latter. A piece at a time, Cole rates a human arm at about 1,800 calories; a leg at 7,150; the lungs, liver, and alimentary canal about 1,500 calories each; the bundle of brain, spinal cord, and nerve and trunk about 2,700 calories. The brave heart? A mere 122.
Of course, while law-abiding folks are unlikely to slice, dice, and serve other folks anytime soon, other species are doing in their fellows day after day. The list of cannibalistic creatures who eat their enemies, their lovers, or their offspring includes fish such as the tiger shark and walleye, cute and cuddly prairie dogs, hamsters, hedgehogs, some snakes, caterpillars, ladybugs, spiders, some toads and tadpoles, hermit crabs, ducklings, cats, dogs, and polar bears (the last three often kill and sometimes consume sickly newborns). Chickens also make the list — but their cannibal dish is eggs not chicks.
And by the way, cannibalism is a species-neutral term. The word for people eating people is anthropophagy from the Greek words anthropos meaning “human being” and phagein meaning “to eat.”
Chapter 2
Digestion: The 24/7 Food Factory
IN THIS CHAPTER
When you see (or smell) something appetizing, your digestive organs leap into action. Your mouth waters. Your stomach contracts. Intestinal glands begin to secrete the chemicals that turn food into the nutrients that build new tissues and provide the energy you need for work, pleasure, and everyday life. This chapter provides a basic primer on the digestive system from start to finish with a few stops along the way to explain how you metabolize everything from apples to zucchini.
Introducing the Digestive System
Your digestive system is a collection of organs specifically designed to turn complex substances (food) into basic components (nutrients). These organs form one long, exceedingly well-organized tube that starts at your mouth, continues down through your throat to your stomach, and then goes on to your small and large intestines to end at your anus.
In between, with the help of the liver, pancreas, and gallbladder, the usable (digestible) parts of everything that you eat are converted to simple compounds that your body can easily absorb to burn for energy or to build new tissue. The indigestible residue is bundled off and eliminated as waste.
Figure 2-1 shows the body parts and organs that comprise your digestive system.
© John Wiley & Sons, Inc.
FIGURE 2-1: Your digestive system in all its glory.
The digestive process run by these organs works in two simple ways, one mechanical and one chemical.
Mechanical digestion takes place in your mouth and your stomach. First, your teeth break food into small, easy-to-swallow pieces that slide quickly from your mouth down through your esophagus (throat) to your stomach. Here, a churning action called peristalsis continues to break food into smaller particles and then moves the particles along to your small intestine, where the churning and breaking continues.
Chemical digestion occurs at every point in the digestive tract where enzymes and other substances, such as hydrochloric acid (from stomach glands) and bile (from the liver), dissolve food, releasing the nutrients inside.
The rest of this chapter explains exactly what occurs and where along the digestive tract.
Digestion: One Step at a Time
Each organ in the digestive system plays a specific role in the digestive drama. But the first act occurs in three places rarely listed as part of the digestive tract: your brain, your eyes, and your nose.
The next acts take place in your mouth, your stomach, and your small and large intestines.
Your brain, eyes, and nose
When you see appetizing food, you experience a conditioned response. (For the lowdown on how your digestive system can be conditioned to respond to food, see Chapter 14; for information on your food preferences, see Chapter 15.) In other words, your thoughts — “Wow! That looks good!” — stimulate your brain to tell your digestive organs to get ready for action.
What happens in your nose is purely physical. The tantalizing aroma of good food is transmitted by molecules that fly from the surface of the food to settle on the membrane lining of your nostrils; these molecules stimulate the receptor cells on the olfactory nerve fibers that stretch from your nose back to your brain. When the receptor cells communicate with your brain, your brain sends encouraging messages to your mouth and digestive tract as the sight and scent of food make your mouth water and your stomach contract in anticipatory hunger pangs.
What if you hate what you see or smell? For some people, even the thought of liver is enough to make them want to leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction. Your mouth purses, and your nose wrinkles as if to keep the food (and its odor) as far away as possible. Your throat tightens, and your stomach turns as muscles contract, not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.
But assume that you like what’s on your plate. Go ahead. Take a bite.
Your mouth
Lift your fork to your mouth, and your teeth and salivary glands swing into action. Your teeth chew, grinding and breaking food into small, manageable pieces. As a result,
You can swallow easily.
You break down the indigestible wrapper of fibers surrounding the edible parts of some foods (fruits, vegetables, whole grains) so that your digestive enzymes can get to the nutrients inside.
At the same time, salivary glands under your tongue and in the back of your mouth secrete the watery liquid called saliva, which performs two important functions:
It moistens and compacts food so your tongue can push it to the back of your mouth and you can swallow, sending the food down your esophagus into your stomach.
It provides amylases, enzymes that start the digestion of complex carbohydrates (starches), breaking the starch molecules into simple sugars. (Check out Chapter 8 for more on carbs.)
No protein digestion occurs in your mouth, and although saliva does contain very small amounts of lingual lipases — fat-busting enzymes secreted by cells at the base of the tongue — the amount is so small that the fat digestion here is insignificant.
TURNING STARCHES INTO SUGARS
Salivary enzymes (like amylases) don’t lay a finger on proteins and leave fats pretty much alone, but they do begin to digest complex carbohydrates, breaking the long, chainlike molecules of starches into individual units