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Cows are ruminants, members of the suborder Ruminantia (order Artiodactyla). Other members of this suborder are the pronghorns, giraffes, deer, antelopes, sheep, and goats. Most ruminants have a four-chambered stomach. (Camels and some other ruminants, however, have a three-chambered stomach.)

Cows and other ruminants have a very different digestive system than humans do which work very differently in order to digest the foods they eat. The stomachs that they have all serve different purposes in order to break down these foods properly. Humans eat so many types of foods that our bodies cannot always break down them correctly. This can lead to fatty build ups amongst other issues. The ruminants use their different stomachs in a series events so that different protozoans and bacteria can properly breakdown the hard to digest foods. Each phase of the ruminants' digestion helps to provide them with valuable nutritional substances like protein.

The first chamber is the large rumen (or paunch). The next two are the reticulum and the omasum (psalterium or manyplies). These first three chambers are believed to be derived from the esophagus. The last chamber is the abomasum (or reed), which corresponds to the stomach of other mammals.

The combined four-chambered stomach is big. In the domestic ox (Bos taurus) the whole stomach occupies nearly three-quarters of the abdominal cavity. In medium sized cattle, the rumen by itself can hold between 25 to 75 gallons. The rumen grows large in early life after the changeover from a milk diet

Ruminants eat fast and store large quantities of grass or foliage in the rumen, where it softens. Many species of minute protozoans and bacteria live without free oxygen in the rumen. These little animals and bacteria digest the cellulose in the plant material, thereby releasing the contents of the plant cells for digestion by the cow. Large amounts of saliva get secreted into the rumen to further the digestion.

The action of the various microbes produces various substances, including fatty acids which are absorbed through the rumen wall. In addition, any protein is converted into fatty acids and ammonia; the ammonia and other simple nitrogen-containing substances are used by the micro-organisms for their own cell-protein synthesis.

After the plant material is processed in the rumen, it is later regurgitated. This material is now called cud, and the ruminant chews it again. The additional chewing breaks down the cellulose content, which is difficult to digest, even more. The regurgitation and chewing of the cud is called rumination.

The chewed cud goes directly to the other chambers of the stomach (the reticulum, omasum, and abomasum, in that order). Additional digestion, with the aid of various essential microorganisms, continues in these other chambers. For example, in the omasum, some fatty acids and 60-70 percent of the water are absorbed. In the abomasum gastric juice containing hydrochloric acid is secreted, as in an ordinary mammalian stomach, futher digesting the food. Also, those micro-organisms that used the ammonia and other nitrogen substances from protein in the rumen, actually get digested by the ruminant in the abomasum and small intestine, thereby providing the cow with protein.

Source: Encyclopedia Brittanica online at, entries Ruminant and Artiodactyl, digestive system.