So, exactly how long is twelve finger breadths? Anatomy textbooks say the length of the duodenum is about 25 cm (just under 10 inches). The width of my four fingers, side by side, is about 7 cm, which means that I would need just over fourteen fingers to reach 25 cm, not twelve. Maybe if early anatomists all had bony fingers like me, they would have dubbed the first part of the small intestine the quattuordenum.
The duodenum's major anatomical claim to fame is its major duodenal papilla. That's the little bump (papilla means "little pimple" in Latin) where two important tubes - the common bile duct and the main pancreatic duct - converge and dump their contents (see the figure below). The common bile duct carries bile, a greenish biodegradable detergent that is manufactured in the liver, concentrated and stored in the gall bladder, and released into the duodenum to digest fats. The main pancreatic duct carries pancreatic juice, which contains bicarbonate (the active ingredient in baking soda) for neutralizing acid from the stomach and at least 19 different enzymes for breaking down proteins, fats, sugars, and nucleic acids.
The flow of bile and pancreatic juice is regulated by a tiny circular muscle called the sphincter of Oddi. At mealtime the sphincter of Oddi relaxes and the juices flow. Incidentally, for years anatomists have been pushing to get rid of eponyms in favor of more descriptive terms, so the sphincter of Oddi is more properly known as the - take a deep breath - sphincter of the hepatopancreatic ampulla. Uh huh.... I think this is one case where the eponym will never die.
Because of my enduring fascination with unintelligent design, I can't help but wonder if there is any good functional reason for both bile and pancreatic juice to enter the duodenum via a single opening. Normally this arrangement isn't a problem: both secretions come in handy whenever the stomach squeezes another glob of partially digested goo into the duodenum. However, things can get ugly if you have gallstones. Although it isn't common, a stone can form that is small enough to travel down the bile duct, but too big to squeeze through the sphincter of Oddi. A stone lodged near the papilla obstructs the flow of bile and pancreatic secretions. Blocking bile flow is bad enough, causing pain and jaundice, but blocking the flow of pancreatic juice can lead to acute pancreatitis, a potentially life-threatening condition in which the pancreas literally starts to digest itself. Acute pancreatis has many causes, but the most common is a gallstone clogging the drain.
"But wait," chimes in the anatomically informed reader, "isn't there a second pancreatic duct - the accessory pancreatic duct - that is connected to main duct but drains into the duodenum at a different papilla? Couldn't that accessory duct serve as an alternate route if the main duct is obstructed?" The answer is yes, but only in about 60% of the population. The remaining 40% would be out of luck, because their accessory duct drains only into the main duct, not into the duodenum directly.
So is there any good functional reason for bile and pancreatic juice to drain at the same point? I can't think of any, and neither can a colleague here at UVM whose does research on gall bladder function for a living. It would make more sense for everyone to have an alternate drainage route for pancreatic juice, not just 60% of us. But, hey, no body is perfect. :-P