Saliva is almost entirely water. Only 0.5 percent of it is anything else, but that tiny portion is full of useful enzymes—proteins that speed up chemical reactions. Among these are amylase and ptyalin, which begin to break down sugars in carbohydrates while they are still in our mouths. Chew a starchy food like bread or potato for a bit longer than normal and you will soon notice a sweetness. Unfortunately for us, bacteria in our mouths like that sweetness, too; they devour the liberated sugars and excrete acids, which drill through our teeth and give us cavities. Other enzymes, notably lysozyme—which was discovered by Alexander Fleming before he stumbled onto penicillin—attack many invading pathogens, but not the ones that cause tooth decay, alas. We are in the rather strange position that we not only fail to kill the bacteria that give us a lot of trouble but actively nurture them.
We produce very little saliva while we sleep, which is why microbes can proliferate then and give you a foul mouth to wake to. It is also why brushing your teeth at bedtime is a good idea because it reduces the number of bacteria you go to sleep with. If you’ve ever wondered why no one wants to kiss you first thing in the morning, it is possibly because your exhalations may contain up to 150 different chemical compounds, not all of them as fresh and minty as we might hope. Among the common chemicals that help to create morning mouth are methyl mercaptan (which smells very like old cabbage), hydrogen sulfide (like rotten eggs), dimethyl sulfide (slimy seaweed), dimethylamine and trimethylamine (rank fish), and the self-explanatory cadaverine.
Professor Joseph Appleton of the University of Pennsylvania School of Dental Medicine, in the 1920s, was the first to study bacterial colonies within the mouth and discovered that, microbially speaking, your tongue, teeth, and gums are like separate continents, each with its own colonies of microorganisms. There are even differences in the bacterial colonies that inhabit the exposed part of a tooth and those beneath the gum line. Altogether, about a thousand species of bacteria have been found in human mouths, though at any one time you are unlikely to have more than about two hundred.
The mouth is not only a welcoming home for germs but an excellent way station for those that want to move elsewhere. Paul Dawson, a professor of food science at Clemson University in South Carolina, has made something of a career of studying the ways people spread bacteria from themselves to other surfaces, as when they share a water bottle or engage in “double dipping” with chips and salsa. In a study called “Bacterial Transfer Associated with Blowing Out Candles on a Birthday Cake,” Dawson’s team found that candle blowing across a cake increased the coverage of bacteria on it by up to 1,400 percent, which sounds pretty horrifying but is in fact probably not much worse than the kinds of exposures we encounter in normal life anyway. There are a lot of germs adrift in the world or wriggling invisibly on surfaces, and those surfaces include a lot of what you put in your mouth and nearly everything you touch.
The most familiar components of the mouth are of course the teeth and the tongue. Our teeth are formidable creations and nicely versatile, too. They come in three varieties: blades (which are pointy), cusps (which are spade-like), and basins, or fossae (which fall somewhere between the other two). The outside of your tooth is the enamel. It is the hardest substance in the human body, but forms just a thin layer and can’t be replaced if it is damaged. That’s why you have to go to the dentist for cavities. Under the enamel is a much thicker layer of another mineralized tissue called dentin, which can renew itself. At the center of it all is the fleshy pulp with nerves and blood supply. Because they are so hard, teeth have been called “ready-made fossils.” When all the rest of you has turned to dust or dissolved away, the last physical trace of your existence on Earth may be a fossilized molar.
We can bite pretty hard. Bite force is measured in units called newtons (in honor of Isaac Newton’s second law of motion, not his oral ferocity), and if you are a typical adult male, you can muster about four hundred newtons of force, which is quite a lot, though nothing like as much as an orangutan, which can bite with five times as much vigor. Still, when you consider how well you can demolish, say, an ice cube (try doing that with your fists and see how far you get) and how little space the five muscles of the jaw occupy, you can appreciate that human chomping is pretty capable.
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The tongue is a muscle, but quite unlike any other. For one thing, it is exquisitely sensitive—think how adroitly you pick out something in your food that shouldn’t be there, like a tiny piece of eggshell or grain of sand—and intimately involved in vital activities like speech articulation and tasting food. When you eat, the tongue darts about like a nervous host at a cocktail party, checking the taste and shape of every morsel in preparation for dispatching it onward to the gullet. As everyone knows, the tongue is coated with taste buds. These are clumps of taste receptor cells found in the bumps on your tongue, which are formally called papillae. They come in three different shapes: circumvallate (or rounded), fungiform (mushroom shaped), and foliate (leaf shaped). They are among the most regenerative of all cells in the body and are replaced every ten days.
For years, even textbooks spoke of a tongue map, with the elemental tastes each occupying a well-defined zone: sweet on the tip of the tongue, sour at the sides, bitter at the back. In fact, that is a myth, traced to a textbook written in 1942 by one Edwin G. Boring, a Harvard psychologist who misinterpreted a paper written by a German researcher forty years before that. Altogether we have about ten thousand taste buds, mostly distributed around the tongue, except in the very middle, where there are none at all. Additional taste buds are found in the roof of the mouth and lower down the throat, which is said to be why some medicines taste bitter as they go down.
As well as the mouth, the body has taste receptors in the gut and throat (to help identify spoiled or toxic substances), but they don’t connect to the brain in the same way as the taste receptors on your tongue, and for good reason. You don’t want to taste what your stomach is tasting. Taste receptors have also been found in the heart, the lungs, and even the testicles. No one knows quite what they are doing there. They also send signals to the pancreas to adjust insulin output, and it may be connected to that.
It is generally supposed that taste receptors evolved for two deeply practical purposes: to help us find energy-rich foods (like sweet, ripe fruits) and to avoid dangerous ones. But it must also be said that they don’t always fulfill either role terribly well. Captain James Cook, the great British explorer, had a salutary demonstration of that in 1774, on his second epic voyage through the Pacific. One of his crew caught a meaty fish, which no one aboard recognized. It was cooked and proudly presented to the captain and two of his officers, but because they had already dined, they merely sampled it and had the remainder put aside for the following day. This was a very lucky thing, for in the middle of the night all three found themselves “seized with an extraordinary weakness and numbness all over our limbs.” Cook was for some hours virtually paralyzed and unable to lift anything—even a pencil. The three men were given emetics, to clear their stomachs. They were lucky to survive, for what they had sampled was puffer fish. These contain a poison called tetrodotoxin, which is a thousand times more powerful than cyanide.