The Private World of the Senses
By any measure, people enjoy a rich sensory world. We are
continually assailed by information from the senses of
vision, hearing, taste, olfaction, and touch. These classic five
senses are supplemented by sensory inputs of cold, warmth,
vibration, and pain, as well as by information from numerous
internal sensory receptors that operate silently and automatically
to keep our interior domain working smoothly.
The world our senses perceive is uniquely human. We share
this exclusive world with no other animal, nor can we venture
into the sensory world of any other animal except as an
abstraction through our imagination.
The idea that each animal enjoys an unshared sensory
world was first conceived by Jakob von Uexküll, a seldom
cited German biologist of the early part of this century. Von
Uexküll asks us to try to enter the world of a tick through our
imagination, supplemented by what we know of tick biolo-gy. It is a world of temperature, of light and dark, and of the
odor of butyric acid, a chemical common to all mammals.
Insensible to all other stimuli, the tick climbs up a blade of
grass to wait, for years if necessary, for cues that will betray
the presence of a potential host. Later, swollen with blood,
she drops to the earth, lays her eggs, and dies. The tick’s
impoverished sensory world, devoid of sensory luxuries and
fine-tuned by natural selection for the world she will
encounter, has ensured her single goal, reproduction.
A bird and a bat may share for a moment precisely the
same environment. The worlds of their perceptions, however,
are vastly different, structured by the limitations of the
sensory windows each employs and by the brain that garners
and processes what it needs for survival. For one it is a
world dominated by vision; for the other, echolocation. The
world of each is alien to the other, just as their worlds are
The nervous system originated in a
fundamental property of life: irritability,
the ability to respond to environmental
response may be simple, such as a
protozoan moving to avoid a noxious
substance, or quite complex, such as a
vertebrate animal responding to elaborate
signals of courtship. A protistan
receives and responds to a stimulus, all
within the confines of a single cell.
Evolution of multicellularity and more
complex levels of animal organization
required increasingly complex mechanisms
for communication between
cells and organs. Relatively rapid communication
is by neural mechanisms and involves propagated electrochemical
changes in cell membranes. The
basic plan of a nervous system is to
code information and to transmit and
process it for appropriate action. These
functions are examined in this section.
Relatively less rapid or long-term
adjustments in animals are governed
by hormonal mechanisms, subject
of the next section.