Chapter 7 The Sensory Systems 287 Check Your Understanding 1. Where are sensors for smell located? 2. Explain the connection between smell and emotions. 3. What is the most common cause of rhinitis? 4. What might cause a perforated septum? perfectly centered a slight deviation to one side is common. However, a large shift in the position of the septum away from the center is called a deviated septum. Injury is usually, but not always, the cause of a deviated septum. When warranted, a deviated septum can be surgically repaired. The septum may also develop one or more holes caused by injury, an ulcer, long-term exposure to toxic fumes, or illegal drug abuse. This condition is known as a perforated septum. A perforated septum can be surgically treated to close the open sores. What Research Tells Us …about Sense of Smell in Animals How does the human olfactory sense compare to that of animals and fish? The human sense of smell is sufficiently powerful that it can detect the presence of a skunk with only 0.000,000,000,000,071 of an ounce of scent present. Because the sense of smell among many species of animals and fish may be related to survival, however, a larger portion of their brains is devoted to receiving and interpreting scents. Dogs, for example, have about 125 to 220 million olfactory receptor cells. This is roughly 20 times the number of receptors that humans have. Dogs bred for hunting may have even more olfactory receptors the bloodhound, for instance, has nearly 300 million. In fact, the percentage of a dog’s brain that is responsible for smell is 40 times greater than the corresponding percentage of the human brain. The wetness of a dog’s nose assists with scent detection by capturing odor particles. With all of these advantages, it has been estimated that a dog’s sense of smell is 100 thousand to 1 million times more sensitive than that of a human. By comparison, the bloodhound’s olfactory sense may be up to 100 million times more sensitive. This is why the bloodhound, sometimes jokingly referred to as “a nose with a dog attached,” is often used in law enforcement to track down missing persons and criminals. A horse’s olfactory sense is not as sharp as that of a dog, but it is still stronger than the human sense of smell. Horses use their olfactory sense to identify other horses, people, predators, pastures, feeds, and water sources. A mare’s sense of smell is so discriminating that it enables her to pick out her own foal from a group of foals. Fish such as salmon also have an extremely well-developed sense of smell. Salmon are born in small, freshwater streams. They then swim to the ocean, where they spend one to three years. After this period, they generally return to the same stream where they were born to lay eggs. How does each salmon locate the exact same stream in which it was born? One hypothesis is that the newborn salmon brain is imprinted with the distinctive odor of the water from the stream of its birth. Researchers tested this hypothesis by tagging a large number of young salmon and recording the location of their births. They then plugged the nostrils of 50 percent of these salmon to block their ability to smell. When the salmon returned to fresh water to lay their eggs, the researchers discovered that the salmon with plugged nostrils did not return to the same stream where they were born. By contrast, most of the salmon with unplugged nostrils returned to the site of their births. Although other factors may influence the salmon’s remarkable ability to return to its birth stream, its sense of smell clearly plays a key role. Taking It Further 1. Do some research to find out how “sniffer” dogs are trained to detect drugs or bombs with their noses. Share your findings with the class. 2. How strong is your own sense of smell? Open an alcohol swab packet near your belly button. Breathe in. Can you detect the odor from the swab? Begin moving it closer to your nose and continue trying to smell it. If you can detect the odor from a distance of 8 to 12 inches from your nose (around chest level), then your sense of smell is normal. If you find that you are having a hard time distinguishing the scent at 4 inches away from your nose, then you may have a loss of the sense of smell. Copyright Goodheart-Willcox Co., Inc.