BIOD 152 - Module 3: Notes - 3.1: Vision and the Eye
The Special Senses
The five special senses vision, smell, taste, hearing, and equilibrium (balance) are the senses that have
specialized organs containing specialized receptor cells, which carry their impulses by way of specialized
somatic and visceral afferents. The other sense, touch, is a somatic sense that does not have a specialized sense
organ. Instead, touch uses general receptors composed of modified dendrites of sensory neurons. Touch
includes pressure, vibration, pain, heat, and the combined information is carried in general somatic afferents and
general visceral afferents.
Vision and the Eye
The organ responsible for sight is the eye, and it consists of three layers. The outer fibrous layer includes the
sclera and cornea. The posterior portion is known as the sclera (Figure 3.1), also known as the “white of
the eye. The anterior portion is known as the cornea (Figure 3.1). The cornea is the transparent part of the
eye where light enters. Aqueous humor fills the spaces between the cornea and the lens.
Figure 3.1 Lateral view of the cornea and sclera of the eye. (Both are highlighted in blue.)
The middle layer of the eye includes choroid, the ciliary muscle, and the iris. The darkly-colored
posterior choroid (Figure 3.2) prevents light from dispersing throughout the eye. The choroid is highly
vascular and supplies blood to the other layers of the eye. The anterior ciliary body (Figure 3.2) changes the
shape of the lens, allowing it to focus. The iris (Figure 3.2) is anterior to the ciliary body and contains the
,circular colored portion of the eye. The iris controls the amount of light let into the pupil, a hole in the center of
the iris. The iris uses its muscle fibers to contract or dilate based on the amount of light in the environment
(Figure 3.2).
The inner sensory layer includes the retina (Figure 3.2). The retina contains containing two types
of photoreceptors, cells that are sensitive to light. The more numerous cells are rods, which are stimulated in
dim light. Rods are more sensitive to light but do not generate sharp or color images. The conesoperate in bright
light, helping to generate sharp color images.
, Figure 3.2 Lateral view of the eye. The choroid and ciliary body are in blue. The anterior portion is the iris
with the pupil in the center. The retina is the inner sensory layer of the eye.
The lens (Figure 3.3) is located posterior to the iris and pupil. The interior of the eye, posterior to the lens, is
called the vitreous body (Figure 3.3). It is a chamber filled with vitreous fluid, which helps to hold the retina
firmly to the choroid.
The Special Senses
The five special senses vision, smell, taste, hearing, and equilibrium (balance) are the senses that have
specialized organs containing specialized receptor cells, which carry their impulses by way of specialized
somatic and visceral afferents. The other sense, touch, is a somatic sense that does not have a specialized sense
organ. Instead, touch uses general receptors composed of modified dendrites of sensory neurons. Touch
includes pressure, vibration, pain, heat, and the combined information is carried in general somatic afferents and
general visceral afferents.
Vision and the Eye
The organ responsible for sight is the eye, and it consists of three layers. The outer fibrous layer includes the
sclera and cornea. The posterior portion is known as the sclera (Figure 3.1), also known as the “white of
the eye. The anterior portion is known as the cornea (Figure 3.1). The cornea is the transparent part of the
eye where light enters. Aqueous humor fills the spaces between the cornea and the lens.
Figure 3.1 Lateral view of the cornea and sclera of the eye. (Both are highlighted in blue.)
The middle layer of the eye includes choroid, the ciliary muscle, and the iris. The darkly-colored
posterior choroid (Figure 3.2) prevents light from dispersing throughout the eye. The choroid is highly
vascular and supplies blood to the other layers of the eye. The anterior ciliary body (Figure 3.2) changes the
shape of the lens, allowing it to focus. The iris (Figure 3.2) is anterior to the ciliary body and contains the
,circular colored portion of the eye. The iris controls the amount of light let into the pupil, a hole in the center of
the iris. The iris uses its muscle fibers to contract or dilate based on the amount of light in the environment
(Figure 3.2).
The inner sensory layer includes the retina (Figure 3.2). The retina contains containing two types
of photoreceptors, cells that are sensitive to light. The more numerous cells are rods, which are stimulated in
dim light. Rods are more sensitive to light but do not generate sharp or color images. The conesoperate in bright
light, helping to generate sharp color images.
, Figure 3.2 Lateral view of the eye. The choroid and ciliary body are in blue. The anterior portion is the iris
with the pupil in the center. The retina is the inner sensory layer of the eye.
The lens (Figure 3.3) is located posterior to the iris and pupil. The interior of the eye, posterior to the lens, is
called the vitreous body (Figure 3.3). It is a chamber filled with vitreous fluid, which helps to hold the retina
firmly to the choroid.
