From CHAPTER TWO: Swallowing Bags, Bones, and Tubes
Everyone has to eat. And the food must be digested somehow in the rumbling, squeezing stomach and intestines before the body can use it. What really happens in there? Over two hundred years ago a famous Italian scientist named Lazzaro Spallanzani tried to find out. He had no X-rays or cameras with fiber-optic tubes to look inside a person's digestive tract, as we do now, but he had his own bold ideas. He could use materials as simple as linen and wood to solve the riddles of digestion. Would he find evidence of grinding, rotting, or chemical changes no one had discovered before?
When young Lazarro was growing up in Scandiano, Italy, he knew nothing of science. At fifteen he went away to a college run by the Jesuit religious order. He studied languages, philosophy, speaking, and writing, and impressed his teachers so much that they asked him to join their order. But his father, a lawyer, wanted him to study law.
So Lazarro went to the University of Bologna. Studying law trained him to think logically, but he didn't like it. Fortunately, one of his cousins, Laura Bassi, was a professor there. She encouraged him to take a wider variety of courses, including chemistry, physics, natural history (nature studies), and math. He was impressed with the power of mathematics, but it was the sciences that awakened his passion. Professor Bassi and another teacher persuaded Lazarro's father to let him drop law. Thus began Lazarro Spallanzani's amazing career in science.
By the time Spallanzani took up his digestion experiments in about 1776, he was a professor of natural history at the university in Pavia, Italy. He was already famous for his extraordinary experiments on other subjects. For example, he had aimed sunbeams at salamanders in a dark room so that he could watch their blood flow under a microscope. He had boldly climbed the Italian mountains to study their gushing spring water. To test the story that there was a giant whirlpool in the middle of Lake Ventasso in the Apennines mountain range, he floated out to the spot on a raft made of tree stumps. (Conclusion: no whirlpool.)
Now about forty-seven years old, Spallanzani was still full of energy, and his brain continued to overflow with questions. How do volcanoes erupt? Are sperm needed to make babies? How do deep-sea animals glow in the dark? How do birds migrate? And so on.
Spallanzani was fascinated by digestion. Very little was known about it then. Some scientists thought stomachs and intestines were like machines, grinding up food into little bits. Others thought there was a sort of bubbly fermentation occurring, like grape juice turning to wine. Another suggestion was that the food in the digestive system was simply rotting.
Only a few scientists had actually done experiments on digestion. In the 1600s, another Italian named Francesco Redi had forced birds to swallow glass balls to test the grinding-machine idea. The glass did indeed get crumbled in the birds' digestive organs. Then, in the 1700s, a Frenchman named Rene de Reaumur studied the chemistry of digestion. He forced some birds to swallow and regurgitate little metal tubes stuffed with sponges and then squeezed their stomach juice out of the sponges for chemical experiments.
Spallanzani used these ideas but went much further. He began with chickens, ducks, turkeys, geese, crows, pigeons, and doves. Then he moved on to frogs, newts, snakes, fish, sheep, oxen, horses, owls, eagles, cats, and dogs.
Why so many? He knew these animals have wildly different diets and eating "equipment." Chickens have no teeth but do have a very tough, muscular organ called a gizzard, usually full of stones they have swallowed. Cats slash their meat with razor-sharp teeth, while horses use huge, flat molars to mash food and snakes swallow their mouse meals whole. Surely there would be differences in digestion, too. And indeed there were. For example, Spallanzani showed that chickens and turkeys were great grinders. A chicken's gizzard could turn solid glass balls into "sand" in a few hours, and a turkey's gizzard could break metal into bits.
Today, some of Spallanzani's animal experiments sound cruel. When he pushed hard objects, such as tin tubes stuffed with food, down the animals' throats, they probably suffered. Some of them even fought back. In his extremely detailed report on the digestion experiments, Spallanzani noted that the snakes became "highly exasperated." One snake, fortunately a nonpoisonous specimen, bit him. Falcons and eagles attempted to attack him. When dogs tried to bite, he decided to hide their food tubes in pieces of meat and throw those on the floor.
We should remember, however, that in the 1700s European scientists generally did not believe that animals could think or feel pain. Ordinary people killed many animals for food, hunted them for sport, and worked them hard on farms without much thought about the animals' feelings. Spallanzani's use of animals for science would not have raised any eyebrows in his own time.
Spallanazani finished his animal experiments. Then, "not neglecting Man, the noblest and most interesting of all," he started in on his own stomach. He knew that humans might feel discomfort from such experiments, even if he did not realize other animals did, too. He decided that he could not ask another person to submit to his tests--he had to try them on himself.
He started with a soft experiment. One morning he chewed a bite of bread, spit it out, and weighed it. He stuffed the chewed bread into a small bag made of linen. The bag would help him track the location of the bread, while letting digestive juices soak into the food. He sewed the opening shut with thread. Then he swallowed the bag.
Spallanzani wasn't too worried. He knew that people sometimes swallow cherry stones and plum pits by accident, and they travel all the way through the digestive tract and come out in the feces without doing any harm. Twenty-three hours after Spallanzani swallowed the bread bag, it came out. He was fine. The bag and thread were intact, but the bread was completely gone.
"The fortunate result of this experiment gave me great encouragement to undertake others," said Spallanzani. So he forged ahead with a carefully planned series of experiments on himself, including dozens of tests, getting more and more daring.
He swallowed bread again inside bags made of two or three layers of cloth to slow down the digestion. The triple-layer bag had a dry speck of bread left in it. Spallanzani tasted it and discovered that it had lost all its flavor.
Was the food getting crushed like the glass in a chicken's gizzard? The human stomach and intestines are certainly muscular. To find out if squeezing was essential for digestion, Spallanzani decided to swallow food inside hard capsules. He carved wood into tubes about the size of a modern multivitamin pill, with holes in the sides to let the digestive juices in. If food still got digested inside such a tube, then digestive juices soaking into the food were more important than the mashing.
Tubes were scarier than bags. A tube might get stuck in his stomach, or it might tear his organs if he vomited it up. Spallanzani was definitely worried, since he often got indigestion after normal meals.
Nevertheless, he filled a wooden tube with chewed veal, put the tube in a linen bag, and swallowed everything. Then he waited. No problem! The food package "exited happily at the end of 22 hours," he reported. Both bag and tube came out intact but empty. His digestive juices seemed to be working even without any squeezing.
Spallanzani sent more wooden tubes on the same journey down the esophagus. To study the effect of chewing he put a chewed piece of pigeon heart in one tub and an unchewed piece in another tube, then swallowed both. Much more of the chewed heart had disappeared when the tubes came out the next day. He repeated these experiments with mutton, veal, and bread and got the same results. Chewed food was digested more quickly, probably because the chewed bits got more thoroughly exposed to his digestive juices.
Spallanzani carved some tubes especially thin, so that they could easily be crushed with the press of a finger. These tubes always came out of his body unbroken. His digestive organs must be squeezing very gently indeed.
To test this idea further, Spallanzani gulped down four firm grapes without chewing. They all came out after a day, still whole. Then he swallowed softer grapes and both firm and soft cherries, and most of them also came out whole. The human digestive system was clearly not a grinding machine.
Copyright © 2005 Leslie Dendy and Mel Boring
This text is from an uncorrected proof.