Omasum

The omasum, also known as the psalterium or book stomach, is the third compartment of the ruminant stomach, located between the reticulorumen and the abomasum. It lies mainly on the right side of the abdominal cavity and is bordered by the looping rumen and reticulum anteriorly and the abomasum posteriorly.

Anatomically, the omasum is characterized by numerous mucosal folds arranged in leaf-like laminae. These laminae create a large surface area covered with papillae, giving the organ its distinctive appearance. The laminar structure functions as a mechanical and absorptive chamber, helping to squeeze ingested material and reduce particle size before it moves to the abomasum.

The primary roles of the omasum are mechanical reduction of digesta and the selective absorption of water, electrolytes, and volatile fatty acids produced by microbial fermentation in the rumen. By absorbing fluids and concentrating ingesta, the omasum helps regulate the rate at which material passes to the abomasum and minimizes the transfer of large particles.

Clinical significance includes the possibility of omasal impaction, usually arising from poor-quality roughage or obstruction, which can lead to reduced feed intake, dehydration, and altered rumination. Diagnosis is based on clinical signs and imaging or exploratory procedures, with treatment ranging from supportive care to surgical intervention in severe cases.

Etymology traces the name to the Latin psalterium, or “book,” reflecting the book-like arrangement of its folds. The omasum is present in ruminants such as cattle, sheep, and goats and is absent in non-ruminant species.