The Five Senses

Scientists recognize five basic tastes: sweet, sour, salty, bitter, and umami (savory). Umami was identified in 1908 by Japanese scientist Kikunae Ikeda.

The pupil is the dark opening in the center of the eye that widens in dim light and narrows in bright light. The colored ring around it, called the iris, is what actually controls the pupil's size.

The stirrup (also called the stapes) is the smallest bone in the entire human body and sits in the middle ear. Along with the hammer and anvil, it forms a chain that carries sound vibrations to the inner ear.

Most adults have on the order of several thousand taste buds, with around 10,000 the figure traditionally quoted. They are replaced about every two weeks, and as we age the number decreases, which is one reason food can taste less intense in later life.

Smell is uniquely wired to the brain's limbic system, which handles emotion and memory, making a familiar scent instantly trigger vivid recollections. This is sometimes called the Proust effect.

The retina lines the back of the eye and contains millions of light-sensitive cells called rods and cones. Rods detect light and dark while cones detect color.

The cochlea is a fluid-filled, spiral-shaped organ in the inner ear that translates sound vibrations into electrical signals sent to the brain. Its name comes from the Latin word for snail.

Cone cells, concentrated in the center of the retina, detect color and work best in bright light. There are three types sensitive to red, green, and blue light respectively.

Meissner's corpuscles are fast-adapting touch receptors found just below the skin surface, especially in fingertips and lips, that are highly sensitive to light, gentle touch and low-frequency vibration. They were named after the German anatomist Georg Meissner. (Pacinian corpuscles sense deep vibration, Ruffini endings sense skin stretch, and Merkel discs sense sustained pressure.)

The olfactory epithelium is a small patch of tissue high inside the nasal cavity packed with millions of smell-detecting nerve cells. Scent molecules dissolve in mucus there and trigger signals sent directly to the brain.

The three semicircular canals in the inner ear are filled with fluid that shifts when you move your head, signaling the brain about your position and movement. They work together with your eyes and muscles to maintain balance.

The cornea is the clear, curved outer layer covering the front of the eye. It performs most of the eye's focusing work, bending light before it even reaches the lens.

Much of what we think of as taste is actually smell — volatile compounds from food travel up the back of the throat to the olfactory receptors. When the nose is blocked, those smell signals cannot get through, so food loses much of its flavor.

A landmark 2014 study published in Science estimated that humans can distinguish at least one trillion different odors, far more than the 10,000 commonly quoted. The human nose has roughly 400 types of olfactory receptors.

Every eye has a small blind spot where the optic nerve connects to the retina, creating a gap with no rods or cones. The brain cleverly fills in this gap using surrounding visual information, so we rarely notice it.