reticulo-rumen mass rumen mass 2026-09-23T15:39:34Z Trait.ontology VT:0011372 The amount of matter in the first compartment of the ruminant stomach. It lies on the left side of the body, occupying the whole of the left side of the abdomen and even stretching across the median plane of the body to the right side. It is capacious, divided into an upper and a lower sac, each of which has a blind sac at its posterior extremity.