The Sense Organs - The nose and tongue
The sense receptors on the tongue and within the nasal cavity work very closely together to give us our sense of taste. These five kinds of receptors—the olfactory cell in the nose and the four special cells or taste buds on the tongue for discriminating salty, sweet, sour, and bitter tastes—also have a functional similarity. All are chemical detectors, and all require moisture in order to function. In the nose, airborne substances must first be moistened by mucus (from the olfactory glands) before they can stimulate olfactory cells. In the mouth, the saliva does the wetting.
The general number and distribution of the four types of taste receptors are described earlier in this chapter under “The Digestive System and the Liver.” These nerve endings, numbering in the hundreds of thousands, merge into two nerve bundles traveling away from the tongue to the brain's “taste center.” The receptors toward the rear of the tongue collect into the glossopharyngeal nerve; those at the front and middle are directed along the lingual nerve .
Our smell receptors are clustered in an area about a half-inch wide on ceiling of the nasal cavity. This is called, appropriately enough, the smell patch. The nerve endings pass upward through the sievelike ethmoid bone , separating the nasal cavity from the brain, and connect to the olfactory bulb, which is the “nose end” of the olfactory nerve . At the other end of the olfactory nerve is the “nose brain” or rhinencephalon— a tiny part of the cerebrum in man, but quite large in dogs and other mammals whose sense of smell is keener than man's.
Although man's sense of smell is probably his least used sense, it still has a quite remarkable sensitivity. With it we can detect some chemicals in concentrations as diluted as one part in 30 billion—for example, the active ingredient in skunk spray. Also, man's ability to smell smoke and to detect gas leaks and other warning scents has prevented many a tragedy.