stomach ruminant stomach The ruminants' stomach consists of four compartments: rumen, reticulum, omasum and abomasum. The first three compartments comprise the forestomach (proventriculus). They are lined with nonglandular membrane. The fourth compartment, the abomasum comprises the true stomach of the ruminants. It is lined with glandular mucous membrane. The rumen lies on the left of the abdominal cavity, the reticulum cranially and the omasum on the right. The abomasum lies ventrally, its proximal end lies ventral to the rumen, reticulum and omasum.