The omasum, also known as the manyplies, is the third compartment of the stomach in ruminants. Little is known about its function, though it appears to primarily aid in the absorption of water, magnesium, and fermentation acids. The numerous folds of its mucosa are thought to trap digesta particles to increase their residence time for this said absorption. A ruminant is any hooved animal that digests its food in two steps, first by eating the raw material and regurgitating a semi-digested form known as cud, then eating the cud. ...

There is some evidence that the contractions of the omasum can propel large particles backwards through the reticulorumen orifice, the junction connecting the omasum with the reticulorumen, into the reticulorumen, the first compartment of the ruminant stomach. In this way, the omasum allows large particles, which still likely contain appreciable amounts of fermentable substrate, to be further digested in the reticulorumen.