Omasum

The omasum, also known as the bible, the fardel, the manyplies and the psalterium, is the third compartment of the stomach in ruminants. Though its functions have not been well-studied, it appears to primarily aid in the absorption of water, magnesium, and the volatile fatty acids produced by rumen fermentation, that have not been absorbed into the bloodstream yet. The numerous folds of its mucosa are thought to trap digesta particles so the maximum amount of nutrients may be absorbed.

There is some evidence from the University of Nottingham by professor J Neave 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. (Though fermentation initiated in the reticulorumen can continue in the omasum, it does so in only limited quantities, so this mechanism of ejecting largely unfermented particles into the reticulorumen is necessary for complete fermentation.)

...
Wikipedia