Overview
The omasum (omasum) is a non-glandular forestomach, almost spherical and firm. It lies in the middle of the cranial abdomen to the right of the median plane, covered by the right ribs and partly by the liver. The reticulum and rumen lie to its left; the abomasum lies ventral to it.
Internal structure
- Omasal laminae (laminae omasi): mucosal folds that arise from the greater curvature and almost fill the cavity. Laminae of several sizes alternate, and their surface is covered with horny papillae.
- Omasal canal (canalis omasi): a free space along the lesser curvature joining the reticulo-omasal orifice to the omasoabomasal orifice. Its floor, the omasal groove, continues the reticular groove; in the suckling calf milk runs along it directly to the abomasum.
- Contents: the material held between the laminae is drier and more compact than that of the rumen.
Function
The omasum receives small portions of fermented contents from the reticulum. Its contractions squeeze out the fluid and push it between the laminae, where water, volatile fatty acids and electrolytes are absorbed. This reduces the volume of fluid reaching the abomasum.
Clinical relevance
- Examination: it cannot be palpated because it is covered by the ribs and liver. It is assessed by ultrasound through the right intercostal spaces.
- Vagal indigestion with failure of omasal transport: damage to the vagus nerve, often after traumatic reticuloperitonitis, stops contents passing to the omasum and abomasum; the rumen distends and the abdomen takes on an "L" shape seen from behind.
- Omasal impaction: accumulation of dry contents between the laminae, associated with very fibrous forage or vagal indigestion.
- Necropsy: dry contents between the laminae are normal and should not be read as impaction.