Rumination is a specialized digestion process found in most hoofed mammals with an even number of toes-such as cattle, sheep, goats, deer, antelope, camels, buffalo, giraffes, and chevrotains. All of these plant-eating animals lack the enzyme cellulase, which is capable of breaking down the tough cellulose in plant cell walls. The stomach of these grazing herbivores consists of four chambers—the rumen, the reticulum, the omasum, and the abomasum—each playing different roles in the digestion process. The ruminant animal swallows its food rapidly without chewing, and later regurgitates it (brings it back up into the mouth), then masticates it (chews), and finally re-swallows it.
When grazing, ruminants swallow their food rapidly, sending large amounts into the largest chamber of the stomach, the rumen, where it is stored and partly digested before regurgitation and chewing when the animal is resting. Rumination is an adaption by which herbivores can spend as little time as possible feeding (when they are most vulnerable to predation) and then later digest their food in safer surroundings. Muscular contractions of the stomach move food back and forth between the rumen and the second stomach chamber, the reticulum, which is often called the honeycomb due to the complex appearance of its inner lining. Bacteria and microorganisms in the rumen (which can digest cellulose) begin the digestion of the plant fibers. Fine fibers are broken down, so providing protein, vitamins, and organic acids which are then absorbed into the bloodstream of the animal. Coarser plant fibers are passed from the rumen to the reticulum, where further bacterial fermentation takes place, and the food is formed into soft chunks called the cud. The cud is regurgitated and ground thoroughly between the molars with an almost circular motion of the lower jaw.
During the chewing process, called chewing the cud, copious quantities of highly alkaline saliva aid in breaking down the fibers, and the food is re-swallowed, this time bypassing the rumen and entering the smallest chamber, the omasum, or third stomach. Here, water and essential acids are reabsorbed. It is the third stomach of a bullock which is eaten as tripe. Muscular contraction by the walls of the omasum mashes and compacts the food still further, passing it directly into the fourth stomach, the abomasum, where gastric secretions further digest the food before it moves into the intestine.
Large amounts of two gases, carbon dioxide and methane, form during bacterial fermentation in the first two chambers—the reticulorumen. Here, frothing occurs as part of the digestive process. Often, however, excessive frothing caused by certain foods traps gas normally eliminated by belching, and bloating occurs. Certain cows are particularly susceptible to this, and farmers often lose animals unless these gases are released. Anti-foaming medications sometimes help, as does an invasive procedure that punctures the stomach wall and allows gases to escape. The methane produced by the digestive systems of the billions of domestic ruminants in the world is considered by some to be a major factor in the destruction of the ozone layer in the upper atmosphere.