Nervous System—The Eyes
Lecture 11
Today we’re going to examine one of the most extraordinary of
the special senses, vision and the human eye. Now the human eye
has a great range of capabilities. It may not be able to see as much
in the dark as an owl can, but it’s a very adaptable and another
extraordinary structure.
W
e begin our study of the eye by examining the anatomy of the
eyeball—its various coverings and their functions (the cornea,
sclera, uvea, and retina), and the photoreceptors of the retina
(“rods” and “cones”) that allow us to perceive different shades and colors of
light. We also review the structure and functions of other components of the
eye (the lens, eyelids, lacrimal glands and ducts, and extrinsic eye muscles).
Next, we consider how the eye perceives light and how the brain converts
those perceptions into meaningful information. Finally, the lecture reviews
the most common eye disorders and their treatments.
Human eyes are not the most advanced in nature, but binocular vision gives
them the ability to judge distances accurately. Loss of vision in one eye
destroys this depth perception.
Lecture 11: Nervous System—The Eyes
The anatomy of the globe (eyeball)
The eyeball has several coverings. The conjunctiva is a thin, transparent
layer that covers the eyeball. It is continuous with the eyelids.
The cornea is the outermost brous layer. It covers the iris and is transparent
and nonvascular. It can help to focus light because it is curved. It is the most
frequently transplanted tissue, because it is readily available from cadavers,
and avascularity prevents circulating antibodies from attacking the transplant.
It contains only pain bers; thus, any contact with the cornea is interpreted
as pain.
60
, The sclera (meaning
“hard”) is the white part of
the eye. It is a rm covering
that maintains the shape of
the globe. It provides some
protection for the inner
© Comstock/Thinkstock.
structures of the eye.
The uvea is the middle
(vascular) layer. It provides
most of the vascularity to The sclera is the white of the eye, and the iris is
the eye’s coverings. The iris the colored portion.
is the colored portion of the
uvea. It contains circular muscles (parasympathetic) that constrict the pupil
and radial muscles (sympathetic) that open the pupil. Legal and illegal drugs
can mimic the parasympathetic system and close the iris.
The ciliary body consists of the ciliary muscle, which changes the shape of
the lens, and the ciliary processes, which secrete the uid that lls the eye.
The choroid provides the vascular supply to the retina. The retina is the inner
(nervous) layer at the back of the eye.
The visual surface of the retina is where the perception of
light begins.
The optic disc contains the blind spot, the egress of the central
retinal artery and central retinal vein, and the egress of the optic
nerve (CN II).
The retina contains 120 million rods—that is, rod-shaped photoreceptors.
They allow one to see shades of gray in dim light and to perceive movement
and shapes. Rods increase in number as one moves peripherally (to the
outside, or temporal side, of the eye). Therefore, the best vision in darkness
is peripheral, a survival advantage in avoiding predators.
61
Lecture 11
Today we’re going to examine one of the most extraordinary of
the special senses, vision and the human eye. Now the human eye
has a great range of capabilities. It may not be able to see as much
in the dark as an owl can, but it’s a very adaptable and another
extraordinary structure.
W
e begin our study of the eye by examining the anatomy of the
eyeball—its various coverings and their functions (the cornea,
sclera, uvea, and retina), and the photoreceptors of the retina
(“rods” and “cones”) that allow us to perceive different shades and colors of
light. We also review the structure and functions of other components of the
eye (the lens, eyelids, lacrimal glands and ducts, and extrinsic eye muscles).
Next, we consider how the eye perceives light and how the brain converts
those perceptions into meaningful information. Finally, the lecture reviews
the most common eye disorders and their treatments.
Human eyes are not the most advanced in nature, but binocular vision gives
them the ability to judge distances accurately. Loss of vision in one eye
destroys this depth perception.
Lecture 11: Nervous System—The Eyes
The anatomy of the globe (eyeball)
The eyeball has several coverings. The conjunctiva is a thin, transparent
layer that covers the eyeball. It is continuous with the eyelids.
The cornea is the outermost brous layer. It covers the iris and is transparent
and nonvascular. It can help to focus light because it is curved. It is the most
frequently transplanted tissue, because it is readily available from cadavers,
and avascularity prevents circulating antibodies from attacking the transplant.
It contains only pain bers; thus, any contact with the cornea is interpreted
as pain.
60
, The sclera (meaning
“hard”) is the white part of
the eye. It is a rm covering
that maintains the shape of
the globe. It provides some
protection for the inner
© Comstock/Thinkstock.
structures of the eye.
The uvea is the middle
(vascular) layer. It provides
most of the vascularity to The sclera is the white of the eye, and the iris is
the eye’s coverings. The iris the colored portion.
is the colored portion of the
uvea. It contains circular muscles (parasympathetic) that constrict the pupil
and radial muscles (sympathetic) that open the pupil. Legal and illegal drugs
can mimic the parasympathetic system and close the iris.
The ciliary body consists of the ciliary muscle, which changes the shape of
the lens, and the ciliary processes, which secrete the uid that lls the eye.
The choroid provides the vascular supply to the retina. The retina is the inner
(nervous) layer at the back of the eye.
The visual surface of the retina is where the perception of
light begins.
The optic disc contains the blind spot, the egress of the central
retinal artery and central retinal vein, and the egress of the optic
nerve (CN II).
The retina contains 120 million rods—that is, rod-shaped photoreceptors.
They allow one to see shades of gray in dim light and to perceive movement
and shapes. Rods increase in number as one moves peripherally (to the
outside, or temporal side, of the eye). Therefore, the best vision in darkness
is peripheral, a survival advantage in avoiding predators.
61
