Overview
The abomasum (abomasum) is the only compartment with a glandular mucosa, equivalent to the stomach of monogastric animals. It secretes hydrochloric acid and pepsin; in the suckling calf it also secretes chymosin, which clots milk. In the newborn it is the largest compartment, because the forestomachs have not yet developed; in the adult it is much smaller than the rumen.
Topography
- Fundus and body: begin at the omasoabomasal orifice and rest on the abdominal floor, ventral to the omasum and caudal to the reticulum, slightly to the left and mostly to the right of the median plane.
- Pyloric part: runs caudally and dorsally on the right, along the right costal arch, and ends at the pylorus, which opens into the duodenum.
- Omenta: the greater omentum attaches to the greater curvature and the lesser omentum to the lesser curvature; these attachments hold it in place, but the abomasum keeps considerable mobility.
Mucosa
- Fundic region: with large spiral folds (plicae spirales abomasi) that enlarge the secretory surface.
- Pyloric region: smoother mucosa; at the pylorus a prominence of the wall, the pyloric torus (torus pyloricus), helps close the outlet.
Clinical relevance
- Left displaced abomasum (LDA): the abomasum, distended with gas and with reduced motility, moves between the rumen and the left abdominal wall. It is typical of high-yielding dairy cows in the first weeks after calving, often with hypocalcaemia, ketosis or metritis. It is diagnosed by the metallic "ping" heard on simultaneous percussion and auscultation over the dorsal third of the last left ribs, and corrected surgically by fixing the abomasum (omentopexy or abomasopexy).
- Right displacement (RDA) and volvulus: the ping is on the right side. Volvulus is a surgical emergency: the twisted abomasum distends, loses its blood supply and sequesters hydrochloric acid, with dehydration and hypochloraemic, hypokalaemic metabolic alkalosis.
- Abomasal ulcers: they can bleed (dark faeces, melaena) or perforate with local or diffuse peritonitis. They occur in adult cows after calving and in calves.
- Ostertagiosis: Ostertagia ostertagi develops in the glands of the mucosa and causes nodules, protein loss and diarrhoea, mainly in young grazing cattle.
- Lymphoma: the abomasum is one of the usual sites of lymphoma in enzootic bovine leukosis.