“…These variations can cause difficulties when carrying out experimental research in this region. This anatomical variability of several tributary vessels leading to the portal vein was confirmed in mammals such as dogs (Kalt and Stump, 1993;Libersa and Laude, 1965;Evans and Christensen, 1979;Duarte 1989), cats (Walker, 1972;Fragoso Neto, 1983), oxen (Maala and Sack, 1983) and in man (Didio, 19.52;1961). In dogs 'the pre-hepatic portion of the portal vein' was described as being formed, invariably, by the confluence of the cranial and caudal mesenteric veins (Kalt and Stump, 1993).…”