Passage of Material from the Reticulum to the Omasum Occurs During Reticular Contraction

The omasum is composed of a body and a canal. The body is filled with multiple muscular folds, or leaves, that project from the greater curvature into the lumen. The canal, which is located on the lesser curvature, connects the reticulum to the abomasum.

Proper functioning of the omasum and reticulum appears to be particularly important to the passage of ingesta out of the rumen. Occasionally, traumatic injury resulting from ingested foreign bodies causes severe adhesions of the reticulum and omasum to the body wall. In addition, damage to vagal fibers entering the organs may occur. In such cases, motility of the rumen proper may continue normally, but the ability to move food out of the forestomachs and into the abomasum is severely impaired. The rumen becomes greatly distended with finely comminuted feed, and the entire rumen becomes a slurry zone. Despite the distended rumen, little movement of ingesta occurs into the abomasum, and the animals eventually suffer severe inanition. This condition is variably known as omasal transport failure and vagal indigestion; usually, little can be done to correct it.

The structure of the omasum, with its many leaves and large mucosal surface area, suggests that it has an absorptive function, but the exact nature of this function is still incompletely understood. One important possibility is that it exists to remove residual VFAs and bicarbonate from ingesta before material is transported to the abomasum. VFAs appear to cause unfavorable reactions in the abomasum, so it is important that the major portion of them be removed before abomasal entry. Also, it appears desirable to absorb, before abomasal entry, any bicarbonate remaining in the ingesta. Bicarbonate remaining in ingesta and entering the abomasum would only neutralize abomasal hydrochloric acid, making the abomasal glands work harder to maintain appropriate abomasal pH.