptors. These are receptor
cells that detect light. The two types of photoreceptors
are rods and cones. Rods do not discriminate
among colors of light. They allow us to see
in dimly lit rooms, at twilight, and
in pale moonlight because they are highly
sensitive to light. Cones give us sharper,
clearer images, but they require intense light. Cones provide us
with color vision. There are three types of
cones– red, blue, and green. Blue cones detect shorter
wavelengths of light from about 400 to
550 nanometers. Green wavelengths detect
mid-range wavelengths of light from 450 to 625 nanometers. And red cones detect
longer wavelengths of light from about 480 to
680 wavelengths. When all three types of
cones are stimulated, a person will see white. So when light from the sun or
a light bulb hits an object, the wavelengths
bounce off the object and stimulate all
three types of cones, making the object appear white. When light from the
sun or a light bulb is absorbed by the
object, no wavelengths bounce back to the cones and
the object appears black. Rods and cones are not equally
distributed in the retina. Rods form a broadband around
the periphery of the retina. As you move towards the
center of the retina, the density of rods decreases. Cones are concentrated in
the macula lutea, which is an area where a visual
image arrives after it passes through the cornea and lens. The center of the macula lutea
is called the fovea centralis. This is the region of
the sharpest vision because it contains the
highest concentration of cones. Rods and cones synapse
with millions of neurons called bipolar cells. Bipolar cells synapse on
neurons called ganglion cells. Axons from an
estimated 1 million ganglion cells converge
on the optic disk. The optic disk is the origin of
the optic nerve, cranial nerve II. It is the point where the
axons from the ganglion cells turn to penetrate
the wall of the eye. The optic disk has
no photoreceptors or any other structures typical
to the rest of the retina. Because there are
no photoreceptors, there are no receptors
to be stimulated when light hits this area. We call this area
the blind spot. You do not notice the blind
spot in your field of vision because involuntary eye
movements keep the visual image moving and allow
your brain to fill in the missing information. There are two cavities of
the eye– the anterior cavity and the posterior cavity. The ciliary body and
the lens together divide the eye into an
anterior and posterior cavity. The anterior cavity is
found between the cornea and the lens. The anterior cavity
can be further divided into the anterior chamber
and the posterior chamber. The anterior chamber is between
the cornea and the iris, and the posterior chamber is
between the iris and the lens. Aqueous humor is a fluid that
circulates within the anterior cavity, passing
from the posterior chamber to the anterior
chamber through the pupil. It also diffuses freely across
the surface of the retina. Aqueous humor provides a
route for nutrients and waste transport, and provides a
fluid cushion for the eye. Aqueous humor creates
intraocular pressure that can be measured in
the anterior chamber. Also, aqueous
humor is constantly produced in the
posterior chamber and then circulates through
the anterior chamber. After it circulates through
the anterior chamber, it will drain through
the scleral venous sinus, which is a passageway
at the base of the iris. From this sinus,
the aqueous humor will be delivered to
the veins in the sclera. The posterior cavity of the
eye contains vitreous body, which is a gelatinous mass. The vitreous body helps
stabilize the shape of the eye. The vitreous body is
formed during development and is never replaced. The lens lies just
posterior to the cornea. It is held in place by
suspensory ligaments that originate in the
ciliary body of the choroid. The lens can change shape
to help focus on images. The lens is transparent. If it loses its transparency,
it is known as a cataract. When light passes through
the cornea, it will bend, and this is called refraction. This helps to focus the
image you are looking at. The greatest amount
of refraction occurs when light passes
through the cornea. Next, the light will
pass through the lens. The lens will also change
shape to direct the light to the fovea centralis
on the retina. Accommodation is the automatic
adjustment of the lens to give us clear vision. When looking from far away
to an object close up, the lens must become rounder to
focus the image on the retina. The lens will become
more around when the ciliary muscles relax as the
suspensory ligaments tighten. When looking at an
object far away, the lens must become flatter. The lens will become
more flat when the ciliary muscles contract
as the suspensory ligaments become loose. The visual pathway describes
the pathway of action potentials from the eye to the brain. The visual pathway begins
at the photoreceptors and ends at the visual
cortex of the cerebrum. The message will
cross two synapses before it heads
towards the brain. The first synapse is
between the photoreceptor and the bipolar cell. The second synapse is
between the bipolar cell and the ganglion cell. The ganglion cells
monitor a specific portion of a field of vision. The axons from
the ganglion cells converge on the optic
disk, and then penetrate the wall of the eye. These axons proceed
toward the diencephalon as the optic nerve,
or cranial nerve II. The visual data from
each optic nerve is shared with the
other optic nerve, meaning half of the fibers
from the left optic nerve crosses over to the
right optic nerve, and half of the fibers
from the right optic nerve crosses over to the
left optic nerve. These newly combined nerves
exit the optic chiasm as the optic tract, and will
extend to the visual cortex in the occipital lobe. This ends chapter 17,
module 3– Structures and Functions of the Eye.

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