Overview
The rumen (rumen) is a non-glandular forestomach lined by keratinised stratified squamous epithelium. Together with the reticulum it forms the reticulorumen, where a population of bacteria, protozoa and fungi ferments plant fibre. The main products are the volatile fatty acids (acetic, propionic and butyric), which are absorbed through the wall and supply most of the animal's energy, and gases (carbon dioxide and methane), which are removed by eructation.
Topography and parts
- Position: it fills the left half of the abdomen, from the diaphragm to the pelvic inlet; its parietal surface contacts the left abdominal wall and the left paralumbar fossa. The ventral sac extends across the median plane to the right. The spleen adheres to its craniodorsal surface, next to the diaphragm.
- Grooves and pillars: the external grooves correspond to the pillars (pilae ruminis), internal muscular folds that divide the cavity. The cranial and caudal pillars, joined by the longitudinal pillars, separate the dorsal sac from the ventral sac.
- Blind sacs: cranially, the ruminal atrium (atrium ruminis) of the dorsal sac and the ruminal recess (recessus ruminis) of the ventral sac; caudally, the caudodorsal and caudoventral blind sacs, outlined by the coronary pillars.
- Ruminoreticular fold: incompletely separates the rumen from the reticulum; above it, contents pass freely between the two.
Mucosa and contents
- Ruminal papillae: mucosal projections that multiply the absorptive surface. They are largest and most numerous in the ventral sac and the blind sacs, and sparse on the roof of the dorsal sac and on the pillars. Their development depends on volatile fatty acids; in the suckling calf the rumen is small and grows once the calf starts eating solid feed.
- Stratification: the contents settle into a dorsal gas cap, a floating fibre mat and a ventral fluid phase. Contractions mix the contents, return boluses to the mouth for rumination and move the gas cap towards the cardia to be eructated.
Clinical relevance
- Motility: contractions are auscultated and palpated in the left paralumbar fossa; in a healthy adult about one or two are heard per minute. Their absence means ruminal atony.
- Bloat (ruminal tympany): frothy bloat is caused by legumes or grain-rich diets that trap gas in a stable foam; free-gas bloat by failed eructation (esophageal obstruction, vagal indigestion, lateral recumbency). Both distend the left paralumbar fossa; free-gas bloat is relieved with a stomach tube or trocar.
- Ruminal acidosis: excessive intake of fermentable carbohydrate lowers the pH and favours lactic acid production. The acute form causes dehydration and collapse; the subacute form causes rumenitis that lets bacteria into the portal blood and leads to liver abscesses.
- Rumenotomy: approached through the left paralumbar fossa, for example to remove foreign bodies from the reticulum.
- Rumen fluid: collected by orogastric tube or by rumenocentesis of the ventral sac to measure pH and assess the flora.
- Left displaced abomasum: the abomasum lies between the rumen and the left wall; its metallic "ping" on percussion must be told apart from that of the ruminal gas cap.