abomasum

The abomasum (fourth stomach) is the fourth and final stomach compartment of ruminant animals such as cattle, sheep, and goats. Unlike the first three forestomachs (rumen, reticulum, reticulum, and omasum), the abomasum is the 'true' stomach where enzymatic digestion of proteins occurs, analogous to the single stomach of monogastric animals like humans.

The abomasum is an elongated pear-shaped sac that lies on the floor of the abdomen on the right-hand side, roughly between the seventh and twelfth ribs. It connects the omasum (third stomach) on one end and leads directly into the beginning of the small intestine (duodenum) on the other.

The abomasum secretes hydrochloric acid and digestive enzymes (particularly pepsin) to break down proteins in the feed that has been fermented and passed through the other stomach compartments. It functions as the principal site of enzymatic digestion in ruminants.

Displaced abomasum (particularly left-displaced abomasum) is a common condition in dairy cattle, especially after calving, where the organ shifts position and fills with gas. Abomasal volvulus (torsion) is a life-threatening emergency. Abomasal ulcers can also occur, particularly in calves.

The rumen is the largest forestomach and functions as a fermentation vat where microorganisms digest plant fiber, while the abomasum is the true glandular stomach that secretes acid and enzymes for protein digestion. Feed passes through the rumen before reaching the abomasum.