The Body: A Guide for Occupants

Craniometry was a popular field of study in the 1800s. Practitioners believed that human intelligence and character could be deduced from the sizes and shapes of human heads. Criminals thus should be detectable by how their facial and cranial features were proportioned; also, nonwhite races were assumed to have inferior heads. 

Today, craniometry is considered a pseudoscience, but during its heyday it was championed by well-known researchers such as Cesare Lombroso, a founder of criminology, and Pierre Paul Broca, discoverer of Broca’s area, the speech center in the brain’s frontal lobe. 

Charles Darwin, who believed all humans share a common ancestor, published The Expression of the Emotions in 1872, arguing that all people everywhere share the same basic facial expressions. In 1960, UC San Francisco professor Paul Ekman tested Darwin’s thesis and found that people do, indeed, share at least six expressions: “fear, anger, surprise, pleasure, disgust, and sorrow” (77). Ekman also found that people unconsciously display microexpressions that betray their true feelings, regardless of the smiles or frowns they put on their faces. The human face is unusual among animals: flat with a tall forehead, expressive eyebrows instead of brow ridges, and prominent noses and chins. 

Of 33 human sensory inputs, including balance, location in space, and passage of time, the most noted are sight, hearing, smell, touch, and taste. Eyes are like cameras, with an opening, the pupil, controlled by the muscles of the iris. A system in front that includes a cornea and lens bends and focuses the light, which then passes through the clear, jelly-like middle of the eye toward the retina, the sensor in the rear. Tears keep the front of the eyeball smooth and germ free. Humans also shed emotional tears, the only known example of such behavior in the animal kingdom. 

The retina has two photoreceptors: rods for dim light and cones for sensing red, blue, and green colors. The strongest sensors are in a small dip called the fovea, which details an area the size of a thumbnail at arm’s length. The eye looks about in rapid jumps, called saccades, 250,000 a day, that give the sense of a larger area of sharp detail. Images are sent to the brain through the optic nerve, a pencil-thick cable that causes a small blind spot in the field of vision that is filled in by the brain’s ability to estimate what should be there. 

Hearing begins at the fleshy outer ear, or pinna, shaped to capture sounds and their source locations. Sounds travel through the ear canal to the eardrum, or tympanic membrane, which vibrates the three tiny bones of the middle ear—called the hammer, anvil, and stirrup for their shapes—that can detect vibrations less than the width of an atom, amplify them, and send them to the cochlea, a snail-shaped chamber filled with tiny hairs, or stereocilia, of different lengths that resonate in tune with the various wavelengths of sound. Modern life contains very loud noises, like jet engines and overly amped earbuds, which can do permanent damage to the stereocilia. 

Attached to the middle ear are a set of gel-filled tubes and sacs, the vestibular system, that gives humans their sense of balance. As people age, the gel thickens, making balance more difficult. 

As for smell, odor molecules, called volatiles, enter the nose and contact a set of nerves called the olfactory epithelium that contains up to 400 separate sensors, several of which are triggered by each aroma. Smells can be extremely complex: Bananas have 300 volatiles, tomatoes emit 400, and coffee has 600. Some researchers believe the human nose can detect a trillion different odors. 

Aromas can evoke strong memories; this may be because smells are processed in the brain’s olfactory cortex, next to the memory-forming hippocampus. Smells are, to some degree, unique for each person, as only about half the receptors in the nose are common to everyone. Humans can detect some odors better than dogs, yet people tend to take their sense of smell for granted. Smell can be lost permanently from head injury or illness, which seriously undermines quality of life. 

The mouth and throat can be places of danger: humans “choke to death more easily than any other mammal” (93). People walk upright, and their necks have a shape different from other primates. Food and air enter the human body through a single tube, the pharynx, at the rear of the mouth. This tube divides at the epiglottis, a tiny door that opens for air and closes to divert food to the esophagus, a muscle-lined tube that pushes food down to the stomach. 

This system has advantages, including improved vocalization and speech, but the danger of choking is ever present. Thousands of Americans die each year from choking—the fourth most common cause of accidental death in the U.S.—and many who apparently die of heart attacks in restaurants are actually choking victims. 

In the 1970s, Dr. Henry Heimlich developed a way to dislodge food trapped in the breathing passageway by embracing the victim from behind and giving a series of sharp hugs just below the ribcage. This movement forces air from the lungs to push up against the obstruction, causing it to pop out. This Heimlich maneuver is now called “abdominal thrusts” by the American Red Cross. 

The greatest choking expert was Dr. Chevalier Quixote Jackson, who, by the time he died in 1958 at age 92, had developed many of the instruments and methods used to relieve choking, saved hundreds of lives, and amassed a collection of more than 2,000 items retrieved from victims. 

The mouth itself is highly complex. An average adult generates nearly a quart and a half of saliva each day through 12 glands in the mouth. Saliva is nearly all water; less than 1% contains enzymes that help pre-digest food but also feed bacteria that emit acids that damage teeth—even their outer enamel, the hardest substance in the body. Most people’s mouths contain about 200 kinds of bacteria, with different types inhabiting tongue, teeth, and gums. 

The tongue, an extremely sensitive muscle, contains 10,000 taste receptors—for sweet, salty, sour, bitter, and umami, or savory—and as many pain receptors, including heat sensors, which are activated by chilies. The Scoville test gives bell peppers a hotness rating of 50 to 100; jalapeños get 2,500 to 5,000; the Carolina Reaper pepper is hottest at 2 million. Pure capsaicin—the chemical responsible for chili’s heat sensation—rates 16 million Scovilles. 

Umami, the fifth taste, comes from glutamate, common in many foods. Discovered by a Japanese chemist in 1909, it became the basis for monosodium glutamate, a popular flavor enhancer. In the late 1960s, it fell from favor in the West due to rumors that it caused illness, but scientists have been unable to find anything wrong with it. 

Flavors derive mainly from smell. For example, “strawberry is actually a smell, perceived nasally, not a taste in the mouth” (106); moreover, it’s a perception that exists only in the brain, not in the strawberry itself. 

Farther down the throat is the larynx, where the vocal cords and other apparatuses of speech perform a complex dance to produce sounds that the mouth can shape into speech. Four percent of children and 1% of adults suffer from stuttering, a still-mysterious ailment with no reliable cure that has affected such luminaries as Aristotle, Darwin, Churchill, and Marilyn Monroe. 

The uvula, a small, fleshy appendage that hangs at the back of the mouth, helps prevent food from entering the nasal passage during a fit of coughing, dispenses some of the mouth’s saliva, and adds to the formation of sounds, including snoring. All of these are minor; in the rare instances when the uvula is removed surgically, the effects on the patient are small. Its bigger purpose, if it has one, remains a mystery.