Colloquially-speaking, cows have four (4) stomachs. Anatomically and biologically speaking though, cows and cattle have only one stomach with three additional fore-stomachs. These fore-stomachs are not true stomachs at all, but rather extensions of the lower esophagus prior to entry into the true stomach. In the ruminant, this true stomach is called the abomasum.
The three fore-stomachs, then, are called the reticulum, rumen, and omasum.
The reticulum is often called the "hardware stomach" since it tends to be a collection vat for foreign objects that the bovine accidentally ingests, from wood to nails. It also functions as a collection chamber prior to partly-digested food from the rumen being regurgitated and chewed as cud. It also functions somewhat as continued breakdown/digestion processes that leaked from the rumen. Some of the digestion juices accumulate in the reticulum to help break down the foreign objects that are prevented from going through the rest of the ruminant digestive tract.
The rumen is the main feature of the ruminant animal, and the largest fore-stomach of them all. It can hold upwards of 50 to 60 gallons of digesta. This is the fermentation vat that houses billions of microbes that call this place home, from bacteria and fungi to protists. The fermentation activity of the rumen makes it an anaerobic environment, where the partly-chewed forages and feeds that the cow consumes ends up in this mixing vat to become further consumed by this ecology of micro-organisms. The primary jobs of these microbes is to feed the cow by producing enzymes to break down as much of the coarse plant material as they possibly can. The nutrients released from this activity is made available to both them and eventually the cow. These microbes produce waste products that are gaseous which needs to be burped up by the cow (gases include carbon dioxide and methane), as well as volatile fatty acids. The microbes' lives are very short, with a half-life of only 15 minutes, and when they die they move on to the omasum and abomasum where they are digested and used as part (nearly half) of the ruminant's protein needs.
The third and final fore-stomach is the omasum. This is a unique chamber which has a lot of epidermis (skin) folded together to maximize the surface area available for liquid absorption. The omasum is the stopping-point between the rumen and the abomasum.
The abomasum functions very, very similar to the stomach that we humans and other non-ruminants have. It secretes similar enzymes and highly acidic hydrochloric acid to further break down the digesta that came from the rumen, and absorb some nutrients, before moving to the small intestine where most nutrient absorption occurs.
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