WE BECOME so accustomed to things we see every day that we don't give them much thought. For example, cows are a part of my daily landscape and, although I always enjoy seeing them, I don't often stop to appreciate the amazing chemical and physical processes going on inside those large, warm bodies. Cows and many other familiar animals, such as sheep and deer, are ruminants. They seem to be always chewing, even when they are not actively eating. This chewing is called rumination, and it is part of a complex process that allows ruminants to extract nutrients from low-quality, high-cellulose food. To digest grass and hay, cows rely on the combination of an extraordinary four-chambered stomach, friendly micro-organisms, and an eight-hour-a-day regimen of cud chewing. When a cow is grazing, she is gathering a reserve rather than eating. Freshly consumed food passes into the first chamber of the cow's stomach, the rumen, where it forms a layer at the rear. Here, inside what is essentially a large fermentation vat, more than 200 types of bacteria and 20 different protozoa attack the surface of food particles with enzymes capable of breaking down the plant cell walls. The cow absorbs the fatty acids released by this process, and they form an important part of her total calorie intake. This is not a small-scale operation: the rumen may hold as much as 200 litres of partially digested food. To help the process along, the cow periodically belches up mushy material from the front of the rumen and grinds it between her molars. This process, called chewing a cud, breaks the food into smaller particles with more surface area for the bacteria to attack; it also adds large amounts of acid buffering saliva to the food mix. We're talking lots and lots of spit here. A cow may produce 40 to 150 litres of saliva a day, depending on what she is eating! When the cow re-swallows her cud, bacteria continue to work on the food particles, which eventually sink to the bottom of the rumen, ready for the next part of the process. The rumen convulses about once a minute, either sending material up the esophagus for further processing, or passing food particles down the pipeline to the next three chambers of the cow's stomach. There, as in the human digestive tract, nutrients such as lipids and carbohydrates are absorbed. This double-dipping digestive design has several advantages for ruminants. It allows them to take advantage of foods other animals cannot use. It also lets them minimize their active grazing time, and therefore the time they are exposed to predators. It's a good survival strategy: quickly cram yourself with food, and then retreat to a nice, safe spot to ruminate placidly. Ah, yes, ruminate - from the Latin word ruminare, meaning "to think." You know, when you watch a big Holstein gazing mildly upon the world as she chews her cud, it's easy to imagine she is contemplating some deep philosophical issues. I ruminate, therefore I am? - REBECCA L. GRAMBO