Sure, hold out your hands in front of you like you're holding a hose with both hands vertically, one fist on top of the other. Imagine the tube is hanging from a bag full of liquid (a stomach). Now squeeze your bottom fist, and relax your top fist, and imagine the liquid filling the top part of the hose. Now relax your bottom fist a bit and squeeze your top fist, and image liquid squirting out both ends of the top part of the hose-- both forward into the bottom part and upward back toward the stomach. That's a good model of your pylorus, except the bottom fist is more like a ring, and the tube isn't vertical but horizontal, and in fact, the bottom of your stomach curves around such that the pylorus is actually squirting liquid (called chyme) upward into your small intestine.

But the vertical fists model helps to visualize the pylorus as a "cylinder" of annular muscle segments that contract and relax with alternating timing to pump liquid from the stomach into the duodenum in precise metered doses. This timed and coordinated squeezing action is called peristalsis, and is the mode of action of all your intestines as well as your esophagus.

Now here's the important part: notice that when you squeezed your top fist, liquid squirted out in both directions. Dynamic studies of the pylorus in action show that these two "squirts" are not the same. The forward squirt is usually smaller and contains only liquid and fine particles of food, whereas the larger backward squirt (called retropulsion) contains all of the large chunks of food that require further digestion.

So how does the pylorus do this? The answer lies largely in the shape and structure of the sphincter wall, and here's where it gets really crazy. The pyloric cylinder has two sets of muscles moving it: the big strong external rings of muscle, and a finer inner layer of muscle in the mucosal lining. When these muscles contract and relax, the inner mucosal lining forms various patterns of folds and wrinkles. When the pylorus is pumping liquid from the stomach to the small intestine, the two sets of muscles work together to shape folds and wrinkles that "grip" chunks of food, prevent them from passing, and then launch them backward, back into the stomach. The shapes of the folds is highly variable, and it appears that your pylorus adapts these intricate patterns in its lining from moment to moment based on the properties of the food in your stomach-- it's texture, viscosity, the shapes and sizes of the chunks in it, etc.

How does your stomach "know" these details about the food in it? I don't think anybody really knows yet.

In case anyone's looking for an exact source, here's the money quote from page 62 of the book:

> A certain contraction of the outer muscular tube is necessary for the formation of macroscopic mucosal folds. The variability of the fold pattern is also dependent on the hydrodynamic action of the fluid content of the submucosa, which in turn depends on the degree of filling of the blood vessels; the mass of mucous membrane, and consequently the volume of its folds, is regulated by the varying vascularity in the submucosa.

> In addition Forssell showed that the surface of the mucosa, or mucosal relief pattern, may vary from moment to moment; these movements are independent of, but co-ordinated with the contractions of the muscularis externa. Especially in the small intestine, but also in the stomach, various active contractile shapes, in some of which the mucosa "grips" particles of food, may be discerned. These consist of digestion chambers, blocking or filtering devices, re-absorption reservoirs and smooth or corrugated transporting tubes. In this way each region may best meet the varying demands placed on it from moment to moment, namely digestion, storage, absorption and transport respectively. One moment the mucosa may be occupied with one task, the next with another. Forsell called the inherent ability of the mucosa to move "mucosal autoplastik", providing a working relief pattern. It also determined to a large extent the number, position and form of the folds. While the coarser breakdown of food particles is effected by contractions of the muscularis externa, the finer dispersion occurs through changes in the relief pattern of the mucosa, which may enhance or counteract effects of contraction of the muscular walls. The special contractile organ of the mucous membrane is the muscularis mucosae; being attached to the mucosa and being incorporated in the submucosa, it is able to displace the former in different directions.

---

EDIT

It's been almost twenty years since I read this book, and re-reading the quote above, I realize that this wild mechanism I described is also used in the small intestine (not just in the distal stomach and pylorus). It makes perfect sense, but it never occurred to me that this is how a whole lot of our guts work to move and process food. It's kind of another epiphany for me.