You might think that the back end of the gut has so many surprises in store for us because it is something we do not think about very much. But I don’t think that’s the real reason. The other end of the gut, the gateway, so to speak, also has no shortage of surprises in store—even though we are directly confronted with it every morning when we clean our teeth.

You can seek out these secrets with your tongue. These are four small points in your mouth. Two of them are located on the inside of your cheeks, opposite your upper molars, more or less in the middle. If you explore the area with your tongue, you will feel two tiny bumps. If they notice them at all, most people assume they must have bitten themselves in the cheek at some point, but they haven’t. These little nubs, which doctors call the parotid papillae, are found in the same position in everybody’s mouth. The other two points are lurking beneath your tongue, just to the right and left of the lingual frenulum, the fold of skin connecting your tongue to the floor of your mouth. These four little nubs supply your mouth with saliva.

The papillae in your cheeks secrete saliva whenever it’s needed right away—for example, when we eat. The two tiny openings under the tongue secrete saliva continuously. If you could somehow enter these channels and swim against the tide of saliva, you would eventually reach the main salivary glands. They produce the most saliva—about 11/2 to 2 US pints (0.7 to 1 liter) a day. If you feel upward from your neck to your cheek, you will notice two soft, round raised areas. May I introduce you? They are the bosses.

The sublingual papillae, those two constant suppliers of saliva, are situated right behind our lower front teeth, which are particularly susceptible to the buildup of tartar. This is because there are substances in our saliva that contain calcium whose sole function it is to make our teeth harder. But if a tooth is constantly bombarded with calcium, it can be a case of too much of a good thing. Tiny molecules floating innocently by are caught up and “fossilized” without so much as a by-your-leave. The problem is not the tartar itself, but the fact that it has such a rough surface, affording a much better foothold for bacteria that cause tooth decay and gum disease than smooth, clean tooth enamel.

But what are fossilizing, calcium-containing substances doing in our saliva? Saliva is basically filtered blood. The salivary glands sieve the blood, keeping back the red blood cells, which are needed in our arteries, not in our mouth. But calcium, hormones, and some products of our immune system enter the saliva from the blood. That explains why each person’s saliva is slightly different. In fact, saliva analysis can be used to test for diseases of the immune system or for certain hormones. The salivary glands can also add extra substances, including those calcium-containing compounds, and even natural painkillers.

Our saliva contains one painkiller that is stronger than morphine. It is called opiorphin and was only discovered in 2006. Of course, we produce only small amounts of this compound, otherwise we would be spaced out on our own spit all the time! But even a small amount has a noticeable effect, since our mouth is such a sensitive thing. It contains more nerve endings than almost anywhere else in the human body. Even the tiniest strawberry seed can drive us crazy if it gets stuck somewhere. We feel every grain of sand in a badly washed salad. A teeny little sore, which we would not even notice if it were on our elbow, hurts like hell and feels monstrously big in our mouth—without our salivary painkiller, it would feel even worse!

When we chew, we produce more saliva and with it more of such analgesic substances, which explains why a sore throat often feels better after a meal and even minor sores in the oral cavity hurt less. It doesn’t have to be a meal—even chewing gum provides us with a dose of our oral anodyne. There are even a handful of new studies showing that opiorphin has antidepressant properties. Is our spit partly responsible for the reassuring effects of comfort eating? Medical research into both pain and depression may deliver the answers in the next few years.

Saliva protects the oral cavity not only from too much pain, but also from too many bad bacteria. That’s the job of mucins, for example. Mucins are proteins that form the main constituent of mucus. They help provide hours of fascination and fun for young children who have just found out they can blow bubbles with their own spit. A more useful function is their ability to envelop our teeth and gums in a protective mucin net. We shoot them out of our salivary papillae like Spider-Man shoots webs from his wrists. These microscopic nets can catch bacteria before they have a chance to harm us. While the bad bacteria are caught in the net, antibacterial substances in our saliva can kill them off.

Like the natural painkillers in our saliva, bactericidal substances are present in our saliva in small concentrations. Our spit is not supposed to disinfect us completely. In fact, we actually need a core team of good little creatures in our mouth. Benign bacteria in the mouth are not totally wiped out by our disinfectant saliva since they take up space—space that could otherwise be populated by more dangerous germs.

Gut: The Inside Story of Our Body's Most Underrated Organ (Revised Edition)