Medial Division of the Left Liver in a Male Observed from the Lateral Position

Description

Viewed from the patient’s left lateral side, the medial division of the left hepatic lobe (Couinaud segment IV) occupies an anteromedial position between the left lateral segments (II and III) and the right liver, forming a broad wedge of parenchyma that reaches superiorly toward the diaphragm. Its inferior margin approaches the porta hepatis, where the transverse fissure separates parenchyma from the hepatoduodenal ligament structures, and the segment’s medial border abuts the plane of the middle hepatic vein. Anteriorly, this territory lies deep to the anterior abdominal wall and costal margin. Segment IV is the bridge. Segment IV matters because it is the practical dividing zone between “left” and “right” functional liver in hepatobiliary surgery, yet it sits in the line of fire for both left hepatectomy planning and extended right resections when tumors cross the middle hepatic vein drainage territory. A lateral position helps you appreciate how far segment IV projects toward the anterior surface and how its cranial extent can sit under the ribs, which affects subcostal incision planning and the working angles for laparoscopic parenchymal transection. This is also the segment that often requires deliberate attention in living donor graft design and in segmentectomy for centrally located colorectal metastases. Use this illustration for teaching Couinaud segmentation in gross anatomy, surgical anatomy, and radiology correlation modules, or as a figure in hepatobiliary oncology manuscripts discussing resection margins, inflow control, and venous outflow preservation. It also fits patient-facing surgical consent materials when explaining why a “left” liver operation may still involve tissue near the midline. Anatomical accuracy verified by SciePro's Medical Advisory Board.