Other Senses

Nearly everyone knows that humans and most animals possess the five senses of sight, hearing, touch, smell, and taste. This is how they learn about the environment around them. Yet modern biologists have recognized that besides these five, a human or animal body has other senses—perhaps up to fifteen—that signal the brain concerning the body's condition and needs.

Over the years scholars and scientists have classified the senses in different ways. We owe the idea that there are five senses to the ancient Greek philosopher Aristotle. Today's scientists generally group the senses into two kinds, external and internal.

Together the external and internal senses provide complete data for the body. The external ones include sight, hearing, taste, and smell. They relay information about what happens outside the body. What we call touch is included under the somatic senses, which are both internal and external. The somatic senses let us feel not only pressure, heat, cold, and pain from external sources but also hunger, thirsty, fatigue, balance, and muscle tension from within the body.

Both external and internal senses depend on the nervous system. Both operate in much the same way. Every sense has special cells receptors, which are connected to nerve cells. Receptors for the external senses are localized in a few areas, such as the retina of the eye for sight and the tongue for taste. Internal sense receptors are found in muscles and various organs, such as the heart and stomach. When stimulated, a receptor sends nerve impulses to the brain. This causes a response.

The senses give us both warning signals and pleasure. If a fire begins at night in a bedroom, one's sense organs send messages to the brain. They report the smell and heat of the foods to help us detect whether foods are beneficial or harmful. Hunger pangs inform us that it is mealtime, and the pleasurable sensation of a full stomach lets us know that we have had enough.