The Nervous System: The Brain, Cranial Nerves, Autonomic Nervous System, and the Special Senses 263
detected. The sense of smell is closely related to the sense
of taste. We use these two senses to decide whether or not
to eat a particular food. Our sense of smell is complex be-
cause a small number of receptors detect a great variety of
odors. It is the brain that then interprets these recep-tor
combinations into a type of olfactory code. The exact
mechanism of how this works is still being investigated by
biologists. However, we do know that olfactory receptors
rapidly adapt to odors and after a short time, we no longer
perceive the odor as intensely as it was initially detected.
that make up the orbits. In addition, the eyebrows help
shade the eye and keep perspiration from getting into the
eye and causing an irritation to the eye. Eyelids and
eyelashes protect the eye from foreign objects. Blinking of
the eyelids lubricates the surface of the eye by spread-ing
tears that are produced by the lacrimal gland. The tears not
only lubricate the eye but also help to combat bacterial
infections through the enzyme lysozyme, salt, and gamma
globulin.The Sense of Taste
Taste buds are the sensory structures found on certain
papillae (pah-PILL-e), which are elevations of the tongue
that detect taste stimuli (Figure 11-9). Taste buds are also
found on the palate of the roof of the mouth, in certain
regions of the pharynx, and on the lips of children. Each
taste bud is composed of two types of cells. The first type
are specialized epithelial cells that form the exterior cap-
sule of the taste bud. The second type of cell forms the
interior of the taste bud. These cells are called taste cells
and function as the receptor sites for taste. The taste bud is
spherical with an opening called the taste pore. Taste hairs
are tiny projections of the taste cells that extend out of the
taste pore. It is these taste hairs that actually function as the
receptors of the taste cell. Cranial nerves VIII, IX, and X
conduct the taste sensations to the brain, which perceives
and interprets the taste.
Before a chemical can be tasted, it must first be dis-
solved in a fluid (just like the odors in the nose). The sa-
liva produced by the salivary glands provides this fluid
medium. Nerve fibers surrounding the taste cells trans-mit
the impulses to the brain for interpretation. The sensory
impulses travel on the facial (VIII), glossopha-ryngeal (IX),
and vagus (X) cranial nerves to the gusta-tory (taste) cortex
of the parietal lobe of the cerebrum for interpretation. The
four major types of taste sensations are sweet, sour, salty,
and bitter. Although all taste buds can detect all four
sensations, taste buds at the back of the tongue react
strongly to bitter, taste buds at the tip of the tongue react
strongly to sweet and salty, and taste buds on the side of the
tongue respond more strongly to sour tastes (see Figure 11-
9). Taste sensations are also influenced by olfactory
sensations. Holding one’s nose while swallowing reduces
the taste sensation. This is a common practice when taking
bad-tasting medicine.
The Sense of Sight
The eyes are our organs of sight. They are protected by the
orbits of the skull. See Chapter 7 to review the bones
Media Link
Watch an animation that illustrates how we
see on the Student Companion
Website..The Anatomy of the Eye
The eye is a sphere filled with two fluids (Figure 11-10).
The skeletal muscles that move the eye are discussed in
Chapter 9. They are the rectus muscles and the oblique
muscles.
The wall of the eye is composed of three layers, or tu-
nics, of tissue. The outermost layer is the sclera
(SKLAIR-ah). It is white and composed of tough
connective tissue. We see it as the white of the eye when
looking in a mir-ror. The conjunctiva (con-JUNK-tee-
vah) is the mucous membrane lining the inner surface of
the eyelids and the anterior part of the sclera. The cornea
(COR-nee-ah) is the transparent part of this outermost
layer that per-mits light to enter the eye. The second layer
is the choroid (KOR-oyd). It contains numerous blood
vessels and pig-ment cells. It is black in color and absorbs
light so that it does not reflect in the eye and impair vision.
The inner-most layer of the eye is the retina (RET-ih-
nah). It is gray in color and contains the light-sensitive cells
known as the rods and cones.
The ciliary (SIL-ee-air-ee) body consists of smooth
muscles that hold the biconvex, transparent, and flexible
lens in place. The iris is the colored part of the eye con-
sisting of smooth muscle that surrounds the pupil. The iris
regulates the amount of light that enters through the
diameter of the pupil. When we go into a dark room, the
iris opens to allow more light to enter. When we go out into
strong sunlight, the iris constricts, letting less light enter the
pupil.The interior of the eye is divided into two compart-
ments. In front of the lens is the anterior compartment that
is filled with a fluid called the aqueous humor. This
fluid helps to bend light, is a source of nutrients for the