Ultrasonogram of the longitudinal groove. Ultrasonogram of the rumen of a three-year-old Holstein Friesian cow viewed at the level of the longitudinal groove at the 12th intercostal space. 1 Abdominal wall, 2 Dorsal sac of rumen, 3 Longitudinal groove, 4 Ventral sac of rumen, Ds Dorsal, Vt Ventral.