Definition of "omasum"

The deer of the northern group are larger in body size and have well-developed rumino-reticulums, smaller omasums, and longer small intestines (Takatsuki 1988).

2009, Seiki Takatsuki, “17: Geographical Variations in Food Habits of Sika Deer: The Northern Grazer vs. the Southern Browser”, in Dale R. McCullough, Seiki Takatsuki, Koichi Kaji, editors, Sika Deer: Biology and Management of Native and Introduced Populations, Springer, page 235

The omasum is a spherical organ filled with muscular laminae (an estimated 90 to 130 in the bovine omasum) that lie in sheets, much like the pages of a book (giving the omasum its colloquial name, book stomach).

2018, Anna Dee Fails, Christianne Magee, Anatomy and Physiology of Farm Animals, Wiley Blackwell, page 385

The role of the omasum is not entirely clear. The organ fills with very fluid digesta leaving the reticulo-rumen, but omasal contents are very dry and tightly compacted. Clearly, fluid absorption takes place in the omasum and in cattle, water, electrolytes and VFA[volatile fatty acids] are absorbed.

2019, Philip Hynd, Animal Nutrition, CSIRO Publishing, unnumbered page

Finally, grazers, with the higher fluid throughput through the reticulorumen, require larger omasa (Table 4.6)—with the main function of omasa being the resorption of fluid, to prevent too diluted digesta reaching the sites of auto-enzymatic digestion (Clauss et al. 2006).

2019, Daryl Codron, Reinhold R. Hofmann, Marcis Clauss, “Chapter 4: Morphological and Physiological Adaptations for Browsing and Grazing”, in Iain J. Gordon, Herbert H. T. Prins, editors, The Ecology of Browsing and Grazing II, Springer, page 109