Stomach of the Ruminants:
1. Shape and Position
Rumen: It is a huge laterally compressed sac. It extends from the diaphragm to the pelvic inlet, on the left half of the abdominal cavity. The parietal surface of the rumen faces to the left and is related to the diaphragm, abdominal floor and the left abdominal wall. The visceral surface faces to the right and is related to the the intestines, the liver, the omasum and the abomasum. The dorsal curvature lies against the diaphragm and the roof of the abdominal cavity. The ventral curvature is in contact with the abdominal floor.
The rumen is divided into many parts by a number of grooves. Two shallow longitudinal grooves, the left on the parietal surface and the right on the visceral surface. Two deep transverse grooves, the cranial and caudal grooves. These four grooves divide the rumen into dorsal and ventral sacs. The dorsal and ventral coronary grooves extend dorsally and ventrally from the caudal end of the longitudinal grooves. They divide the caudal part of the rumen into caudodorsal and caudoventral blind sacs. The dorsal coronary grooves are absent in sheep and goat.
The atrium ruminis or the cranial sac of the rumen is the dorsal cranial projection of the rumen, it is continuous caudally with the dorsal sac and communicates cranially with the reticulum through the ruminoreticulum opening. The cranial end of the ventral sac is called the recessus ruminis.
Reticulum: Is the most cranial compartment of the ruminants stomach, lies between the rumen and the diaphragm at the level of the sixth to ninth intercostal spaces. Ventrally the reticulum is demarcated from the cranial sac of the rumen by the ruminoreticular fold internally and the ruminoreticular groove externally.
Omasum: The omasum of the ox is spherical in shape. It is demarcated from the reticulum by a constriction the collum omasi and from the abomasum by the sulcus omasoabomasicus. The omasum of the small ruminants is oval in shape and smaller than the reticulum.
Abomasum: The abomasum is the true stomach of ruminants. It is pear-shaped sac that is divided into fundus, body and pyloric part. The abomasum is closely related on the left to the recessus ruminis.
The capacity of the four compartments would rank as follow:
Ox: rumen, abomasum, omasum, and reticulum.
Sheep and goat: rumen, abomasum, reticulum, and omasum.